Glossary of Abbreviations

Glossary of Abbreviations and Acronyms

We use some abbreviations on our website to refer to different beauty and aesthetic treatments. If you are not familiar with these terms, you can find their meanings and explanations here.

AA: Arachidonic Acid

Arachidonic acid is an omega-6 fatty acid (distinct from arachidic acid, a saturated fat with a similar name) produced in the body from DGLA and found in animal-source foods. It is a precursor to a range of signalling molecules - including prostaglandins, leukotrienes, and thromboxanes - that mediate inflammatory responses in the skin and throughout the body. Whilst these inflammatory pathways serve important functions in wound healing and immune defence, chronically elevated AA activity is associated with persistent skin inflammation and conditions such as atopic dermatitis.

ACC: Acetyl-CoA Carboxylase

Acetyl-CoA carboxylase is the enzyme that converts acetyl-CoA into malonyl-CoA — a critical step in the regulation of fat oxidation. When glucose and insulin are elevated, ACC activity increases, raising malonyl-CoA levels, which in turn inhibit CPT-1 and prevent long-chain fatty acids from entering the mitochondria. This is the mechanism by which high carbohydrate intake suppresses fat burning at the cellular level, forming the glucose-to-fat-oxidation direction of the Randle Cycle. ACC is also the rate-limiting enzyme in de novo fatty acid synthesis, linking it to both fat oxidation regulation and endogenous fat production.

ACD: Allergic Contact Dermatitis

Type IV (delayed-type) hypersensitivity reaction requiring prior sensitisation. Reaction typically peaks 48–96 hours post-exposure. Diagnosed by patch testing. Distinguish from ICD (irritant contact dermatitis), which requires no sensitisation and is non-immunological.

ACTH: Adrenocorticotropic Hormone

Pituitary peptide hormone released in response to CRH, acting on the adrenal cortex to stimulate cortisol and adrenal androgen (DHEA-S) production. Relevant to skin via two pathways: cortisol-mediated barrier disruption, and adrenal androgen-driven sebum overproduction in stress-related acne. Also known as corticotropin.

AD: Atopic Dermatitis

Chronic relapsing inflammatory skin condition characterised by barrier dysfunction, Th2/IL-4/IL-13 immune polarisation, and filaggrin deficiency. Distinct from contact dermatitis and seborrhoeic dermatitis.

AGA: Androgenetic Alopecia

Androgenetic alopecia is the most common form of progressive hair loss, affecting both men and women. It is driven by the action of dihydrotestosterone (DHT) on genetically susceptible hair follicles, causing dermal papilla cells to progressively shorten the active growth phase and reduce hair shaft diameter - a process known as miniaturisation. In women it typically presents as diffuse thinning across the crown rather than the defined recession pattern more common in men.

AGE: Advanced Glycation End-product

See: Advanced Glycation End-products (AGEs)

AGEs: Advanced Glycation End-products

Advanced Glycation End-products are compounds formed when sugar molecules bind to proteins or fats in your body, a process called glycation. In skin, AGEs primarily damage collagen and elastin by creating cross-links between protein fibres, which reduces their flexibility and contributes to wrinkles, loss of elasticity, and premature ageing. High sugar diets accelerate AGE formation, whilst environmental factors like UV exposure and smoking also increase their production. Understanding AGEs is important because managing dietary sugar intake can help slow their formation and preserve skin health.

AHA: Alpha-Hydroxy Acid

Alpha Hydroxy Acids are a group of water-soluble acids derived from natural sources like fruits and milk that gently exfoliate the skin's surface. Common examples include glycolic acid (from sugar cane), lactic acid (from milk), citric acid (from citrus), malic acid (from apples), and tartaric acid (from grapes). These acids work by dissolving the bonds between dead skin cells, allowing them to be lifted away and revealing fresher, brighter skin underneath. Regular use of AHAs can improve skin texture, reduce fine lines, even out pigmentation, and enhance moisture retention, making them particularly effective for dull, sun-damaged, or ageing skin.

ALA: Alpha-Linolenic Acid

Alpha-linolenic acid is the essential omega-3 fatty acid — meaning the body cannot produce it and it must be obtained from food. It is the parent omega-3, found in flaxseed oil, chia seeds, walnuts, and rapeseed oil, and serves as the dietary starting point for conversion to the longer-chain omega-3s EPA and DHA, though this conversion is inefficient in the body. In skin biology, ALA is incorporated into membrane phospholipids where it contributes to the anti-inflammatory balance between omega-3 and omega-6 fatty acids, making dietary omega-3 sources relevant to both skin barrier competency and inflammatory skin conditions.

AR: Androgen Receptor

Androgen receptors are nuclear receptor proteins found in dermal fibroblasts, keratinocytes, sebocytes, and hair follicle dermal papilla cells that mediate the effects of androgens - primarily testosterone and dihydrotestosterone (DHT) - on skin tissue. When bound by an androgen, AR translocates to the cell nucleus and acts as a transcription factor, altering gene expression to produce effects including sebocyte proliferation, follicle miniaturisation, and modulation of collagen synthesis. DHT binds androgen receptors with approximately five times greater affinity than testosterone, meaning local 5-alpha reductase activity in skin tissue can substantially amplify AR signalling without any change in circulating hormone levels.

ATGL: Adipose Triglyceride Lipase

Adipose triglyceride lipase is the enzyme that initiates triglyceride breakdown in adipocytes, cleaving the first fatty acid from a stored triglyceride molecule to produce diacylglycerol (DAG). It is responsible for the majority of triglyceride hydrolysis under basal conditions and acts as the first step in the lipolysis cascade, with hormone-sensitive lipase (HSL) completing the process. ATGL activity is the rate-limiting factor for overall fat mobilisation from adipose tissue between meals and during exercise.

BAT: Brown Adipose Tissue

Brown adipose tissue is a metabolically specialised form of fat whose primary function is heat generation rather than energy storage. Its adipocytes are dense with mitochondria and express uncoupling protein-1 (UCP-1), which converts the energy that would normally produce ATP into heat instead. BAT is most abundant in newborns and persists in adults around the neck and upper back; its activity declines with age and obesity. Unlike white adipose tissue, BAT burns energy rather than storing it, making it a focus of interest in metabolic health research.

BHB: Beta-Hydroxybutyrate

The predominant circulating ketone body produced by hepatic fatty acid oxidation during carbohydrate restriction, fasting, or caloric deficit. Functions both as an alternative fuel substrate and as a signalling molecule — an endogenous HDAC inhibitor (class I and IIa), a direct suppressant of the NLRP3 inflammasome, and an upstream activator of FOXO3a-mediated oxidative stress resistance pathways.

CAP: Cold Atmospheric Plasma

CAP refers to ionized gas at or near room temperature and atmospheric pressure. This ionized state is achieved by applying a strong electric field to a gas mixture, typically air. The resulting plasma contains charged particles, ultraviolet radiation, and various reactive chemical species, including ROS. The "cold" designation differentiates CAP from high-temperature plasmas used in industrial applications. CAP's unique properties make it a promising tool in various medical and cosmetic applications, including promoting wound healing and stimulating skin regeneration.

CCE: Cornified Cell Envelope

See: cornified envelope (CE)

CCK: Cholecystokinin

Cholecystokinin is a satiety hormone released by I-cells in the upper small intestine in response to dietary fat and protein. It acts on receptors in the vagus nerve and brainstem to generate one of the earliest fullness signals of a meal — typically within 15 to 30 minutes of eating — and simultaneously triggers the release of digestive enzymes from the pancreas and bile from the gallbladder. CCK works alongside GLP-1 and PYY as part of the body's natural appetite regulation system, and its release is activated most effectively by whole food sources of fat and protein rather than rapidly absorbed refined carbohydrates.

CCO: Cytochrome c oxidase

The terminal enzyme of the mitochondrial electron transport chain (Complex IV), responsible for accepting electrons from cytochrome c and transferring them to molecular oxygen, completing the ETC and driving the proton gradient that powers ATP synthesis. The primary photoacceptor for red (630–660nm) and near-infrared (810–850nm) wavelengths in photobiomodulation. Contains copper and haem centres that absorb photons at these wavelengths; red and NIR photon absorption photodissociates inhibitory nitric oxide (NO) from CCO's oxygen-binding site, restoring electron transfer rate and increasing ATP production. Central to the mechanism of LED therapy in dermal fibroblasts and dermal papilla cells.

CE: Cornified Envelope

The insoluble proteinaceous shell that forms the structural core of the corneocyte, replacing the plasma membrane during terminal keratinocyte differentiation. Assembled from involucrin, loricrin (approximately 70% of CE mass), SPRRs, envoplakin, and periplakin, crosslinked by transglutaminase-1 (TGM1) in a calcium-dependent reaction. The CE outer surface is subsequently esterified with ceramides and other lipids by TGM1 to form the lipid envelope — the covalently bound lipid layer that bridges the protein envelope and the extracellular intercellular lipid matrix. CE structural integrity depends on programme completeness through the entire mid-to-terminal differentiation phase; IL-4/IL-13 suppression of loricrin and involucrin, cortisol-driven premature cornification, and calcium gradient disruption each produce structurally incomplete CEs independently.

CGRP: Calcitonin Gene-Related Peptide

37-amino acid neuropeptide co-released with substance P from sensory C-fibres and Aδ-fibres at peripheral skin nerve terminals. The most potent endogenous vasodilator in the peripheral microcirculation. Acts via CLR/RAMP1 heterodimer receptors on vascular smooth muscle, endothelial cells, mast cells, and dendritic cells, producing vasodilation, increased vascular permeability, and immune cell recruitment. Serum CGRP is significantly elevated in neurogenic rosacea and correlates with clinical severity and flushing frequency.

