Skin, Appendages & Function
A complete guide to the body’s largest organ system — from the five epidermal strata and four specialised epidermal cell types through dermal architecture, hypodermis, hair follicle biology, sweat and sebaceous glands, nail structure, cutaneous sensory receptors, melanin pigmentation, thermoregulation, vitamin D synthesis, wound healing phases, skin aging, burns classification, and the pathophysiology of key dermatological conditions.
Pull back the outermost layer of the human body and you find not a passive wrapping but a metabolically active, immunologically sophisticated, sensory-rich organ system that is simultaneously the largest by surface area (~1.5–2 m²), the heaviest by percentage of body mass (~16%), and arguably the most functionally diverse organ in the human body. The integumentary system — skin and its appendages — is your first line of defence against infection, your thermal management system, your vitamin D factory, and your sensory interface with the physical world. It hosts a microbiome of roughly 1,000 bacterial species, continuously regenerates itself every 40–56 days, and synthesises multiple hormones and signalling molecules. Damage to just 20% of its surface can be life-threatening. Understanding the integumentary system in structural and functional detail is foundational knowledge for every student of biology, anatomy, nursing, and medicine.
The Integumentary System — Scale, Significance, and Scope
The integumentary system is defined by skin and its structurally and functionally related appendages: hair follicles, nails, sebaceous (oil) glands, eccrine and apocrine sweat glands, and the specialised sensory receptors embedded throughout the dermis and epidermis. Together, these components form a continuous, dynamic organ that covers the entire external body surface and transitions at body orifices (mouth, nostrils, anus, urethra, vagina) into specialised mucous membranes with distinct but related histological organisation. Understanding the integumentary system is not simply a matter of memorising anatomical layers — it is about understanding how a multilayered barrier maintains internal homeostasis against a constantly challenging external environment.
Skin is not uniform across the body surface. Thick skin — found on the palms and soles — has five epidermal strata including the stratum lucidum, is hairless (glabrous), contains exceptionally high densities of eccrine sweat glands and mechanoreceptors, and can be up to 1.5 mm thick in the epidermis alone. Thin skin — covering the rest of the body — lacks a stratum lucidum, has four epidermal strata, bears hair follicles and sebaceous glands, and has a much thinner epidermis (0.1–0.15 mm). The thickness of the dermis is not correlated with epidermal thickness — the back has relatively thin epidermis but the thickest dermis in the body (~4 mm), providing tensile strength and protection. As the NCBI reference on skin anatomy confirms, skin’s variable structure across body regions reflects precise adaptation to local mechanical, environmental, and functional demands.
Skin as Architecture — Three Layers, One Unified System
0.1–1.5 mm thick
Stratified squamous epithelium; avascular; renewed every 40–56 days; five strata in thick skin, four in thin; primary physical and chemical barrier; site of melanin production and vitamin D synthesis; contains keratinocytes (~95%), melanocytes, Langerhans cells, Merkel cells.
1–4 mm thick
Dense irregular connective tissue; highly vascularised; contains papillary (superficial, loose collagen) and reticular (deep, type I collagen, elastin) layers; houses hair follicles, sebaceous glands, sweat glands, nerve endings, mechanoreceptors, blood vessels, and lymphatics; provides structural strength and elasticity.
Variable thickness
Loose connective tissue and adipose tissue below the dermis; not part of the skin proper; insulates body, stores energy (subcutaneous fat — the largest fat depot), anchors skin to deep fascia, contains larger blood vessels and lymphatics; thickness varies greatly by body region and individual metabolic state.
The relationship between these three layers is not simply one of stacking — they are structurally interdependent. The epidermis and dermis interdigitate through the dermal-epidermal junction (DEJ), where epidermal rete ridges project down into the dermis and dermal papillae project up into the epidermis, dramatically increasing the contact surface area and mechanical adhesion between the two layers. The DEJ is a specialised basement membrane zone containing type IV collagen, laminin-332, nidogen, perlecan, and anchoring structures (hemidesmosomes connecting keratinocytes to the lamina densa; anchoring fibrils of type VII collagen connecting the lamina densa to the dermis). Genetic defects in these anchoring proteins cause a family of severe blistering diseases called epidermolysis bullosa, where mechanical trauma causes the epidermis to shear from the dermis because the DEJ components required for adhesion are absent or dysfunctional.
The Epidermis — Five Strata and the Journey from Basale to Corneum
The epidermis is organised as a stratified squamous epithelium in which the deepest cells actively divide and the superficial cells are terminally differentiated, dead, and continuously shed. This organisation creates a conveyor-belt system: new keratinocytes produced by stem cell division in the stratum basale progressively move upward, undergoing a precisely orchestrated programme of differentiation and keratinisation — expressing different keratin isoforms, synthesising lipid-filled lamellar bodies, losing their nuclei and organelles, and eventually becoming the dead, keratin-filled squames of the stratum corneum that are shed into the environment. The entire journey from basale to shedding takes approximately 40–56 days in normal skin; this accelerates to 3–4 days in psoriasis, producing the characteristic scale.
Stratum Basale — the Proliferative Foundation
The deepest epidermal stratum, resting on the basement membrane. A single layer of columnar or cuboidal basal keratinocytes attached to the basement membrane by hemidesmosomes (integrin-laminin interactions) and to each other by desmosomes. This layer contains keratinocyte stem cells — slow-cycling cells that divide asymmetrically to self-renew and produce transit-amplifying cells committed to terminal differentiation. Basal cells express keratins K5 and K14, which form the major intermediate filament network in this stratum. Melanocytes and Merkel cells are also found here. Basal cell carcinoma (BCC) — the most common skin cancer — arises from mutations in this layer, typically driven by Hedgehog pathway dysregulation and chronic UV exposure.
Stratum Spinosum — the Spiny Layer of Intercellular Bridges
Several layers of polyhedral keratinocytes above the basale, named for their spiny appearance under light microscopy — caused by the shrinkage of cells during fixation while desmosomes maintain intercellular connections, creating short projecting “spines.” Keratinocytes here are still mitotically active (particularly in the lower spinosum) and begin synthesising keratins K1 and K10 (the differentiation-specific keratins) alongside K5/K14. Langerhans cells reside in the spinosum and extend long dendritic processes between keratinocytes, forming an immunological surveillance network. Lamellar granules (also called Odland bodies) begin to appear in the upper spinosum — membrane-bound organelles packed with lipid bilayer precursors, ceramide precursors, and hydrolytic enzymes that will be secreted into the intercellular space at the spinosum-granulosum interface.
Stratum Granulosum — the Granular Layer of Barrier Assembly
Two to five layers of flattened keratinocytes characterised by numerous basophilic keratohyalin granules — protein complexes containing profilaggrin (precursor of filaggrin, which cross-links keratin filaments into the dense macrofibrils of the corneum), loricrin (the major structural protein of the cornified cell envelope, ~70% of total mass), and involucrin (early marker of cornification, cross-linked to the plasma membrane by transglutaminase enzymes). The granulosum is the site of active barrier assembly: lamellar bodies are transported to the apical surface and exocytosed, releasing lipid bilayers (ceramides, free fatty acids, cholesterol) into the extracellular space — forming the lamellar body-derived lipid mortar of the stratum corneum that is essential for skin permeability barrier function. Cells approaching the top of the granulosum begin programmed death (cornification, distinct from apoptosis) — the nucleus and organelles are degraded by caspase-14 and other proteases.
Stratum Lucidum — the Clear Layer (Thick Skin Only)
A narrow, translucent zone of flattened, densely packed, anucleate cells found only in thick skin (palms and soles). Cells are filled with eleidin — a transformation product of the keratohyalin granule proteins (filaggrin precursors) — and appear clear or translucent under haematoxylin and eosin staining. The lucidum represents a transitional zone between the granular and cornified layers, with cells already at an advanced stage of differentiation. Its presence in thick skin provides additional structural reinforcement in the highest-wear body regions. Histologically, the absence of a stratum lucidum is one of the features distinguishing thin skin from thick skin sections under examination — a key fact for histology practical assessments.
Stratum Corneum — the Dead, Keratinised Outermost Barrier
The outermost and by far the most functionally important stratum — 15–20 layers of flattened, anucleate, dead corneocytes (fully differentiated keratinocytes) in thin skin, up to 100+ layers on palms and soles. Each corneocyte is a “brick” — a keratin-packed, protein-crosslinked cell enclosed in the cornified cell envelope (a 15 nm-thick layer of cross-linked proteins — loricrin, involucrin, cornifin — assembled by transglutaminases). The “mortar” between the bricks is the lamellar body-derived lipid matrix: stacked, organised lipid bilayers of ceramides (50%), free fatty acids (25%), and cholesterol (25%). This “bricks and mortar” architecture creates the skin permeability barrier — the primary physical barrier against water loss (transepidermal water loss, TEWL) and against penetration of chemicals, allergens, and microorganisms. Corneocytes are continuously shed from the surface (desquamation) via serine proteases (kallikrein-5, kallikrein-7) that degrade the corneodesmosomes holding them together; defects in this process cause icthyosis (scaling disorders).
Four Epidermal Cell Populations — Specialisation at Every Level
Despite keratinocytes dominating the epidermis numerically, three additional cell populations are essential for its sensory, immune, and pigmentary functions. Each has a distinct developmental origin, morphology, distribution, and function — and each is implicated in specific diseases when dysregulated.
Keratinocytes — Barrier Builders
Derived from surface ectoderm; connected by desmosomes (intercellular junctions containing desmoplakin, desmoglein, and desmocollin) and hemidesmosomes (attaching to the basement membrane via integrins). Produce keratin intermediate filaments — K5/K14 in basal cells, K1/K10 in differentiating cells — and the cornified cell envelope proteins (loricrin, involucrin, profilaggrin). Secrete cytokines (IL-1α, IL-1β, IL-6, IL-18, TNF-α, TSLP) that orchestrate cutaneous immune responses. Autoimmune targeting of desmoglein-1 (Staphylococcal scalded skin syndrome, pemphigus foliaceus) or desmoglein-3 (pemphigus vulgaris) disrupts intercellular adhesion, causing blistering from acantholysis. Squamous cell carcinoma arises from malignant transformation of spinosum keratinocytes.
Melanocytes — the UV Shield System
Derived from neural crest cells that migrate to the DEJ during embryogenesis. Highly dendritic cells sitting in the stratum basale, never forming desmosomes with adjacent keratinocytes (instead attaching via E-cadherin and integrins). Each melanocyte serves ~36 keratinocytes in a “epidermal melanin unit.” Synthesise melanin in membrane-bound melanosomes via the enzyme tyrosinase (tyrosine → DOPA → dopaquinone → melanin), requiring the transcription factor MITF (melanocyte-inducing transcription factor). Transfer melanosomes to keratinocytes via phagocytosis of dendritic tips. The melanin cap that accumulates supranuclearly in keratinocytes absorbs UV radiation before it reaches nuclear DNA. Vitiligo: autoimmune destruction of melanocytes. Melanoma: malignant transformation driven by BRAF V600E mutation (~50% of cases), NRAS mutations, and UV-induced driver mutations in CDK4, CDKN2A, TERT promoter.
Langerhans Cells — the Epidermal Immune Patrol
Bone marrow-derived monocyte-lineage dendritic cells that reside in the stratum spinosum, identified by their characteristic Birbeck granules (tennis racket-shaped pentalaminar organelles containing langerin/CD207) visible on electron microscopy. They form a dense patrol network, with dendritic processes extending between keratinocytes to sample the extracellular environment. Upon antigen encounter, Langerhans cells undergo maturation (upregulating MHC II, CD80, CD86), retract their dendrites, and migrate via lymphatics to draining lymph nodes where they present antigen to T lymphocytes — initiating adaptive immune responses, including contact hypersensitivity reactions (e.g., poison ivy dermatitis, nickel allergy). Langerhans cells also contribute to peripheral tolerance by inducing regulatory T cell responses under non-inflammatory conditions. Langerhans cell histiocytosis (LCH): clonal proliferation of CD1a+ Langerhans cell-like cells causing bone lesions, diabetes insipidus, and skin rashes.