CLE: Corneocyte Lipid Envelope

A covalently bound monolayer of omega-hydroxyceramides esterified to specific glutaminyl residues (Q107, Q118, Q122, Q133, Q496) on the outer surface of the cornified protein envelope (CPE) by transglutaminase-1 (TGM1). Distinct from the intercellular lamellar lipid bilayers — the CLE is covalently attached rather than organised by non-covalent forces, and is resistant to the lipid solvents that extract extracellular ceramides. Its primary function is architectural: it serves as the covalent anchor point through which the extracellular lamellar bilayer lipids organise themselves against the corneocyte surface. Without a complete CLE, lamellar bilayer organisation fails downstream regardless of extracellular ceramide availability. The CLE precursor pathway requires acylglucosylceramide delivery via lamellar body secretion, GBA-mediated deglucosylation, esterase-mediated linoleate release, FATP4, CYP4F22-mediated omega-hydroxylation, and ELOVL4-mediated ultra-long-chain fatty acid elongation — making it a non-redundant, multi-step biosynthetic pathway.

CMC: Carboxymethylcellulose

Carboxymethylcellulose is a synthetic food additive used widely as an emulsifier and thickening agent in ultra-processed foods, including many sauces, dressings, processed meats, and low-fat dairy products. Research published in Gut demonstrated that CMC directly disrupts the gut's protective mucus layer and alters microbiome composition in ways that increase pro-inflammatory bacterial activity — independently of a food's macronutrient profile. It is one of several food additives whose gut-disruptive effects appear to operate through the microbiome rather than through direct intestinal tissue contact.

CoQ10: Coenzyme Q10

Coenzyme Q10 is the only endogenously synthesised lipid-soluble antioxidant in the human body — a fat-soluble molecule present in every cell that functions simultaneously as an essential component of mitochondrial ATP production and as a direct scavenger of reactive oxygen species, with both functions declining measurably in skin with age and UV exposure, and both replenishable through topical application at concentrations that reach the viable epidermis.

CPE: Cornified Protein Envelope

The insoluble, covalently crosslinked protein shell that forms the structural core of the corneocyte, assembled in four sequential stages from involucrin (scaffold initiator), envoplakin and periplakin (rigidity platform), loricrin (approximately 70% of protein mass), and SPRRs (elasticity), crosslinked by transglutaminases 1, 3, and 5. The CPE replaces the plasma membrane entirely during terminal keratinocyte differentiation. Its assembly is calcium-dependent at every stage via TGM1's requirement for calcium as a direct cofactor. CPE completeness is directly correlated with barrier resilience: a higher proportion of incompletely crosslinked corneocytes produces measurably greater TEWL responses to equivalent barrier insult.

CPT-1: Carnitine Palmitoyltransferase-1

Carnitine palmitoyltransferase-1 is an enzyme located on the outer mitochondrial membrane that controls the entry of long-chain fatty acids into the mitochondria for oxidation. It acts as a gatekeeper for fat burning: when CPT-1 is active, fatty acids cross into the mitochondria and are used for energy; when it is inhibited, they cannot enter and are rerouted toward storage as triglycerides or accumulate as lipid intermediates. CPT-1 is inhibited by malonyl-CoA, which rises when glucose and insulin are elevated — making it the key molecular switch in the Randle Cycle through which high carbohydrate intake suppresses fat oxidation.

CRH: Corticotropin-Releasing Hormone

Hypothalamic peptide hormone that initiates the HPA axis stress response by stimulating pituitary ACTH release. Also produced locally in skin by keratinocytes and sebocytes, where it acts via CRH-R1 receptors to stimulate sebocyte lipid synthesis — a direct mechanism linking stress to acne. Synonyms: corticotropin-releasing factor (CRF).

DAG: Diacylglycerol

Diacylglycerol is a lipid molecule and signalling intermediate produced when fatty acids cannot be fully oxidised — typically when fat accumulates in cells unable to process it efficiently due to impaired mitochondrial function or metabolic inflexibility. In this context, DAG activates protein kinase C (PKC), which interferes with insulin receptor signalling by phosphorylating IRS-1 at serine residues rather than tyrosine residues — blocking the insulin cascade and contributing to insulin resistance. DAG accumulation in muscle and liver tissue is one of the primary mechanisms by which the fat-sugar combination characteristic of ultra-processed foods drives metabolic disease.

DCA: Deoxycholic Acid

Deoxycholic acid is a bile acid produced naturally in the gut from primary bile acids secreted by the liver, where its endogenous role is emulsifying dietary fat to aid digestion. In aesthetic medicine, a synthetic form of the same molecule is used as an injectable fat reduction agent — it disrupts the cell membranes of adipocytes in subcutaneous tissue, causing permanent adipocyte destruction followed by inflammatory clearance and connective tissue remodelling. Unlike treatments that reduce fat cell size, DCA destroys fat cells permanently; the treated cells are replaced by fibrous tissue rather than regenerating. In the UK, DCA-based injection lipolysis is most commonly delivered as Aqualyx, a CE-marked formulation combining deoxycholic acid with a polymer matrix that controls diffusion at the injection site.

DEJ: Dermo-Epidermal Junction

The dermo-epidermal junction is the structural interface between the skin's outer epidermis and the deeper dermis, made up of anchoring proteins including Type IV and Type VII collagen and laminin. It acts as a communication channel, allowing fibroblasts in the dermis to send growth and differentiation signals up to the keratinocytes above, and vice versa. This junction weakens with age and UV exposure, reducing the mechanical anchoring of the outer skin layer and impairing the cross-layer signalling that supports renewal quality. Many professional rejuvenation treatments — including microneedling and fractional laser — work in part by stimulating repair and remodelling at the DEJ.

DGLA: Dihomo-Gamma-Linolenic Acid

DGLA is an omega-6 fatty acid formed when gamma-linolenic acid (GLA) is elongated in the body, sitting at a metabolic crossroads between anti-inflammatory and pro-inflammatory pathways. It can be converted into prostaglandin E1, which has anti-inflammatory effects, or it can be further converted to arachidonic acid, which contributes to inflammatory signalling. The balance between these two outcomes is influenced by diet, stress, and overall metabolic health, making DGLA relevant to conditions where skin inflammation plays a role.

DHA: Docosahexaenoic Acid

DHA is an omega-3 fatty acid that plays a crucial structural role in the brain, eyes, and nervous system, making up about 40% of the polyunsaturated fats in your brain. It supports normal brain function, vision, and cognitive health, with research linking adequate DHA levels to improved memory and mental clarity. DHA is also important during pregnancy and early childhood for proper neurological development. Like EPA, DHA is primarily found in oily fish and marine sources, though it can also be derived from algae for vegetarian and vegan supplements.

DHT: Dihydrotestosterone

Dihydrotestosterone is a potent androgen converted from testosterone by the enzyme 5α-reductase. In hair follicles, DHT binds to androgen receptors on dermal papilla cells, triggering a signalling cascade that progressively miniaturises genetically susceptible follicles - shortening the anagen growth phase, reducing hair shaft thickness, and eventually causing follicle regression. It is the primary hormonal driver of androgenetic alopecia and the target of medications such as finasteride.

DP: Dermal Papilla

The dermal papilla is a compact cluster of specialised fibroblasts located at the base of each hair follicle. It functions as the follicle's signalling command centre, directing whether the follicle actively grows, rests, or regresses through paracrine communication with hair follicle stem cells and follicular keratinocytes. It is the primary site of action for both DHT in androgenetic alopecia and growth factors delivered through PRP and iPRF hair treatments.

DPC: Dermal Papilla Cell

Dermal papilla cells are the specialised fibroblasts that make up the dermal papilla at the base of each hair follicle. They direct the hair growth cycle by secreting paracrine signals, including FGF-7, VEGF, and GAS6, that activate hair follicle stem cells, sustain follicular keratinocyte proliferation, and maintain the perifollicular blood supply during active growth. Their signalling output is suppressed by DHT in androgenetic alopecia and by glucocorticoids during chronic stress.

DPCs: Dermal Papilla Cells

See: Dermal Papilla Cell (DPC)

DPP-4: Dipeptidyl Peptidase-4

DPP-4 is an enzyme found throughout the body that rapidly breaks down and inactivates GLP-1 and other incretin hormones after they are released. It is the reason the body's natural GLP-1 signal lasts only around two minutes — a short window that matches normal meal digestion. GLP-1 receptor agonist medications are specifically designed to resist DPP-4 breakdown, which is what allows them to remain active for days rather than minutes. A separate class of diabetes medications called DPP-4 inhibitors (gliptins) work by blocking this enzyme to extend the natural GLP-1 signal instead.

dWAT: Dermal White Adipose Tissue

Dermal white adipose tissue is a distinct layer of adipocytes situated between the lower dermis and the subcutaneous fat, developmentally and functionally separate from the subcutaneous white adipose tissue beneath it. Unlike subcutaneous fat, dWAT has direct paracrine interactions with the dermis and hair follicles — its thickness tracks the hair growth cycle, and signals from dWAT adipocytes regulate hair follicle stem cell activity and follicle cycling. With age, dermal fibroblasts progressively acquire adipogenic characteristics, contributing to dWAT expansion and a corresponding reduction in the matrix-producing fibroblast population available for collagen maintenance.

ECM: Extracellular Matrix

The structural framework of proteins, proteoglycans, and ground substance that surrounds and supports cells within tissues. In the dermis, the ECM is composed primarily of collagen fibres, elastic fibres, hyaluronic acid, and proteoglycans such as decorin, collectively providing tensile strength, elasticity, and the mechanical environment in which fibroblasts operate. The ECM is not a passive scaffold. It sequesters growth factors, transmits mechanical signals to resident cells, and is continuously remodelled by the same fibroblasts embedded within it.

EDC: Epidermal Differentiation Complex

A large genomic locus on chromosome 1q21 containing the majority of genes encoding structural proteins of the cornified envelope and terminal keratinocyte differentiation markers, including filaggrin (FLG), loricrin (LOR), involucrin (IVL), small proline-rich proteins (SPRRs), S100 proteins, and late cornified envelope (LCE) proteins. The EDC is not simply a gene cluster — its chromatin state is actively remodelled by SATB1 and BRG1 in a calcium- and differentiation-stage-dependent manner, with gene loci opening sequentially as keratinocytes progress through the programme. IL-4 and IL-13 disrupt EDC chromatin accessibility via STAT6, producing global terminal differentiation suppression across the full locus rather than selective individual gene inhibition.