Merkel Cells — Touch-Sensitive Mechanoreceptors
Oval, clear-cytoplasm cells located at the dermal-epidermal junction in the stratum basale, associated with myelinated Aβ sensory nerve terminals to form Merkel discs. Contain dense-core granules with peptide neurotransmitters (vasoactive intestinal peptide, somatostatin, met-enkephalin) released in response to mechanical deformation. Merkel discs are slowly-adapting Type I mechanoreceptors with small receptive fields — ideal for detecting sustained pressure and fine spatial detail (used in reading Braille, discriminating textures). Especially dense in fingertips (~1,000/cm²), lips, and genitalia. Merkel cell carcinoma: rare but aggressive neuroendocrine skin carcinoma associated with Merkel cell polyomavirus (MCPyV) integration in ~80% of cases and immunosuppression; Merkel cells express CK20 (cytokeratin 20) — a useful diagnostic immunohistochemical marker distinguishing it from other neuroendocrine tumours.
The Dermis — Structural Core of Skin Function
The dermis is the skin’s powerhouse of structural support, vascular supply, and sensory infrastructure. Its two layers have distinct tissue architecture and different functional roles, and the ratio of papillary to reticular dermis changes with age, chronic sun exposure, and disease — making it the primary site of visible skin aging changes including wrinkles, laxity, and texture alterations.
Hypodermis — Insulation, Energy Storage, and Mechanical Cushioning
Composition and Architecture
The hypodermis (subcutaneous layer, superficial fascia) lies deep to the dermis and is composed of loose connective tissue interspersed with adipocytes organised into lobules separated by fibrous septa. It is not histologically part of the skin but is clinically and functionally inseparable from it — subcutaneous injections are delivered here, lidocaine diffuses through it, and the depth of burns is measured through to it. Thickness varies enormously by body region (near-zero over the shin, several centimetres over the abdomen) and by sex (women have proportionally more subcutaneous fat, particularly over the gluteal, femoral, and breast regions, while men accumulate more visceral fat). Blood vessels and nerves supplying the dermis travel through the hypodermis.
Adipocytes in the hypodermis are not metabolically inert — they synthesise leptin (appetite-regulating adipokine), adiponectin (insulin-sensitising), and resistin; convert androgens to oestrogens (aromatase activity); and produce inflammatory cytokines in obesity. Lipodystrophy (loss of subcutaneous fat, as in HIV-associated lipodystrophy from antiretroviral therapy) produces not just cosmetic changes but metabolic dysregulation, insulin resistance, and hyperlipidaemia.
Functional Significance
The hypodermis performs three primary functions: thermal insulation (subcutaneous fat has low thermal conductivity, approximately 0.21 W/m·K compared to 0.6 W/m·K for water — providing a significant insulating layer that reduces core heat loss in cold environments); mechanical cushioning (protecting underlying structures — bones, muscles, neurovascular bundles — from compressive and shear forces); and energy storage (the subcutaneous fat depot stores triglycerides as a readily mobilisable energy reserve, accessed during caloric deficit via hormone-sensitive lipase and adipose triglyceride lipase under adrenergic stimulation).
Panniculitis — inflammation of the hypodermis — produces tender subcutaneous nodules and is associated with conditions including erythema nodosum (immune-mediated, associated with sarcoidosis, inflammatory bowel disease, and streptococcal infection), lupus profundus, and pancreatic panniculitis (from enzymatic fat necrosis in pancreatitis). Cellulitis, though primarily a dermal infection, commonly extends into the hypodermis, particularly when caused by group A Streptococcus or Staphylococcus aureus.
Skin Appendages — Hair, Nails, and Glandular Structures
Skin appendages are epidermal-derived specialised structures that extend into the dermis and sometimes the hypodermis. They retain the capacity for epidermal regeneration — following burns that destroy the epidermis, surviving epidermal cells in hair follicles and sweat glands re-epithelialise the wound surface, which is why superficial and partial-thickness burns can heal without grafting. Their structural diversity reflects the range of functions they serve: thermal regulation, chemical communication, mechanical protection, and lubrication.
Hair Follicles
Approximately 5 million hair follicles cover the human body (excluding palms, soles, lips, glans penis, labia minora). Each follicle is a downgrowth of the epidermis into the dermis, terminating in a hair bulb surrounding the dermal papilla — a cluster of specialised fibroblasts that controls follicle cycling. Follicles produce terminal hair (scalp, beard, pubic, axillary) or vellus hair (fine, unpigmented body hair). Each follicle has an associated arrector pili smooth muscle and sebaceous gland — together forming a pilosebaceous unit.
Sebaceous Glands
Holocrine glands associated with hair follicles (pilosebaceous units), absent from palms, soles, and dorsal foot. Produce sebum — a complex lipid mixture of triglycerides, wax esters, squalene, and free fatty acids — by holocrine secretion (entire secretory cells disintegrate, releasing their contents into the follicular canal). Sebum lubricates and waterproofs the hair and skin surface, has antimicrobial properties, and contributes to the skin microbiome environment. Stimulated by androgens (dihydrotestosterone) — explaining pubertal onset of acne. Hyperactivity in acne vulgaris leads to comedone formation and Cutibacterium acnes colonisation.
Sweat Glands
Eccrine glands (~2–4 million, distributed across all body regions) are simple coiled tubular glands opening directly at the skin surface. Produce hypotonic sweat (mostly water, NaCl, KCl, urea, lactic acid) for thermoregulation; innervated by sympathetic cholinergic fibres. Apocrine glands (axillae, groin, areolae, external auditory meatus — the ceruminous glands) open into hair follicles, produce viscous protein-lipid secretions that are odourless until metabolised by skin bacteria producing the characteristic body odour; activated at puberty by androgens; modified in the breast (mammary glands) and eyelid (Moll’s glands).
Nails
Hard, translucent plates of compacted, tightly adherent dead keratinocytes (onychocytes) covering the dorsal surface of the terminal phalanges. The nail plate grows from the nail matrix (the proliferative zone beneath the proximal nail fold — the lunula marks its visible distal extent). The nail bed underlies the plate; the nail folds flank it laterally; the hyponychium seals the junction between the nail plate and the distal digit. Fingernails grow ~3 mm/month; toenails ~1 mm/month. Nail changes are diagnostically important: koilonychia (spoon nails → iron deficiency), clubbing → chronic hypoxia, pitting → psoriasis, leuconychia → hypoalbuminaemia.
Hair Follicle Biology and the Growth Cycle
Each hair follicle independently cycles through three phases — anagen, catagen, and telogen — governed by signalling from the dermal papilla and the follicle bulge (where epithelial stem cells reside). The dermal papilla is the master regulator: it secretes growth factors (VEGF, FGF-7, insulin-like growth factor-1) that maintain the anagen phase, and inhibitory signals (BMP4, DKK1) that drive catagen and telogen entry. Understanding follicle biology is essential for understanding the causes of hair loss (alopecia) and the rationale for treatments targeting these pathways.
Anagen — Active Growth Phase (2–7 Years for Scalp Hair)
The follicle is at maximal depth in the dermis/hypodermis. Matrix keratinocytes at the base of the hair bulb divide rapidly (~24-hour cell cycle — one of the fastest-dividing cell populations in the body), pushing the hair shaft upward at ~1 cm/month. Melanocytes in the upper hair bulb transfer melanin to cortical hair shaft cells, pigmenting the growing hair. Anagen length is genetically determined and is the primary determinant of maximum achievable hair length. Chemotherapy agents targeting rapidly dividing cells (cisplatin, doxorubicin, cyclophosphamide) arrest matrix cell division within days, causing the hair shaft to constrict (Pohl-Pinkus constriction) and break — producing anagen effluvium, the acute hair loss of chemotherapy that occurs within weeks of treatment rather than the 2–4 month delay of telogen effluvium.
Catagen — Involution Phase (~2–3 Weeks)
The follicle undergoes controlled regression driven by apoptosis of matrix keratinocytes. The lower two-thirds of the follicle involutes and retracts upward; the hair bulb separates from the dermal papilla; the melanocytes stop producing melanin; the dermal papilla condenses below the shrinking follicle but remains viable, maintaining its capacity to reinitiate anagen. The hair shaft becomes a club hair — a dead, depigmented hair attached at its proximal end by a brush-like club shape. At any given time, approximately 1–2% of scalp hairs are in catagen. The molecular signals driving catagen include TGF-β1/β2, FGF-5, and BDNF. Disruption of catagen regulation contributes to various forms of alopecia.
Telogen — Resting Phase (~2–4 Months)
The follicle is short, the club hair is retained, and the follicle bulge harbours quiescent epithelial stem cells. The dermal papilla sits close to the bulge, maintaining a signalling dialogue that will eventually trigger re-entry into anagen. Normal daily shedding of 50–100 telogen club hairs occurs during this phase as new anagen hairs push them from the follicle. Approximately 10–15% of scalp hairs are in telogen at any time. Telogen effluvium — sudden diffuse hair shedding 2–4 months after a physiological trigger (postpartum, severe illness, major surgery, crash dieting, high fever) — occurs when a large number of follicles simultaneously shift into telogen, then synchronously shed their club hairs months later. The delay reflects the time spent in telogen before shedding.
Exogen — Active Shedding (Overlapping Telogen)
Sometimes described as a distinct fourth phase, exogen is the active shedding of the club hair driven by matrix metalloproteinase and cysteine protease activity within the follicle that loosens the club hair’s attachment. Exogen is not simply passive telogen shedding — it is an actively controlled process, and abnormalities in exogen (failure to shed club hairs) can cause retained hair shaft disorders. Simultaneously, a new anagen hair begins to grow beneath the shed telogen club hair — the new hair often visible as a short, pigmented hair at the mouth of the follicle while the longer club hair is still attached proximally. This temporal overlap of exogen and new anagen is what allows hair density to remain relatively constant during normal cycling.
Androgenetic alopecia (male and female pattern baldness) — affecting up to 50% of men by age 50 and ~25% of women by age 50 — results from progressive follicle miniaturisation driven by dihydrotestosterone (DHT) acting on androgen-sensitive follicles in the vertex and frontal scalp. DHT shortens anagen duration progressively with each cycle, reduces dermal papilla size, and eventually converts terminal follicles to vellus-producing miniaturised follicles. DHT is produced locally in the follicle by type 2 5α-reductase from testosterone; finasteride (5α-reductase inhibitor) and topical minoxidil (mechanism includes KATP channel opening and VEGF upregulation, prolonging anagen) are the primary evidence-based treatments.
Seven Functions of the Integumentary System
Physical and Chemical Barrier Protection
The stratum corneum “bricks and mortar” barrier prevents desiccation (water loss), excludes pathogens, allergens, and chemicals, and resists mechanical trauma. Skin surface pH (~4.5–5.5, the “acid mantle” from sweat and sebum) inhibits pathogen colonisation. Tight junctions in the stratum granulosum provide a second permeability barrier. Psoriasis and atopic dermatitis both involve barrier disruption — FLG (filaggrin) gene mutations are the strongest genetic risk factor for atopic eczema, acting by weakening the stratum corneum structure.
Thermoregulation
Eccrine sweating (evaporative cooling), cutaneous vasodilation/vasoconstriction, and piloerection collaboratively regulate body temperature. The hypothalamic preoptic thermostat triggers these responses via sympathetic nervous system pathways. At 37°C core temperature, skin blood flow is ~250 mL/min; at 40°C, it rises to ~8 L/min. At maximum exercise in heat, sweat rates reach 2–3 L/hour, losing ~1.2 kJ/mL evaporated — the dominant heat dissipation mechanism in hot environments.