EFA: Essential Fatty Acid

Essential fatty acids are fatty acids the body cannot synthesise and must obtain from food — specifically linoleic acid (an omega-6) and alpha-linolenic acid (an omega-3). The word "essential" carries its strict nutritional meaning here: absence leads to a recognised deficiency syndrome, including impaired skin barrier function, increased water loss through the skin, and poor wound healing. In practice, genuine EFA deficiency is rare on any normal dietary pattern, since both fatty acids are present in virtually all foods containing fat; the more clinically relevant question for skin health is typically the balance between omega-6 and omega-3 intake rather than adequacy of either in isolation.

EGF: Epidermal Growth Factor

EGF is a growth factor that primarily targets keratinocytes, the predominant cell type in your skin's outer layer (epidermis). It promotes their multiplication and migration, which accelerates wound closure and helps restore the skin's protective barrier. In facial rejuvenation treatments using PRP or iPRF, EGF contributes to improved skin texture and surface renewal, complementing the deeper structural effects of other growth factors. This makes it particularly valuable for addressing fine lines and overall skin quality.

ENS: Enteric Nervous System

The enteric nervous system is a vast network of approximately 500 million neurons embedded in the walls of the digestive tract, often described as the body's "second brain." It operates largely independently of the central nervous system, controlling gut motility, secretion, and local immune responses. The ENS communicates continuously with the brain via the gut-brain axis, influencing appetite, mood, and how the body responds to hormones like GLP-1. Disruption to ENS function — through poor diet, stress, or ultra-processed food additives — can have consequences beyond the gut, contributing to systemic inflammation and impaired appetite regulation.

EPA: Eicosapentaenoic Acid

EPA is one of the two main omega-3 fatty acids found in fish oil, known for its anti-inflammatory properties and cardiovascular benefits. It works by influencing the production of signalling molecules called eicosanoids, which help regulate inflammation, blood clotting, and immune function throughout the body. Research shows EPA can have relatively rapid effects on mood regulation, with some studies demonstrating improvements in depressive symptoms within 2-3 weeks of supplementation. EPA is particularly abundant in cold-water oily fish like sardines, mackerel, and anchovies.

FFA: Free Fatty Acid

See: free fatty acids (FFAs)

FFAs: Free Fatty Acids

Free fatty acids are fat molecules that have been released from their stored form in fat cells and are circulating freely in the bloodstream, available for the body to use as energy. They are produced when stored body fat is broken down - a process that occurs during exercise, caloric restriction, and body contouring treatments such as fat cavitation and cryolipolysis. In the context of aesthetic treatments, free fatty acids are the by-product of disrupted fat cells that are subsequently processed and eliminated by the liver via the lymphatic system. Staying well hydrated after fat reduction treatments supports the body's clearance of FFAs efficiently.

FLG: Filaggrin

Filaggrin is a structural protein that plays a central role in skin barrier function. It physically aggregates the keratin filaments inside skin cells to form the dense, flattened architecture of the stratum corneum, and then breaks down to release the amino acids and compounds that make up natural moisturising factor (NMF). Filaggrin levels can be reduced by genetic mutations in the FLG gene or suppressed by inflammatory signals — both routes lead to a weakened barrier, reduced NMF, and increased skin sensitivity. The abbreviation FLG refers specifically to the gene that encodes filaggrin.

FPHL: Female Pattern Hair Loss

Female pattern hair loss is the most common form of hair loss in women, characterised by gradual thinning across the crown and top of the scalp while the frontal hairline is typically preserved. Unlike male hair loss, FPHL does not always involve excess androgens - many women with this condition have entirely normal hormone levels, making the underlying causes more varied and complex. It becomes significantly more common after the menopause, when changes in oestrogen levels affect the signals that support healthy hair growth. Treatment options range from topical and oral medications to regenerative approaches such as iPRF scalp treatment.

FTL: Fractional Thulium Laser

FTL is a professional skin resurfacing treatment that uses a 1927nm wavelength laser delivered in a fractional pattern — treating thousands of microscopic columns of skin whilst leaving the surrounding tissue intact for faster healing. The wavelength's intermediate water absorption allows the device to be calibrated across a wide spectrum of intensity, from gentle collagen-stimulating sessions with minimal downtime to more intensive resurfacing for significant photoaging, pigmentation, and acne scarring. New collagen and elastin production continues for three to six months after each session as the skin's repair process completes. The treatment is particularly well-evidenced for melasma and sun damage across all skin tones, including darker skin types where many resurfacing lasers carry a higher risk of pigmentation changes.

GAG: Glycosaminoglycan

See: Glycosaminoglycans (GAGs)

GAGs: Glycosaminoglycans

Glycosaminoglycans are long-chain sugar molecules found in the skin's dermis that attract and hold water, contributing to skin's volume, hydration, and structural resilience. Hyaluronic acid is the most well-known GAG, but others including versican, decorin, and heparan sulphate also support the environment in which skin cells produce collagen and elastin. GAG content in the dermis declines with age and hormonal change, contributing to the loss of skin plumpness and firmness. Professional treatments and topical formulations that support GAG levels help maintain the hydrated, supportive tissue environment that healthy skin depends on.

GAS6: Growth arrest-specific protein 6

Growth arrest-specific protein 6 is a vitamin K-dependent signalling protein secreted by dermal papilla cells at the base of each hair follicle. It acts as the paracrine activation signal for hair follicle stem cells, binding to TAM receptors on their surface to initiate the transition from resting to active growth phase. Chronic elevation of stress hormones, including cortisol during periods of psychological stress or rapid weight loss, suppresses GAS6 secretion, stalling the hair cycle in the resting phase and contributing to telogen effluvium.

GHS-R: Growth Hormone Secretagogue Receptor

GHS-R is the receptor that ghrelin binds to in order to produce its effects, including stimulating hunger, triggering growth hormone release, and influencing the brain's reward pathways. It is found most densely in the appetite-regulating region of the hypothalamus, but also in the brainstem and the brain's reward circuitry — which is why elevated ghrelin doesn't just create hunger, but can increase the appeal and motivation to seek out palatable foods. GHS-R is sometimes called the ghrelin receptor, and its activity in multiple brain regions helps explain why managing hunger involves more than simple willpower.

GLA: Gamma-Linolenic Acid

GLA is an omega-6 fatty acid produced in the body by converting linoleic acid through an enzyme called delta-6 desaturase. It is found in evening primrose oil, borage oil, and hemp seed oil, and is also available as a dietary supplement. GLA plays a role in the skin's inflammatory regulation, and research suggests that impaired conversion of linoleic acid to GLA — common in atopic dermatitis — contributes to barrier dysfunction and skin sensitivity.

GLP-1: Glucagon-Like Peptide-1

GLP-1 is a hormone produced naturally in the small intestine and colon in response to eating, particularly when a meal contains protein and fat. It signals to the brain that you are full, slows digestion, and prompts the pancreas to release insulin in a glucose-dependent way. GLP-1 is the hormone that injectable weight management medications such as semaglutide and tirzepatide are designed to mimic — but the body produces its own version through food choices that activate the gut's natural satiety systems.

GLP-1 RA: GLP-1 Receptor Agonist

A GLP-1 receptor agonist is a medication that mimics the action of the body's natural GLP-1 hormone by activating the same receptor, but with a much longer duration of action. Where natural GLP-1 is active for only a couple of minutes after a meal, GLP-1 RAs are engineered to resist rapid breakdown, providing continuous appetite suppression and blood sugar regulation. Semaglutide (Wegovy/Ozempic) and tirzepatide (Mounjaro) are the most widely used GLP-1 RAs in the UK for weight management and type 2 diabetes.

GLP-1 RAs: GLP-1 Receptor Agonists

See: GLP-1 Receptor Agonist (GLP-1 RA)

GLP-1R: Glucagon-Like Peptide-1 Receptor

The GLP-1 receptor is the protein on cell surfaces that GLP-1 binds to in order to produce its effects. It is found on pancreatic cells, brain and nerve cells, the stomach, and the heart, which is why GLP-1 influences such a wide range of functions — from insulin release and appetite suppression to digestion speed and cardiovascular health. GLP-1 receptor agonist medications work by activating this same receptor, replicating and extending the effects of the body's own GLP-1.

GOAT: Ghrelin O-Acyltransferase

GOAT is the enzyme responsible for activating ghrelin, the body's primary hunger hormone. It does this by attaching an eight-carbon fatty acid chain to ghrelin in a process called acylation — without which ghrelin cannot bind to its receptor or stimulate appetite. GOAT is unique in human biology as the only known enzyme that performs this specific type of fatty acid modification on a hormone, making it a potential target for future appetite-regulation research.

HA: Hyaluronic Acid

Hyaluronic acid is a naturally occurring molecule found throughout the body, with particularly high concentrations in the skin's dermal layer where it holds water at up to 1,000 times its own weight. It contributes to skin volume, cushioning, and the hydrated environment that fibroblasts need to produce collagen and elastin effectively. HA levels in the dermis decline with age and hormonal change, contributing to skin that appears less plump and resilient. In aesthetics, HA is used both as a key ingredient in topical products and skin boosters, and as the basis for dermal filler treatments.

HFSC: Hair Follicle Stem Cell

Hair follicle stem cells are a population of adult stem cells located in a region of the hair follicle called the bulge, situated in the outer root sheath midway along the follicle. They are responsible for initiating each new hair growth cycle, activating in response to paracrine signals (primarily GAS6) secreted by dermal papilla cells. Chronic glucocorticoid elevation suppresses GAS6 output from dermal papilla cells, leaving hair follicle stem cells in prolonged quiescence and extending the resting phase of the hair cycle.

HFSCs: Hair Follicle Stem Cells

See: Hair Follicle Stem Cell (HFSC)

HPA: Hypothalamic-Pituitary-Adrenal Axis

The hypothalamic-pituitary-adrenal axis is the body's primary hormonal stress response system, coordinating the release of cortisol from the adrenal glands in response to physical or psychological stress signals originating in the brain. In skin biology, sustained HPA activation — and the chronically elevated cortisol that results — is directly relevant because cortisol suppresses several skin repair mechanisms simultaneously: it inhibits HMG-CoA reductase, the enzyme the skin activates first when barrier repair is needed; reduces fibroblast collagen synthesis; and impairs the ceramide production pathways that maintain barrier integrity. This makes chronic stress a genuine biological driver of skin deterioration rather than simply an indirect lifestyle factor, and explains why periods of prolonged stress often produce measurable, multi-layered changes in skin quality that persist beyond the stressful period itself.