Sensory Reception
Specialised mechanoreceptors (Meissner corpuscles, Pacinian corpuscles, Ruffini endings, Merkel discs), thermoreceptors (cold: TRP M8/A1 channels; warm: TRP V1/V4), and nociceptors (C fibres: slow burning pain; Aδ fibres: sharp, fast pain) provide continuous sensory data about the mechanical, thermal, and noxious environment. Fingertip two-point discrimination is as fine as 2–3 mm, enabled by high receptor density (~2,500 Meissner corpuscles/cm²).
Vitamin D Synthesis
UVB radiation (290–315 nm) photoisomerises epidermal 7-dehydrocholesterol to pre-vitamin D3, then thermally to cholecalciferol (vitamin D3). Sequentially hydroxylated in liver (25-OH vitamin D3) and kidney (1,25-dihydroxyvitamin D3 / calcitriol). Calcitriol regulates calcium absorption, bone mineralisation, immune modulation, and cell differentiation. Vitamin D deficiency (serum 25-OH D3 <20 ng/mL) is the world’s most prevalent nutritional deficiency, affecting ~1 billion people — linked to osteoporosis, immune dysfunction, and increased cancer risk.
Immune Surveillance and Defence
Langerhans cells (adaptive immunity), dermal dendritic cells, mast cells, NK cells, and skin-resident memory T cells form a multi-layer immunological network. The skin microbiome (Staphylococcus epidermidis and others) competes with pathogenic organisms and trains cutaneous immunity. Antimicrobial peptides (defensins, cathelicidins — produced by keratinocytes and neutrophils) provide innate chemical defence. The skin is the first site of sensitisation for many allergens and plays a key role in establishing systemic immune responses to environmental antigens.
Excretion and Absorption
Eccrine sweat eliminates small quantities of urea (~0.5 g/L), lactic acid, ammonia, uric acid, and electrolytes. In renal failure, urea crystallises on the skin surface (“uraemic frost” — a late sign of severe uraemia). Transdermal absorption is exploited pharmacologically in patches (nicotine, fentanyl, scopolamine, testosterone, oestrogens, glyceryl trinitrate) and by occupational and environmental toxin exposure. Absorption depends on skin barrier integrity, molecular size and lipophilicity of the penetrant, vehicle formulation, and body site (scrotal skin has highest permeability; palm and sole lowest).
Melanin — UV Absorption, Pigmentation, and Skin Colour
Skin colour is determined not by the number of melanocytes (which is approximately equal across all ethnic groups) but by the amount, type, and distribution of melanin produced and the packaging of melanosomes within keratinocytes. This understanding is fundamental for interpreting racial and ethnic variation in skin cancer risk, vitamin D synthesis capacity, and the biological rationale for tanning as an adaptive photoprotective response.
Melanin Synthesis (Melanogenesis) — occurs within melanosomes: Step 1: Tyrosine ↓ Tyrosinase (rate-limiting enzyme) + Cu²⁺ cofactor Step 2: DOPA (3,4-dihydroxyphenylalanine) ↓ Tyrosinase Step 3: Dopaquinone (reactive intermediate — branch point) Branch 1 — Eumelanin (brown/black, photoprotective): Dopaquinone → Leucodopachrome → Dopachrome ↓ Dopachrome tautomerase (DCT/TYRP2) DHI + DHICA → polymerised to eumelanin Associated with: MC1R wild-type alleles, darker skin, lower skin cancer risk Branch 2 — Pheomelanin (red/yellow, poor photoprotection): Dopaquinone + cysteine → Cysteinyldopa → polymerised to pheomelanin Associated with: MC1R loss-of-function alleles (R151C, R160W, D294H), red hair, freckles Pheomelanin may generate ROS on UV exposure — not purely protective Regulation of melanogenesis: α-MSH → MC1R → cAMP → PKA → CREB → MITF upregulation → tyrosinase, DCT, TYRP1 expression ASIP (agouti signalling protein) → MC1R antagonist → switches to pheomelanin UV → p53 → POMC → α-MSH → tanning response (delayed, 48–72 hours post-UV) Melanosome transfer to keratinocytes: Melanocyte dendrite contacts keratinocyte → phagocytosis of dendritic tip Melanosomes accumulate supranuclearly in keratinocytes → "melanin cap" Absorbs and scatters UV before it reaches nuclear DNA → photoprotection SPF (sun protection factor) of darkly pigmented skin: ~13.4 versus ~3.4 in lightly pigmented skin
Cutaneous Sensory Receptors — Touch, Pressure, Vibration, Temperature, and Pain
The skin is the body’s primary interface with the tactile world — its sensory receptor population is the basis of everything from reading Braille to detecting the burn of a hot surface before it causes injury. Cutaneous receptors are classified by the stimulus they detect (mechanoreceptors, thermoreceptors, nociceptors) and by their adaptation rate (rapidly adapting: respond to stimulus onset and offset; slowly adapting: respond continuously to maintained stimulus). Their spatial distribution across the body surface reflects the functional demands of each region.
The two-point discrimination test (the minimum distance between two simultaneous touch stimuli that can be perceived as separate rather than single) is the clinical measure of spatial acuity and is directly related to the density of slowly-adapting mechanoreceptors and the size of their cortical representation. Fingertips: 2–3 mm; palm: 10–15 mm; back: 40–70 mm — reflecting both receptor density and the proportional somatosensory cortex representation (Penfield’s “cortical homunculus” dramatically enlarges the hand, lips, and face relative to their body surface area).
Thermoregulation — the Skin’s Role in Temperature Homeostasis
The skin’s thermoregulatory function is among the most physiologically critical — deviation of core body temperature beyond the narrow range of 36–37.5°C impairs enzymatic function, disrupts neurological signalling, and is rapidly life-threatening. The integumentary system is the primary effector organ for heat dissipation, responding to signals from the hypothalamic preoptic anterior hypothalamic area (POAH) — the body’s central thermostat — through sympathetic nervous system pathways.
Relative contribution of thermoregulatory mechanisms at different environmental and activity conditions
When cutaneous thermoregulation is overwhelmed — by extreme ambient temperature, high humidity (preventing sweat evaporation), excessive physical exertion, or pharmacological interference (anticholinergic drugs block eccrine sweating; diuretics reduce sweat volume) — core temperature rises dangerously. Heat exhaustion occurs when cardiovascular and fluid demands of maximal sweating are not met (hypotension, tachycardia, excessive sweating, dizziness, nausea); core temperature is elevated but below 40°C. Heat stroke is a medical emergency defined by core temperature >40°C with neurological dysfunction (confusion, seizures, coma) — caused by direct thermal cellular injury and systemic inflammatory response from gut bacterial translocation and cytokine storm. The elderly are most vulnerable because both eccrine sweat gland output and cutaneous vasodilation capacity decline significantly with age.
Conversely, hypothermia (core temperature <35°C) results from insufficient cutaneous vasoconstriction, piloerection, and shivering thermogenesis to compensate for environmental heat loss — causing progressive enzymatic failure, cardiac arrhythmias, and eventually cardiac arrest.
Vitamin D Synthesis in Skin — the Photochemical Pathway
The Complete Vitamin D Synthesis and Activation Pathway — From UV to Calcitriol
Step 1 — Photoisomerisation (skin): UVB radiation (wavelength 290–315 nm) absorbed by 7-dehydrocholesterol (7-DHC, stored in the plasma membranes of epidermal keratinocytes) breaks the B ring of the steroid nucleus, producing pre-vitamin D3 (previtamin D3). This is a purely photochemical reaction requiring no enzymatic activity. Prolonged UV exposure does not cause toxicity because excess previtamin D3 is photodegraded to tachysterol and lumisterol — inactive isomers. This explains why sun exposure alone cannot cause vitamin D toxicity, unlike oral supplementation.
Step 2 — Thermal isomerisation (skin): Previtamin D3 spontaneously rearranges over 2–3 days at body temperature to vitamin D3 (cholecalciferol). This thermally-driven isomerisation is enhanced at 37°C; stored under the skin for days, providing a sustained post-UV supply. Cholecalciferol enters the circulation bound to vitamin D-binding protein (DBP).
Step 3 — First hydroxylation (liver): CYP27A1 (mitochondrial 27-hydroxylase) or CYP2R1 (microsomal 25-hydroxylase) add a hydroxyl group at carbon 25, producing 25-hydroxyvitamin D3 (25(OH)D3, calcidiol) — the major circulating form and the serum marker used to assess vitamin D status. Half-life: 2–3 weeks. Not biologically active — binds nuclear receptor only weakly.
Step 4 — Second hydroxylation (kidney, immune cells, placenta): CYP27B1 (1α-hydroxylase) in proximal tubular cells adds a hydroxyl at carbon 1, producing 1,25-dihydroxyvitamin D3 (calcitriol) — the biologically active form. Half-life: 4–6 hours. Regulated by PTH (stimulates), FGF-23 from osteocytes (inhibits), calcitriol itself (feedback inhibition), calcium, and phosphate. Calcitriol acts on the vitamin D receptor (VDR, a nuclear receptor) in intestinal cells, osteoblasts, parathyroid cells, immune cells, and many other tissues. In CKD, reduced functional nephron mass → reduced CYP27B1 activity → calcitriol deficiency → secondary hyperparathyroidism → renal osteodystrophy.
Wound Healing — Four Overlapping Phases from Injury to Scar
Wound healing is one of the most complex, coordinated biological processes in the human body — requiring the timed activation of platelets, neutrophils, macrophages, fibroblasts, endothelial cells, and keratinocytes, governed by an intricate network of growth factors, cytokines, and extracellular matrix signals. Failure of any phase produces pathological outcomes: non-healing chronic wounds (most commonly in diabetic foot ulcers, pressure injuries, venous leg ulcers) or excessive healing (hypertrophic scars, keloids). According to the American Academy of Dermatology, chronic wounds affect an estimated 6.5 million patients in the United States alone, with an annual treatment cost exceeding $25 billion.
Haemostasis (Seconds to Hours) — Plugging the Breach
Immediately upon vascular injury, injured vessels constrict (vasospasm, mediated by endothelin and thromboxane A2) to slow blood loss. Exposed subendothelial collagen activates platelet adhesion (via GP Ib/IX/V binding to von Willebrand factor and GP VI direct collagen binding), spreading, and aggregation (platelet-activating factor, thromboxane A2, ADP amplification). Activated platelets degranulate, releasing growth factors (PDGF — a potent fibroblast and smooth muscle cell chemoattractant and mitogen; TGF-β1 — induces matrix production and fibroblast activation; EGF, IGF-1), ADP, serotonin, and coagulation factors. The coagulation cascade (intrinsic pathway activated by contact with collagen; extrinsic pathway activated by tissue factor) converges on thrombin generation → fibrinogen cleavage → fibrin polymerisation. The fibrin clot serves as the initial wound scaffold and provisional matrix for cell migration. This phase is also the primary site of derangement in coagulation disorders (haemophilia, thrombocytopenia) that impair wound healing.
Inflammation (Hours to Days, Peak 48–72 Hours) — Recruiting the Repair Crew
Histamine and prostaglandins released by mast cells and platelets cause vasodilation and increased vascular permeability — the cardinal signs of inflammation (rubor/redness, calor/heat, tumor/swelling). Neutrophils are the first leucocytes to arrive (chemotaxis along IL-8, LTB4, fMLP gradients) — they phagocytose bacteria, release reactive oxygen species (oxidative burst), and produce proteases (elastase, cathepsin G, MMP-8) that debride necrotic tissue. Neutrophil numbers peak at 24–48 hours then decline. Monocytes, following 48–96 hours later, differentiate into tissue macrophages under CSF-1, TGF-β1, and IL-4 signalling. Macrophages are the critical orchestrators of the subsequent repair process — they switch from pro-inflammatory M1 phenotype (producing TNF-α, IL-1β, IL-6, NO — antimicrobial and debridement functions) to pro-regenerative M2 phenotype (IL-10, TGF-β, VEGF, IGF-1, FGF — angiogenesis and fibroplasia stimulation). Failure of this M1→M2 switch, as in diabetic wounds, is a key mechanism of impaired healing.