HS: Heparan Sulphate

Heparan sulphate is a sulphated glycosaminoglycan chain found covalently attached to heparan sulphate proteoglycans (HSPGs) in the dermal extracellular matrix and at the dermo-epidermal junction. Its highly variable sulphation pattern determines which growth factors (including FGF2, FGF7, and VEGF) it can bind and retain. Chronic UV exposure and MMP-driven matrix degradation both degrade heparan sulphate chains, depleting the skin's on-site growth factor reservoir.

HSL: Hormone-Sensitive Lipase

Hormone-sensitive lipase is the primary enzyme through which hormonal signals control fat release from adipocytes. When catecholamines such as adrenaline are elevated — during fasting, exercise, or stress — they activate HSL via a cAMP signalling cascade, triggering the breakdown of stored triglycerides into free fatty acids that enter circulation as fuel. Insulin directly suppresses HSL activity, promoting fat storage rather than release. HSL is therefore the molecular switch through which the body's fed and fasted states control fat mobilisation, and its regulation is central to understanding how dietary patterns influence body composition.

HSPG: Heparan Sulphate Proteoglycan

Heparan sulphate proteoglycans are proteins bearing long sugar chains called heparan sulphate glycosaminoglycans, distributed throughout the dermal extracellular matrix and at the dermo-epidermal junction. Their primary role in skin is to bind and store growth factors including FGF2, FGF7, and VEGF in a protected on-site depot, releasing them when tissue repair signals demand it. UV exposure and age-related matrix degradation both deplete heparan sulphate chains, reducing the skin's capacity to store and deploy these repair signals effectively.

HSPGs: Heparan Sulphate Proteoglycans

See Heparan Sulphate Proteoglycan (HSPG)

ICD: Irritant Contact Dermatitis

Non-immunological skin inflammation caused by direct cytotoxic disruption of the stratum corneum. No prior sensitisation required. Distinguish from ACD (allergic contact dermatitis), which is type IV hypersensitivity. ICD is the most common pattern in cosmetic product reactions.

IDO: Indoleamine 2,3-Dioxygenase

Indoleamine 2,3-dioxygenase is an enzyme that diverts dietary tryptophan towards the kynurenine pathway during immune activation. It is upregulated by pro-inflammatory cytokines and by lipopolysaccharide, making its activity a direct reflection of systemic inflammatory state. When IDO is active, less tryptophan is available for conversion to the gut-protective metabolite IPA or for serotonin synthesis in the gut. Measuring the ratio of kynurenine to tryptophan (KYN:TRP) in the blood provides a practical blood-based indicator of IDO activity and systemic inflammatory burden.

IGF-1: Insulin-like Growth Factor

IGF-1 is a growth factor that promotes cell survival and supports tissue regeneration, often amplifying the effects of other growth factors when they work together. For hair restoration, IGF-1 appears to prolong the active growth phase of the hair cycle whilst delaying the regression phase, resulting in longer, thicker hair shafts. It also helps protect cells from programmed death (apoptosis), potentially rescuing struggling hair follicles that might otherwise stop functioning. IGF works synergistically with other growth factors in PRP and iPRF treatments.

IPA: Indole-3-Propionic Acid

Indole-3-propionic acid is a metabolite produced exclusively by commensal gut bacteria — principally Clostridium sporogenes — from dietary tryptophan. It protects gut barrier integrity by binding to receptors on intestinal epithelial cells that upregulate tight junction proteins, and it exerts direct antioxidant activity within the gut lining. Unlike many health markers, IPA cannot be produced by the body independently of its gut bacteria, making blood IPA concentration a direct functional indicator of commensal bacterial activity and gut barrier competency.

IPL: Intense Pulsed Light

IPL stands for Intense Pulsed Light, which is a type of light therapy that can treat various skin issues, such as sun damage, age spots, freckles, rosacea, acne, hair removal, and more. It uses a device that emits flashes of light of different wavelengths to target the pigment cells, blood vessels, or hair follicles in the skin. The light energy is converted into heat, which destroys the unwanted cells or follicles, without harming the surrounding skin. IPL can improve the colour and texture of the skin, reduce redness and inflammation, and stimulate collagen and elastin production. IPL can be performed on the face, neck, chest, hands, or legs. It is suitable for most skin types, but it may not work well on very dark or tanned skin, or very light or gray hair. IPL is not the same as laser treatment, which uses a single wavelength of light to target a specific skin condition. IPL is more versatile and can treat multiple skin concerns at once. IPL is also known as photofacial, photorejuvenation, or broadband light therapy.

iPRF: Injectable Platelet-Rich Fibrin

iPRF is an advanced form of platelet therapy that preserves more of your blood's natural regenerative components than standard PRP. The processing uses lower centrifuge speeds, which retains white blood cells, stem cells, and creates a fibrin matrix that releases growth factors gradually over 7-14 days rather than just a few hours. This sustained release means your cells receive continuous healing signals throughout the critical early regeneration period, often producing more robust results than PRP. iPRF is particularly effective for hair restoration and facial rejuvenation treatments.

JAK2: Janus Kinase 2

JAK2 is an enzyme that acts as an intracellular relay, transmitting signals from hormone and cytokine receptors on the cell surface into the nucleus where gene expression is regulated. It is activated by a range of important receptors — including the leptin receptor in appetite regulation and cytokine receptors involved in inflammation and tissue repair. When JAK2 is activated, it typically phosphorylates and activates STAT proteins, particularly STAT3, which then travel to the nucleus to switch specific genes on or off. JAK2 inhibitors are used clinically in conditions involving overactive inflammatory signalling.

KGF: Keratinocyte Growth Factor

Keratinocyte growth factor is a member of the fibroblast growth factor family (also known as FGF-7) produced primarily by dermal fibroblasts and dermal papilla cells. It acts parabolically on keratinocytes and hair follicle stem cells, stimulating their proliferation, migration, and differentiation. In hair follicles, KGF output peaks during the active growth phase and plays a key role in prolonging anagen and protecting follicular keratinocytes from apoptosis.

KYN: Kynurenine

Kynurenine is a metabolite produced when the enzyme indoleamine 2,3-dioxygenase (IDO) diverts dietary tryptophan away from protective pathways towards an inflammatory cascade. IDO activity is upregulated by pro-inflammatory signals — including LPS and certain cytokines — as well as by glucocorticoids during chronic stress. An elevated kynurenine-to-tryptophan (KYN:TRP) ratio in the blood is an established marker of immune activation, and is associated with chronic inflammatory conditions, metabolic disease, and altered gut-brain signalling. Understanding the KYN:TRP ratio alongside IPA provides a more complete picture of gut and immune function than either measure alone.

LA: Linoleic Acid

Linoleic acid is an essential omega-6 fatty acid found in plant oils such as sunflower, safflower, and rosehip oil, which the body cannot synthesise and must obtain through diet or topical application. In the skin, it is a key structural component of the barrier lipids that hold the stratum corneum together, and a deficiency is associated with impaired barrier function and increased water loss. It is also the precursor to other omega-6 metabolites involved in the skin's inflammatory and anti-inflammatory responses.

LADD: Laser-Assisted Drug Delivery

LADD is a technique that uses a laser or energy-based device to temporarily increase the skin's permeability, allowing topical active ingredients to reach deeper skin layers than they could through intact skin alone. The laser creates microscopic channels through the skin's outer barrier layer, opening a short delivery window — typically minutes to a few hours — during which ingredients such as tranexamic acid, vitamin C, hyaluronic acid, or polynucleotides are applied and absorbed at significantly higher concentrations than standard topical use achieves. This approach is used to enhance the results of treatments like thulium fractional laser and microneedling by combining their structural collagen benefits with the direct delivery of skin-improving actives in a single session.

LCFA: Long-Chain Fatty Acid

See: Long-Chain Fatty Acids (LCFAs)

LCFAs: Long-Chain Fatty Acids

Long-chain fatty acids are fatty acids with carbon chain lengths of 14 to 22 carbons, and encompass the majority of biologically significant fatty acids in the human body — including palmitic acid, stearic acid, oleic acid, linoleic acid, and the omega-3 fatty acids EPA and DHA. Unlike shorter-chain fatty acids, LCFAs require emulsification by bile salts and travel via the lymphatic system as chylomicrons before entering systemic circulation. They serve as the primary structural components of membrane phospholipids, triglycerides, and ceramide chains throughout the body, including in skin tissue.

LEDT: Light-emitting diode therapy

Non-ionising, non-thermal light-based treatment using light-emitting diodes at specific wavelengths to modify biological processes in skin tissue. Distinct from laser therapy (coherent light source) in that LED produces incoherent, diverging light across a defined wavelength band. Used clinically for collagen stimulation, acne reduction, erythema management, and post-procedure recovery support.

LPP: Long Periodicity Phase

The long periodicity phase is a critical structural arrangement within the skin's stratum corneum (outermost layer) that's essential for maintaining barrier integrity. Whilst the LPP makes up a relatively small portion of the skin's lipid structure, it plays a disproportionately important anchoring role by creating stable molecular arrangements between ceramides and other lipids. This phase is primarily formed by specific ceramides like Ceramide EOS and EOP, which help create the tightly organised layers that prevent water loss and protect against environmental damage. The LPP's structural importance explains why certain ceramide deficiencies can severely compromise barrier function despite affecting only a small percentage of total ceramides.

LPS: Lipopolysaccharide

Lipopolysaccharide is a structural molecule found in the outer membrane of gram-negative gut bacteria. Under normal conditions it remains contained within the gut, but when intestinal barrier integrity is compromised, LPS can enter the bloodstream — a state known as metabolic endotoxaemia. Once in circulation, LPS binds to immune receptors on macrophages and other cells, triggering the release of pro-inflammatory signals including TNF-α, IL-1β, and IL-6. This pathway connects gut barrier disruption to systemic low-grade inflammation, with documented associations to metabolic disease and skin health.