Proliferation (Days to Weeks) — Building the Repair Tissue
Three simultaneous processes restore tissue: Angiogenesis — VEGF and FGF-2 from macrophages, platelets, and hypoxic tissue drive endothelial sprouting, restoring the oxygen supply essential for fibroblast and keratinocyte activity; Fibroplasia — fibroblasts migrate into the fibrin scaffold (using integrin-dependent traction on fibronectin and collagen), proliferate under PDGF stimulation, synthesise type III collagen, fibronectin, and hyaluronic acid — producing the granulation tissue (red, granular-appearing tissue visible in healing wounds, named for its granular surface caused by new capillary loop tips); subsequently, fibroblasts differentiate into myofibroblasts (expressing α-smooth muscle actin, contractile) that actively pull wound edges together (wound contraction); and Re-epithelialisation — keratinocytes at the wound margin become migratory (downregulating E-cadherin, upregulating integrins for fibronectin and collagen), migrate across the granulation tissue surface, and proliferate to resurface the wound; stem cells from hair follicles and sweat glands within the wound also contribute to re-epithelialisation.
Remodelling (Weeks to Years) — Optimising the Scar
The initially weak type III collagen-rich granulation tissue is replaced by stronger type I collagen over months to years — a process mediated by matrix metalloproteinases (MMP-1, MMP-2, MMP-3 — collagenases and gelatinases) balanced by tissue inhibitors of metalloproteinases (TIMPs). Collagen fibres reorganise from random to parallel bundles aligned along tension lines. Myofibroblasts undergo apoptosis as remodelling proceeds, reducing scar vascularity (the initially red, raised scar becomes flat and pale). Wound tensile strength increases from ~20% of normal at 3 weeks to ~80% at 3–6 months — never reaching pre-injury levels. Pathological remodelling produces: hypertrophic scars (raised scars confined to the wound margins, containing excess type III collagen — more common after deep burns; treat with pressure garments, silicone gel, corticosteroid injection) and keloids (raised scars extending beyond wound margins, containing thick type I collagen bundles, lacking myofibroblasts — more common in darker skin phototypes, have genetic component; treat with excision plus corticosteroid injection, radiotherapy).
Skin Immune Functions — the Outer Immune Fortress
Physical and Chemical Innate Barriers
The stratum corneum physically excludes most pathogens. The acid mantle (pH 4.5–5.5 from lactic acid in sweat and free fatty acids from sebaceous gland secretions and microbiome) inhibits alkaline-preferring pathogens. Antimicrobial peptides (AMPs) — defensins (β-defensin-1 constitutively expressed; β-defensin-2, -3 induced by bacterial products and injury), cathelicidin (LL-37, derived from the CAP18 precursor produced by keratinocytes and neutrophils), dermcidin (in eccrine sweat) — provide broad-spectrum antimicrobial activity via membrane disruption and immunomodulation. Tight junctions in the stratum granulosum form a second barrier. Sebum fatty acids are directly bactericidal and fungicidal. Loss of any of these layers — as in atopic dermatitis — dramatically increases skin colonisation by Staphylococcus aureus (~90% of patients) and risk of secondary infection.
Skin-Associated Lymphoid Tissue (SALT)
The skin hosts a complex resident immune cell population: Langerhans cells (~700–1,000/mm² in epidermis), dermal dendritic cells, mast cells (~7,000/mm³ dermis), tissue-resident memory T cells (TRM — CD8+ and CD4+ cells that remain in the skin long-term after resolved infection, providing rapid local recall responses), regulatory T cells, NK cells, and γδ T cells. Keratinocytes themselves actively participate in immune signalling — producing TSLP (thymic stromal lymphopoietin, key driver of Th2-skewing in atopic dermatitis), IL-33, IL-25, IL-1α, IL-6, IL-18, and TNF-α in response to damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs). The concept of skin-associated lymphoid tissue (SALT) reflects the skin’s role as an organised immune organ rather than simply a passive barrier.
The Skin Microbiome — Commensal Defence
The skin microbiome comprises approximately 1,000 bacterial species, along with fungi (Malassezia predominates on sebaceous sites), viruses (phageome), and mites (Demodex folliculorum on face). Dominant bacteria vary by body site: Staphylococcus epidermidis and Cutibacterium acnes (formerly Propionibacterium acnes) predominate on sebaceous sites; Corynebacteria and micrococci on moist sites; Staphylococci on dry skin. S. epidermidis actively competes with S. aureus by producing phenol-soluble modulins and serine protease Esp; Cutibacterium acnes produces propionic acid inhibiting other microorganisms; some commensals produce bacteriocins and induce AMP production in keratinocytes. Dysbiosis of the skin microbiome — reduction in commensal diversity and S. epidermidis colonisation with increase in S. aureus — is a feature of atopic dermatitis and contributes to its pathogenesis. Microbiome-based therapies (topical application of commensal strains) are in clinical trials for eczema.
Intrinsic and Extrinsic Skin Aging
Skin aging has two distinct but interacting mechanisms that produce converging but distinguishable histological and clinical changes. Intrinsic (chronological) aging is universal, genetically determined, and inevitable; extrinsic aging — primarily driven by cumulative UV exposure (photoaging) — is environmentally acquired, preventable, and responsible for most of the visible differences in skin appearance between individuals of the same age. According to MedlinePlus, skin aging is primarily driven by reduced cellular renewal rates, loss of structural proteins, and cumulative environmental damage.
Intrinsic skin aging is a systemic biological programme — every centimetre of skin ages at the same rate, driven by telomere attrition in epidermal stem cells, progressive reduction in epidermal turnover rate, declining dermal fibroblast numbers, and reduced collagen synthesis. This produces uniformly smooth, thinned, pale, dry skin with fine surface lines — quite distinct from the deep wrinkles, uneven pigmentation, and solar elastosis of photoaged skin.