LVL: Length, Volume, Lift

LVL stands for Length, Volume, Lift, which are the three main benefits of this lash treatment. LVL is a type of lash lift that works with your natural lashes to give them a curl and a tint that makes them look longer, thicker, and more defined. LVL is done with a special device that applies a lifting balm and a volumizing fix to your lashes, followed by a lash tint. The result is a natural-looking enhancement that lasts up to 8 weeks with proper aftercare. LVL is suitable for most lash types, but it may not be recommended for people with very short, sparse, or damaged lashes. LVL is also known as LVL Enhance or LVL Lash Lift.

MC1R: Melanocortin 1 Receptor

G-protein-coupled receptor expressed on melanocytes, keratinocytes, and immune cells, functioning as the primary receptor for α-MSH and ACTH in the skin pigmentation pathway. MC1R activation by α-MSH triggers adenylyl cyclase-mediated cAMP production, which activates PKA and upregulates MITF (microphthalmia-associated transcription factor) — the master regulator of melanocyte differentiation and eumelanin synthesis. MC1R variants are among the most common functional polymorphisms in human skin biology: loss-of-function variants (particularly R151C, R160W, D294H) reduce eumelanin synthesis capacity, producing the red hair, fair skin, and increased UV sensitivity of the RHC (red hair colour) phenotype, and significantly increasing melanoma risk independently of UV exposure. MC1R is also expressed on dermal fibroblasts and immune cells, where its activation by α-MSH and ACTH exerts anti-inflammatory effects via NF-κB suppression. Relevant to Fitzpatrick skin typing, UV sensitivity assessment, and pre-treatment risk stratification for laser and energy-based procedures in fair-skinned clients.

MCFA: Medium-Chain Fatty Acid

See: Medium-Chain Fatty Acids (MCFAs)

MCFAs: Medium-Chain Fatty Acids

Medium-chain fatty acids are dietary fats with carbon chain lengths of 8 to 12 carbons, found primarily in coconut oil, palm kernel oil, and human breast milk. Unlike long-chain fatty acids, MCFAs bypass the lymphatic transport system and are absorbed directly into portal circulation, reaching the liver rapidly for mitochondrial oxidation — making them a fast-metabolising energy source. They sit between the gut-derived short-chain fatty acids (SCFAs, C2–C6) and the long-chain fatty acids (LCFAs, C14–C22) that form the majority of dietary and structural fats in the body.

McSCs: Melanocytic Stem Cells

Melanocytic stem cells are the regenerative precursor cells responsible for producing the melanocytes that give hair its colour during each growth cycle. They reside in the bulge region of the hair follicle — the same stem cell niche that houses hair follicle stem cells — and are activated at the start of each new anagen phase to generate fresh pigment-producing melanocytes that migrate into the hair matrix. With each successive hair cycle, a proportion of McSCs are lost through activation without replacement, a process accelerated by UV damage and oxidative stress; when the McSC reservoir in a follicle is depleted below a functional threshold, that follicle produces unpigmented hair — the cellular mechanism of greying. McSCs are distinct from the melanocytes found in the basal layer of the epidermis, which serve a separate function in skin pigmentation rather than hair colouration.

MDA: Microdermabrasion

Microdermabrasion, which is a type of exfoliating treatment that uses a device to spray fine crystals or a diamond tip to remove the dead skin cells and improve skin texture and tone. It can also help to reduce the appearance of fine lines, wrinkles, pores, scars, pigmentation, and sun damage. MDA is done with a special device that gently abrades the surface of the skin under sterile conditions. The device also vacuums the crystals and the dead skin cells, leaving the skin smooth and refreshed. MDA is not painful, but it may cause some mild redness or sensitivity after the treatment. MDA is suitable for most skin types, but it may not be recommended for people with active acne, rosacea, eczema, or other skin conditions. MDA is a quick and easy treatment that can be done in less than an hour, and it does not require any downtime. MDA can be done as a standalone treatment or in combination with other treatments such as chemical peels, facials, or masks. MDA is also known as micro-resurfacing, skin polishing, or power peel.

MMP: Matrix metalloproteinase

See: Matrix metalloproteinases

MMPs: Matrix Metalloproteinases

Matrix metalloproteinases are enzymes produced by skin cells that break down structural proteins including collagen and elastin as part of normal tissue remodelling. In healthy skin, MMP activity is carefully balanced with new protein synthesis; problems arise when UV exposure, inflammation, cortisol, or oestrogen decline tips this balance toward excessive degradation. Elevated MMP activity is a primary driver of collagen fragmentation, elastin disorganisation, and visible skin ageing. A number of professional treatments and topical actives — including retinoids, vitamin C, and polynucleotides — work specifically by reducing MMP activity to protect the structural proteins the skin is simultaneously trying to rebuild.

MN: Microneedling

A minimally invasive treatment using fine needles to create controlled micro-injuries in the epidermis and dermis, initiating the three-phase wound healing cascade — inflammation, proliferation, and remodelling — and stimulating fibroblast synthesis of Type I and III collagen, elastin, and glycosaminoglycans. A secondary and clinically significant function is transient dermal microporation: open microchannels substantially increase skin permeability, enabling direct dermal delivery of topically applied actives (polynucleotides, HA preparations, vitamin C, PRP). Distinct from MNRF in both mechanism and clinical indication; MN operates primarily in the papillary dermis targeting skin quality, texture, scarring, and drug delivery, without the RF thermal component responsible for senescent fibroblast clearance, HSP-mediated collagen chaperone activity, and tropoelastin/fibrillin upregulation.

MNRF: RF Microneedling

A minimally invasive treatment combining fine-needle percutaneous micro-injury with bipolar radiofrequency energy delivered at insulated needle tips into the dermis. The dual mechanism activates both the wound healing cascade (via needle micro-injury) and a distinct thermal pathway (via RF energy) that induces HSP47-mediated collagen synthesis, HSP70-mediated NF-κB and MMP suppression, tropoelastin and fibrillin upregulation, and apoptotic clearance of senescent dermal fibroblasts. Distinct from standard microneedling (MN) in both mechanism and clinical indication; standard microneedling targets skin quality and drug delivery via the papillary dermis, while MNRF targets structural laxity and senescent fibroblast remodelling via the reticular dermis.

MOSH: Mineral Oil Saturated Hydrocarbons

MOSH are chemical contaminants found in some petroleum-based cosmetic products. Research shows 68% of lip products exceed recommended MOSH limits, which is concerning because lip products are largely ingested. MOSH accumulate in the body over time, prompting safety concerns and calls for reduced levels in formulations. See also: POSH.

MSH: Melanocyte-Stimulating Hormone

Family of peptide hormones cleaved from POMC, comprising α-MSH, β-MSH, and γ-MSH. α-MSH is the most skin-relevant isoform — produced locally in keratinocytes and melanocytes as part of the peripheral cutaneous HPA axis, acting primarily via MC1R on melanocytes to upregulate melanogenesis through cAMP-mediated MITF transcription factor activation and eumelanin synthesis. α-MSH additionally exerts anti-inflammatory effects by suppressing NF-κB activation and reducing pro-inflammatory cytokine production — establishing it as a natural cutaneous immunomodulator co-produced alongside ACTH under stress and UV stimulation. The balance between α-MSH-driven eumelanin and pheomelanin synthesis is a primary determinant of Fitzpatrick skin type and UV sensitivity. β-endorphin and ACTH are co-produced from the same POMC cleavage event, linking pigmentation signalling to stress response and opioid-mediated itch/pain modulation.

MsSC: Melanocytic Stem Cell

See: Melanocytic Stem Cells (MsSCs)

MTZs: Microscopic Thermal Zones

Microscopic thermal zones are the small, precisely controlled columns of heat damage created by fractional laser treatments such as the thulium 1927nm laser. Rather than treating the entire skin surface, fractional devices create a pattern of these tiny injury zones whilst leaving surrounding tissue intact, which acts as a reservoir of healthy cells for rapid repair. This approach allows meaningful skin remodelling — stimulating collagen production, clearing pigmentation, and renewing the epidermal surface — whilst significantly reducing recovery time compared to fully ablative treatments. The density and depth of MTZs can be adjusted to suit different skin concerns and client tolerances.

MUFA: Monounsaturated Fatty Acid

See: Monounsaturated Fatty Acids (MUFAs)

MUFAs: Monounsaturated Fatty Acids

Monounsaturated fatty acids are dietary fats containing a single double bond in the carbon chain, placing them between saturated fats (no double bonds, maximum stability) and polyunsaturated fats (multiple double bonds, highest oxidative vulnerability). The most common MUFA is oleic acid, found in olive oil, avocado, and animal fats, and it is also a major component of human sebum. In skin biology, MUFAs are more oxidatively stable than polyunsaturated fatty acids but can disrupt the tightly ordered lipid packing in the stratum corneum when present in excess — a finding that is relevant both to certain inflammatory skin conditions and to the choice of oils in topical skincare formulations.

NAD⁺: Nicotinamide Adenine Dinucleotide

NAD⁺ is a coenzyme found in every living cell that plays a central role in energy metabolism and DNA repair. In skin, it is particularly important for maintaining the function of keratinocytes and fibroblasts, the cells responsible for barrier integrity and collagen production respectively. NAD⁺ levels decline with age and can be replenished via niacinamide (vitamin B3), which the body converts directly into NAD⁺. Adequate NAD⁺ supports cellular repair processes and helps reduce the accumulation of senescent cells that contribute to skin ageing.

NMF: Natural Moisturising Factor

Natural moisturising factor is a collection of water-attracting compounds found inside the outermost skin cells that keep them hydrated from within, independently of any moisturiser applied to the surface. It is produced when filaggrin, a key barrier protein, breaks down during the final stages of skin cell maturation — releasing amino acids, urocanic acid, pyrrolidone carboxylic acid, and other hygroscopic molecules. When NMF levels are low, skin cells lose water more easily, contributing to tightness, flakiness, and a compromised barrier that does not respond well to topical hydration alone. Inflammation, genetic filaggrin variants, and age-related slowdown in skin cell renewal are the most common reasons NMF levels fall.

NPY: Neuropeptide Y

36-amino acid neuropeptide co-released with noradrenaline from sympathetic nerve terminals, acting through Y1, Y2, and Y5 receptor subtypes on cutaneous cells. Pro-regenerative profile — promotes endothelial proliferation, angiogenesis, and wound healing via Y2/Y5 receptors. Modulates macrophage TNF-α secretion via β-arrestin 2 signalling, contributing to the pro-inflammatory to anti-inflammatory macrophage transition in wound healing. Promotes hair follicle growth in animal models. Distinct from substance P and CGRP in that its primary skin role is regenerative rather than inflammatory. Sympathetic co-transmitter alongside noradrenaline and catecholamines.

NTS: Nucleus Tractus Solitarius

The nucleus tractus solitarius is a region in the brainstem that serves as the primary processing centre for signals travelling from the gut to the brain via the vagus nerve. It receives information about meal composition, gut distension, and hormonal satiety signals — including GLP-1, CCK, and leptin — and relays this to the hypothalamus to regulate hunger and fullness. The NTS is one of the key reasons early satiety signals can reach the brain within minutes of eating, well before blood glucose or insulin changes would be detectable. It plays a central role in the gut-brain axis that GLP-1 receptor agonist medications like Mounjaro engage.

ORS: Outer Root Sheath

The outer root sheath is the outermost cellular layer of the hair follicle, surrounding the hair shaft from the follicle opening down to the bulb. It contains a population of keratinocytes with stem cell characteristics, and plays an active role in hair cycle regulation — ORS cells undergo programmed cell death during catagen, releasing keratin proteins that signal to the dermal papilla to initiate the next growth phase. The ORS is also the source of the keratinocyte reservoir that drives re-epithelialisation of the skin surface after injury, making it relevant to both hair biology and wound healing.

P80: Polysorbate 80

Polysorbate 80 is a synthetic emulsifier used in processed foods, some pharmaceutical preparations, and certain cosmetic formulations to blend oil and water-based ingredients. In the context of gut health, the same Gut research that examined CMC found that polysorbate 80 also alters microbiome composition and increases the pro-inflammatory potential of the microbial community, with effects that are transferable to germ-free models — confirming the microbiome as the operative mechanism. Its widespread presence in ultra-processed food products makes it one of the additives most consistently associated with microbiome disruption in current research.

PAH: Paradoxical Adipose Hyperplasia

Paradoxical adipose hyperplasia is a rare adverse event associated with cryolipolysis (fat freezing) in which the treated area develops an increase in fat volume rather than the expected reduction, typically becoming visible as a firm, well-demarcated enlargement 2–6 months after treatment. The mechanism is not fully established but is thought to involve a paradoxical proliferative response in fat cell precursors triggered by the cold stimulus. PAH does not resolve spontaneously and requires surgical correction — typically liposuction — if it occurs. Male sex is the most consistently identified risk factor; the condition is rare but its incidence is higher than early estimates suggested, making pre-treatment disclosure an ethical and clinical requirement.

PBM: Photobiomodulation

The use of non-ionising light at specific wavelengths and doses to modify biological processes in living tissue through photochemical rather than photothermal mechanisms. Encompasses LED therapy, low-level laser therapy, and broadband light applications in the visible and near-infrared spectrum (approximately 400–1100nm). The preferred current scientific term, adopted over LLLT because it does not imply laser as a requirement and accommodates both stimulatory and inhibitory dose-response effects. Biological effect is dose-dependent and follows the Arndt-Schulz biphasic curve. Primary photoacceptor for red and NIR wavelengths is cytochrome c oxidase (CCO) in the mitochondrial electron transport chain.

PC: Phosphatidylcholine

Phosphatidylcholine is a phospholipid that forms the primary structural component of cell membranes throughout the body, including the bilayer membranes of adipocytes and skin cells. It consists of a glycerol backbone, two fatty acid chains, a phosphate group, and a choline head group — the amphiphilic structure that allows phospholipids to self-organise into the bilayer architecture of biological membranes. In aesthetic medicine, phosphatidylcholine has historically been used alongside deoxycholic acid in injection lipolysis formulations; research has since established that the two agents work through distinct mechanisms — PC induces adipocyte apoptosis via inflammatory signalling pathways, while deoxycholic acid causes direct membrane lysis and necrosis.

PCA: Pyrrolidone Carboxylic Acid

Pyrrolidone carboxylic acid is a naturally occurring compound produced when filaggrin, a key skin barrier protein, breaks down in the outer layers of the stratum corneum. It is one of the largest components of natural moisturising factor (NMF) — the collection of hygroscopic molecules inside skin cells that bind and retain water. PCA draws moisture into the corneocyte and is also one of the primary contributors to the skin's acidic surface pH, helping to maintain the acid mantle environment that supports ceramide production and inhibits harmful bacteria.

PDGF: Platelet-Derived Growth Factor

PDGF is one of the primary signalling molecules stored inside platelets that drives tissue repair and regeneration. When released, it stimulates fibroblasts (the cells that produce collagen and elastin) to multiply and migrate to areas where healing is needed. For hair restoration, PDGF specifically influences the dermal papilla cells at the base of hair follicles, which regulate the hair growth cycle. This growth factor is present in high concentrations in both PRP and iPRF treatments, making it central to their regenerative effects.

PDH: Pyruvate Dehydrogenase

Pyruvate dehydrogenase is an enzyme complex that converts pyruvate — the end product of glycolysis — into acetyl-CoA for entry into the mitochondrial citric acid cycle. In the context of the Randle Cycle, PDH is inhibited when fatty acid oxidation is active, causing elevated acetyl-CoA and NADH ratios that signal the cell to suppress glucose oxidation. PDH inhibition is one of the three sequential points at which fatty acid oxidation blocks glucose utilisation in the original glucose-fatty acid cycle mechanism.

PDRN: Polydeoxyribonucleotide

PDRN is a biological compound derived from DNA fragments, typically sourced from salmon sperm, that is used in injectable skin treatments to stimulate the skin's natural repair and regeneration processes. It works primarily by activating receptors on skin cells that trigger new blood vessel formation, reduce inflammation, and encourage fibroblasts to produce collagen — improving skin quality, hydration, and texture over a course of treatments. PDRN is closely related to the broader term polynucleotides (PN), and the two are often used interchangeably in aesthetics, though PDRN technically refers to shorter DNA fragments whilst PN covers a wider molecular weight range. It is considered well-tolerated across all skin types and is increasingly used alongside other treatments such as microneedling and laser resurfacing to support healing and enhance results.

PKA: Protein Kinase A

Protein kinase A is an enzyme that acts as an intracellular messenger, relaying signals from hormones like GLP-1 into specific cellular actions. When GLP-1 binds to its receptor on pancreatic cells, it triggers a rise in cyclic AMP (cAMP), which activates PKA — and it is PKA that then drives insulin secretion. Beyond the pancreas, PKA is involved in a wide range of signalling pathways throughout the body, including those that regulate cell metabolism and gene expression. In the context of GLP-1 and weight management, it is the key intracellular link between hormone signal and physiological response.

PKC: Protein Kinase C

Protein kinase C is a family of enzymes activated by diacylglycerol (DAG) that play a central role in the cellular mechanism of insulin resistance. When DAG accumulates — as occurs when fatty acids cannot enter the mitochondria due to CPT-1 inhibition — PKC is activated and phosphorylates IRS-1 (insulin receptor substrate-1) at serine rather than tyrosine residues. This serine phosphorylation impairs the insulin signalling cascade downstream, reducing glucose uptake and contributing to insulin resistance independently of the insulin receptor itself.

PLLA: Poly-L-Lactic Acid

Poly-L-lactic acid is a biodegradable synthetic polymer used in aesthetic medicine as a collagen biostimulator — meaning it works by encouraging the skin to produce its own collagen rather than adding volume directly. Injected as tiny microspheres into the deeper layers of the skin, it triggers a controlled healing response that activates collagen-producing fibroblast cells over several months. Results build gradually across a course of treatment sessions and can last two to three years, making it a longer-horizon option than traditional fillers. It is most often used for structural collagen decline in the midface, temples, and jawline, and is not suitable for the periorbital area or lips.

PMU: Permanent Makeup

Permanent makeup is a type of cosmetic tattooing that creates designs on the skin that look like makeup, such as eyeliner, lip colour, and eyebrow shape. It can also be used to fill in areas of the skin that have lost colour or hair due to aging, illness, or surgery, such as the eyebrows, scars, or the area around the nipples. Permanent makeup is done with a special device that injects pigment into the skin under sterile conditions. The pigment stays in the skin for a long time, but it may fade over time and need to be touched up. Permanent makeup is also known as SPMU, derma-pigmentation, micro-pigmentation, or cosmetic tattooing.

PN: Polynucleotides

POMC: Pro-opiomelanocortin

Large precursor protein produced primarily in anterior pituitary corticotroph cells and — critically for skin biology — locally in keratinocytes, melanocytes, and hair follicle cells as part of the peripheral cutaneous HPA axis. Cleaved by prohormone convertases PC1 and PC2 into multiple bioactive peptides including ACTH, α-MSH, β-MSH, and β-endorphin. In skin: ACTH drives local cortisol synthesis via melanocortin receptors; α-MSH stimulates melanogenesis via MC1R and exerts anti-inflammatory effects; β-endorphin modulates pain and itch signalling. UV radiation upregulates POMC expression in the epidermis in a wavelength-dependent pattern, independently activating the cutaneous HPA axis. The co-production of melanogenic (α-MSH) and glucocorticoid-stimulating (ACTH) peptides from a single precursor establishes POMC cleavage as a mechanistic link between stress, UV exposure, pigmentation, and local immunosuppression.

POSH: Polyolefin Oligomeric Saturated Hydrocarbons

POSH are petroleum-derived contaminants chemically similar to MOSH that can be present in cosmetic products. Like MOSH, they accumulate in the body when ingested through lip product use. Safety researchers measure MOSH and POSH levels together when assessing petroleum-based lip care products. See also: MOSH.

PPARα: Peroxisome Proliferator-Activated Receptor Alpha

A ligand-activated nuclear receptor expressed in keratinocytes, sebaceous glands, and Langerhans cells that functions as the skin's endogenous lipid sensor. When activated by fatty acids or their derivatives, PPARα drives coordinated transcription of ceramide synthesis, free fatty acid production, and cholesterol synthesis gene programmes, alongside terminal keratinocyte differentiation markers including filaggrin and transglutaminase-1. Active PPARα simultaneously suppresses NF-κB through multiple mechanisms, coupling barrier lipid production and anti-inflammatory signalling to the same receptor activation event. Suppressed by IL-4/IL-13 via STAT6, chronological ageing, and FFA depletion. Activated by EPA, DHA, and topical oat-derived lipid agonists.

PPARγ: Peroxisome Proliferator-Activated Receptor Gamma

Peroxisome proliferator-activated receptor gamma is the master transcription factor controlling adipocyte formation and fat storage. Its activation in preadipocytes is sufficient to drive their differentiation into mature adipocytes, making it the central molecular switch of adipogenesis. PPARγ also regulates insulin sensitivity in adipose tissue — thiazolidinedione drugs used in type 2 diabetes act by activating PPARγ, improving insulin signalling in fat cells. Its expression is suppressed by the inflammatory environment produced by senescent adipocytes, which is one mechanism by which ageing impairs the adipose tissue's ability to replace lost fat cells.

PRP: Platelet-Rich Plasma

PRP is a regenerative treatment that uses concentrated platelets from your own blood to stimulate healing and tissue repair. The blood is processed in a centrifuge to separate and concentrate the platelets, which are then injected back into areas like the scalp for hair restoration or the face for skin rejuvenation. These concentrated platelets release growth factors that trigger your body's natural repair processes, promoting collagen production and tissue regeneration. PRP has been used for decades in medical settings and is now widely applied in aesthetic treatments.

PSU: Pilosebaceous Unit

The pilosebaceous unit (PSU) is a microscopic structure within the skin that plays a vital role in hair growth and skin health. It functions as a miniature factory, consisting of two key components:

Hair Follicle: This is a tube-shaped indentation in the skin that houses the hair root and is responsible for hair growth and development.
Sebaceous Gland: This is a small oil gland located near the base of the hair follicle. Sebaceous glands produce sebum, an oily substance that lubricates the skin and hair, keeping them hydrated and supple.

These two components work together within the PSU. The hair follicle generates hair, while the sebaceous gland secretes sebum that travels up the hair shaft and onto the skin's surface. This sebum helps maintain a healthy moisture barrier, protecting the skin from dehydration and irritation. However, excess sebum production can contribute to acne development.

PSUs are primarily found in hair-covered areas of the body, such as the scalp, face, neck, and upper back. Their number and activity vary depending on body location and hormonal influences. Understanding the function of the pilosebaceous unit is crucial for managing various skin conditions, particularly acne.

PUFA: Polyunsaturated Fatty Acid

See: Polyunsaturated Fatty Acids (PUFAs)

PUFAs: Polyunsaturated Fatty Acids

PUFAs are a class of dietary fats found in oily fish, nuts, seeds, and plant oils, characterised by having multiple double bonds in their chemical structure. They include the omega-3 and omega-6 fatty acid families, both of which are essential - meaning the body cannot produce them and they must be obtained through diet. In skin health, PUFAs play a central role in maintaining the barrier, regulating inflammation, and supporting the production of structural lipids in the outer skin layers.

PYY: Peptide YY

Peptide YY is a satiety hormone released by the same intestinal cells (L-cells) that produce GLP-1, and the two hormones work together as the body's main post-meal fullness signal. PYY acts on the hypothalamus to reduce appetite and slows the movement of food through the digestive tract, extending the feeling of satisfaction after eating. Like GLP-1, PYY is released most strongly in response to protein and fat reaching the lower small intestine — which is part of why protein-rich, fat-containing meals tend to produce longer-lasting fullness than meals high in refined carbohydrates.

RNS: Reactive Nitrogen Species

Reactive nitrogen species are a family of nitrogen-derived molecules (most notably nitric oxide) produced in skin tissue by specialised enzymes called nitric oxide synthases. At normal levels, they play essential roles in regulating blood flow, supporting wound healing, and providing antimicrobial defence at the skin barrier. Like reactive oxygen species, they operate on a dose-dependent basis: the same molecules that support tissue repair at controlled concentrations can contribute to inflammation and structural damage when produced in excess.

RONS: Reactive Oxygen and Nitrogen Species

RONS is the collective term for the two families of reactive molecules that act as the skin's primary chemical signalling system: reactive oxygen species (ROS) and reactive nitrogen species (RNS). Together, they coordinate essential skin functions including wound repair, immune defence, and the response to UV exposure — acting as beneficial signals at physiological concentrations, and as drivers of ageing and inflammation when produced in excess. Several professional aesthetic treatments, including cold atmospheric plasma, work by delivering carefully controlled RONS to activate the skin's natural repair mechanisms.

ROS: Reactive Oxygen Species

Reactive oxygen species are chemically active molecules produced naturally by skin cells during normal metabolism, UV exposure, and immune responses. At low levels, they act as important signalling molecules, helping to coordinate skin repair, collagen remodelling, and defence against bacteria. At higher concentrations, caused by sun damage, chronic inflammation, or the natural ageing process, they can degrade collagen, accelerate visible skin ageing, and slow the skin's healing response. Understanding ROS is central to the science behind antioxidant skincare and many professional skin treatments.

RQ: Respiratory Quotient

The respiratory quotient is a measurement used to identify which fuel — glucose or fat — the body is primarily oxidising at any given time. It is calculated as the ratio of carbon dioxide produced to oxygen consumed: an RQ near 0.7 indicates predominantly fat oxidation; an RQ near 1.0 indicates predominantly glucose oxidation. Metabolically healthy individuals shift fluidly between these values in response to fasting, feeding, and exercise — a capacity known as metabolic flexibility. A blunted RQ shift is a measurable indicator of metabolic inflexibility and an early functional marker of insulin resistance.

SASP: Senescence-associated secretory phenotype

The pro-inflammatory signalling profile adopted by senescent cells that have permanently exited the cell cycle. Rather than remaining inert, senescent fibroblasts secrete a mix of inflammatory cytokines, MMPs, and growth factors that actively degrade surrounding collagen and elastin whilst suppressing synthesis in neighbouring cells. SASP accumulation in ageing dermis is one of the mechanisms through which the dermal environment becomes simultaneously less productive and more destructive over time.

SC: Stratum corneum

Stratum corneum, the outermost layer of the epidermis composed of corneocytes embedded in a lipid matrix, forming the primary transcellular barrier that limits water loss and protects against external insults.

SCFA: Short-Chain Fatty Acid

See: Short-Chain Fatty Acids (SCFAs)

SCFAs: Short-Chain Fatty Acids

Short-chain fatty acids are small metabolites — principally butyrate, propionate, and acetate — produced when gut bacteria ferment dietary substrates or derived directly from certain foods such as ruminant dairy fat. Butyrate is the primary energy source for the cells lining the colon and plays a central role in maintaining gut barrier integrity by supporting tight junction assembly. SCFAs also regulate systemic immune function and reduce inflammatory signalling, making microbiome-derived SCFA production one of the most clinically meaningful outputs of gut health.

SD: Seborrhoeic Dermatitis

Chronic inflammatory condition at sebaceous-rich sites (scalp, central face, external ear) associated with Malassezia yeast and Th17/Th22 immune dysregulation. Distinguish from SD as standard deviation in statistical contexts — disambiguate by context. Spectrum includes dandruff (pityriasis capitis) at milder end.

SFA: Saturated Fatty Acid

See: Saturated Fatty Acids (SFAs)

SFAs: Saturated Fatty Acids

Saturated fatty acids are a class of dietary fats in which the carbon chain contains no double bonds, making them chemically stable and resistant to oxidation. Despite their reputation in general nutrition guidance, they are the dominant fatty acid species in healthy skin — the free fatty acid fraction of the stratum corneum is composed predominantly of saturated species including palmitic and stearic acid, which maintain the tight lipid packing that underpins barrier function. Palmitic acid also serves as the direct substrate for ceramide synthesis, the process that produces the skin's primary barrier lipids. In the context of skin health, saturated fats are structural components rather than passive dietary bystanders.

SG: Stratum granulosum

Stratum granulosum, the granular layer of the viable epidermis immediately beneath the stratum corneum, where keratinocytes complete terminal differentiation through profilaggrin assembly, lamellar body secretion, tight junction formation, and cornification initiation.

SHBG: Sex Hormone Binding Globulin

SHBG is a protein produced by the liver that binds to sex hormones (testosterone, oestrogen, and dihydrotestosterone) in the bloodstream, controlling how much of these hormones are available for the body to use. When SHBG levels are low, more free androgens circulate in the blood, which can contribute to hair loss and other hormonal imbalances. SHBG levels can be affected by rapid weight loss, insulin resistance, and certain medications including GLP-1 receptor agonists. Maintaining healthy SHBG levels is important for hormonal balance and may influence hair health, particularly in women experiencing weight loss.

SIPS: Stress-Induced Premature Senescence

SIPS describes the process by which a cell enters senescence not because it has exhausted its replicative lifespan, but because it has been driven into permanent arrest prematurely by external stress. In skin, the most relevant triggers are UV radiation, reactive oxygen species (ROS), pollution-derived oxidative damage, and chronic inflammatory cytokine exposure. SIPS bypasses the normal telomere-shortening countdown and activates the senescence programme directly through oxidative DNA damage pathways — which is why UV-exposed skin accumulates senescent fibroblasts and keratinocytes at a substantially higher rate than chronologically matched, sun-protected skin of the same age. The distinction between SIPS and replicative senescence matters clinically: SIPS-driven senescent cell burden reflects accumulated environmental insult rather than chronological age alone, and it is meaningfully modifiable through UV protection, antioxidant support, and barrier maintenance from the point those habits are adopted — even if the existing accumulated burden is not quickly reversible.

SMP: Scalp Micropigmentation

Scalp micropigmentation is a type of cosmetic tattooing that creates the appearance of hair follicles on the scalp. It can be used to cover bald spots, thinning hair, receding hairlines, or scars. It can also create the illusion of a shaved head or a fuller hair density. Scalp micropigmentation is done with a special device that injects tiny dots of pigment into the skin under sterile conditions. The pigment matches the natural hair colour and blends with the existing hair. Scalp micropigmentation is a permanent solution, but it may require touch-ups over time as the pigment fades. Scalp micropigmentation is also known as hair tattooing, hair simulation, or tricopigmentation.

SP: Substance P

Substance P is an 11-amino acid neuropeptide produced by primary sensory C-fibre neurons and released both centrally (modulating pain transmission) and peripherally at skin nerve terminals. Its skin-relevant actions are mediated primarily through NK1 (neurokinin 1) receptors expressed on mast cells, keratinocytes, fibroblasts, endothelial cells, and dermal papilla cells of the hair follicle. SP is the principal neuropeptide mediator of stress-related hair follicle disruption.

SPMU: Semi-Permanent Makeup

Semi-Permanent Makeup is a type of cosmetic tattooing that creates designs on the skin that look like makeup, such as eyeliner, lip colour, and eyebrow shape. It can also be used to fill in areas of the skin that have lost colour or hair due to aging, illness, or surgery, such as the eyebrows, scars, or the area around the nipples. The pigment matches the natural skin tone and blends with the existing hair or makeup. SPMU is not permanent, but it can last for several months or years, depending on the type of pigment, the depth of the injection, and the skin type. SPMU may require touch-ups over time as the pigment fades or the skin changes. SPMU is also known as PMU, micro-pigmentation, derma-pigmentation, or cosmetic tattooing.

SPT: Serine Palmitoyltransferase

Serine palmitoyltransferase is an enzyme found in keratinocytes - the skin's primary epidermal cells - that initiates the first and rate-limiting step in ceramide synthesis, combining the amino acid serine with a fatty acid to begin building the skin's barrier lipids. Without sufficient SPT activity, ceramide production slows regardless of raw material availability, leading to a less effective skin barrier and increased moisture loss. SPT activity can be upregulated by niacinamide and is suppressed by inflammatory cytokines IL-4 and IL-13, making it a key target in barrier repair protocols.

STAT3: Signal Transducer and Activator of Transcription 3

STAT3 is a transcription factor — a protein that, once activated, moves into the cell nucleus and directly switches genes on or off. It is activated by JAK2 in response to a wide range of hormonal and inflammatory signals, including leptin, cytokines such as IL-6, and growth factors involved in tissue repair. In the context of skin and aesthetics, STAT3 activation by polynucleotide treatment contributes to increased collagen synthesis in fibroblasts; in metabolic health, STAT3 mediates leptin's appetite-suppressing effects in the hypothalamus. Its activity in both repair and inflammatory pathways makes it a recurring player across skin biology, wound healing, and metabolic signalling.

SVF: Stromal Vascular Fraction

The stromal vascular fraction is the non-adipocyte cellular component of adipose tissue, comprising preadipocytes, fibroblasts, endothelial cells, pericytes, resident macrophages, and adipose-derived stem cells (ADSCs). It is the reservoir from which new adipocytes differentiate throughout adult life and the compartment in which age-related adipose inflammation is initiated. ADSCs within the SVF have attracted significant research interest for their ability to differentiate into multiple cell lineages and their paracrine secretion of growth factors relevant to tissue regeneration.

TDO: Tryptophan 2,3-Dioxygenase

Tryptophan 2,3-dioxygenase is an enzyme that, like IDO, diverts dietary tryptophan into the kynurenine pathway — but where IDO is driven by immune activation and inflammatory signals, TDO is activated specifically by glucocorticoids, including cortisol. This makes TDO the direct biochemical link between chronic psychological stress and reduced tryptophan availability for protective pathways. TDO activation contributes to the lower IPA concentrations and elevated KYN:TRP ratios observed in individuals under sustained psychological or physiological stress, independent of gut microbiome disruption.

TEWL: Transepidermal Water Loss

TEWL is a scientific measurement of how much water evaporates from the skin's surface. It's used to assess skin barrier function—higher TEWL indicates the skin is losing moisture more quickly, which happens when the protective barrier is compromised. Lips naturally have significantly higher TEWL than facial skin due to their thinner structure and lack of oil-producing glands. Understanding TEWL helps explain why lips require different moisturising strategies than the rest of your skin.

TGF-β: Transforming Growth Factor-Beta

TGF-β is a powerful growth factor that stimulates collagen synthesis and extracellular matrix production, which gives skin its structure and firmness. Whilst it's essential for wound healing and tissue regeneration, the body tightly regulates it because too much can lead to excessive scarring. In PRP and iPRF treatments, TGF-β is delivered in controlled amounts that promote healthy collagen production without causing fibrosis. This growth factor is largely responsible for the gradual improvements in skin firmness and thickness that emerge months after treatment as new collagen accumulates.

TJ: Tight Junction

Tight junction, an intercellular protein complex forming continuous paracellular seals between epithelial cells. In the epidermis, TJs are assembled in the stratum granulosum and constitute the paracellular barrier system operating independently of the stratum corneum lipid bilayer.

TLR4: Toll-Like Receptor 4

Toll-Like Receptor 4 is a pattern recognition receptor found on the surface of immune cells, endothelial cells, and adipocytes. Its primary role is detecting the presence of lipopolysaccharide (LPS) from gram-negative bacteria, triggering an immune response via NF-κB signalling. When LPS enters systemic circulation from a disrupted gut barrier, TLR4 activation drives the production of inflammatory cytokines including TNF-α and IL-1β — linking gut microbiome disruption to chronic systemic inflammation relevant to skin, metabolic, and immune health.

TRP: Tryptophan

Tryptophan is an essential amino acid — meaning the body cannot produce it and it must be obtained from food — found in protein-rich sources including meat, fish, eggs, dairy, and legumes. In the gut, tryptophan is the metabolic starting point for two competing pathways: conversion by commensal bacteria into the protective metabolite indole-3-propionic acid (IPA), or conversion via the enzyme IDO into kynurenine (KYN) under inflammatory or stress conditions. Tryptophan is also the precursor for approximately 95% of the body's serotonin, produced by gut enterochromaffin cells — linking tryptophan metabolism to both gut barrier function and neurological signalling.

UCA: Trans-Urocanic Acid

Trans-urocanic acid is a compound derived from histidine, one of the amino acids released when filaggrin degrades in the stratum corneum. As a component of natural moisturising factor (NMF), it contributes to the skin's acidic surface pH alongside pyrrolidone carboxylic acid (PCA), helping to maintain the acid mantle that regulates ceramide-processing enzyme activity. UCA also undergoes a structural change on UV exposure — converting from its trans to cis form — which is thought to play a role in the skin's natural photoprotective response.

UCP-1: Uncoupling Protein-1

Uncoupling protein-1 is a protein found in the inner mitochondrial membrane of brown and beige adipocytes that dissipates the proton gradient used to generate ATP, releasing energy as heat instead. It is the molecular mechanism behind the thermogenic function of brown adipose tissue and the inducible heat-generating capacity of beige adipocytes activated by cold or adrenergic stimulation. UCP-1 expression is the defining marker distinguishing thermogenic adipocytes from energy-storing white adipocytes.

VDR: Vitamin D Receptor

The vitamin D receptor is a protein found inside skin cells, particularly keratinocytes, that acts as a molecular switch for vitamin D's biological effects. When calcitriol, the active form of vitamin D, binds to VDR, it activates genes responsible for building a healthy skin barrier, producing antimicrobial peptides that defend against harmful bacteria, and regulating the skin's innate immune responses. VDR is also part of the signalling network linking gut health to skin immunity, meaning that gut microbiome health and vitamin D levels interact through the same receptor system. Impaired VDR signalling — from deficiency, reduced receptor expression, or both — is associated with weakened barrier function and increased susceptibility to inflammatory skin conditions.

VEGF: Vascular Endothelial Growth Factor

VEGF specialises in creating new blood vessels, a process called angiogenesis that's essential for tissue health and regeneration. When VEGF is released during PRP or iPRF treatment, it signals cells lining blood vessels to multiply and form new capillary networks, improving nutrient delivery and waste removal in the treated area. This is particularly important for hair follicles, which are highly vascular structures requiring robust circulation to support the energy-intensive process of hair growth. Enhanced blood supply from VEGF helps maintain long-term health of both hair follicles and facial skin.

VIP: Vasoactive Intestinal Peptide

28-amino acid neuropeptide released from both sensory and autonomic nerve terminals in skin, acting through VPAC1 and VPAC2 receptors on mast cells, T-lymphocytes, dendritic cells, and keratinocytes. Primarily anti-inflammatory and immunosuppressive at physiological concentrations — inhibits Th1 cytokine production, promotes T-regulatory cell induction, and reduces dendritic cell activation. Vasodilatory and increases vascular permeability, complementing CGRP's vascular actions. Can stimulate mast cell degranulation at higher concentrations. Functions as a natural brake on neurogenic inflammation, counterbalancing pro-inflammatory SP and CGRP signals.

VLCFA: Very-Long-Chain Fatty Acid

See: Very-Long-Chain Fatty Acids (VLCFAs)

VLCFAs: Very-Long-Chain Fatty Acids

Very-long-chain fatty acids are fatty acids with carbon chain lengths above 22 carbons, synthesised endogenously by elongase enzymes — principally ELOVL1 and ELOVL4 — in the endoplasmic reticulum rather than obtained directly from diet. In skin biology, VLCFAs are the defining structural components of the stratum corneum's free fatty acid fraction and the acylceramide subtypes responsible for the long periodicity phase of the lamellar lipid matrix. Their synthesis is degraded by peroxisomal rather than mitochondrial β-oxidation, a distinction that becomes clinically significant in peroxisomal disorders associated with VLCFA accumulation.

WAT: White Adipose Tissue

White adipose tissue is the dominant form of body fat in adults, found in subcutaneous and visceral depots throughout the body. Its adipocytes store energy as triglycerides in a single large lipid droplet and secrete hormones — including leptin and adiponectin — that regulate appetite, insulin sensitivity, and inflammatory tone. WAT is the primary target of non-invasive fat reduction treatments and the adipose compartment most affected by age-related senescence and metabolic dysfunction.