Reflecting Kligman’s foundational work distinguishing intrinsic from extrinsic aging, elaborated in Yaar and Gilchrest’s comprehensive review of skin aging biology
Photoaging is estimated to account for approximately 80–90% of visible facial aging in fair-skinned individuals — the accumulated UV damage of a lifetime compressing decades of “biological age” onto the histology of the face while the buttock skin of the same individual may appear decades younger. The single most effective anti-aging intervention remains daily broad-spectrum sunscreen.
Reflecting the classic Griffiths/Voorhees work on retinoid reversal of photoaging and the clinical-epidemiological evidence for sun protection efficacy in preventing premature photoaging
Chronological Aging — the Inevitable Programme
Driven by: telomere shortening in epidermal stem cells (progressively limiting proliferative capacity with each division); accumulation of reactive oxygen species and mitochondrial DNA mutations; reduced fibroblast numbers and collagen synthesis rate (type I collagen production decreases ~1% per year after age 30); increased expression of MMP-1 and MMP-3 (collagen-degrading enzymes activated by AP-1 transcription factor); reduced elastin synthesis; declining epidermal turnover (from ~3 weeks at age 20 to ~6–8 weeks at age 80, causing delayed wound healing). Clinical features: smooth surface, fine wrinkling, loss of skin elasticity (collagen and elastin loss), pallor (reduced vascularity, fewer melanocytes), xerosis (dryness, reduced sebum production, reduced water-holding capacity from declining NMF — natural moisturising factor from filaggrin degradation), reduced sweat gland output (thermoregulatory compromise). No inflammatory component; limited to sun-protected sites in its pure form.
Photoaging — the Environmental Overlay
UVA (320–400 nm, penetrates to dermis, constant throughout the day) and UVB (290–320 nm, penetrates only epidermis, strongest at midday) each damage the skin by distinct but synergistic mechanisms. UVB: directly damages epidermal DNA (cyclobutane pyrimidine dimers → mutations in p53, CDKN2A — key tumour suppressors in skin cancer); stimulates epidermal thickening (stratum corneum); induces tanning (delayed, 48–72h — mediated by UV-induced p53 → POMC → α-MSH → MC1R → MITF → melanogenesis). UVA: penetrates dermis; generates reactive oxygen species (singlet oxygen, superoxide) via chromophore absorption → oxidative damage to collagen, elastin, and DNA; activates AP-1 transcription factor → MMP-1, MMP-3, MMP-9 upregulation → matrix degradation → deep wrinkles, laxity, “solar elastosis” (tangled, degraded elastin fibres replacing normal dermis — visible histologically as basophilic amorphous material). Clinical features of photoaging: coarse, deep wrinkling; irregular pigmentation (lentigines, dyspigmentation); telangiectasia; cutis rhomboidalis nuchae; actinic keratoses (premalignant epidermal dysplasia). Confined to sun-exposed skin; dramatically more pronounced in Fitzpatrick phototypes I–II.
Burns Classification — Depth, Surface Area, and Clinical Management
Burns are classified by the depth of tissue destruction and the extent of body surface area involvement — two independent dimensions that together determine severity, prognosis, and treatment. A small deep burn (1% TBSA full thickness) requires skin grafting but is not immediately life-threatening; a large superficial burn (30% TBSA superficial partial-thickness) may not require grafting but requires aggressive fluid resuscitation to prevent hypovolaemic shock. Understanding burns requires integrating integumentary anatomy — knowing exactly which skin layers are destroyed at each depth — with systemic physiology of the burn response.
Superficial (First-Degree) Burns
Depth: Epidermis only (no blister). Appearance: Red, dry, painful, blanches with pressure. Examples: Mild sunburn, brief flash flame. Healing: 3–7 days, complete epidermal regeneration from basal layer, no scarring. Management: Moisturiser, analgesia, no dressing required. Not included in TBSA calculations for fluid resuscitation.
Superficial Partial-Thickness (Second-Degree, Superficial)
Depth: Epidermis + papillary dermis. Appearance: Moist, red/pink, blistered, extremely painful, blanches with pressure. Healing: 10–14 days without grafting, from surviving dermal appendages. Minimal scarring. Management: Antimicrobial dressings (silver sulfadiazine, Mepilex Ag), analgesia, monitor for infection. Included in TBSA.
Deep Partial-Thickness (Second-Degree, Deep)
Depth: Epidermis + reticular dermis (deep). Appearance: Pale, mottled, blistered or leathery, reduced pain sensation (deep nerve damage), does not blanch. Healing: >3 weeks if managed conservatively, with significant hypertrophic scarring; skin grafting generally recommended. Management: Early excision and split-thickness skin graft.
Full-Thickness (Third-Degree) Burns
Depth: Entire dermis destroyed; all epidermal appendages lost. Appearance: White, brown, black, or leathery; painless (nerve endings destroyed); does not blanch; may feel leathery on palpation (eschar). Healing: Cannot re-epithelialise without surgical skin grafting — no surviving epidermal cells remain. Management: Urgent excision and split-thickness autograft; consider cultured epithelial autografts (CEA) for extensive burns.
The Rule of Nines and Parkland Formula — Clinical Burns Assessment
Rule of Nines (TBSA estimation, adults): Head and neck = 9%; each arm = 9%; anterior trunk (chest + abdomen) = 18%; posterior trunk (upper and lower back) = 18%; each leg = 18%; perineum = 1%. Total = 100%. For children, the Lund and Browder chart is used instead, accounting for the proportionally larger head and smaller legs. The patient’s palm (excluding fingers) represents approximately 1% TBSA — useful for estimating irregular burn areas. Only partial-thickness and full-thickness burns are included in TBSA calculations for resuscitation.
Parkland Formula for fluid resuscitation (burns ≥15–20% TBSA): Total volume (first 24 hours) = 4 mL × weight (kg) × % TBSA burned. Fluid used: Hartmann’s solution (Ringer’s lactate). Timing: give half in the first 8 hours from time of burn (not from hospital arrival), remainder over the following 16 hours. Second 24 hours: colloid (5% human albumin) + maintenance crystalloid. Adequate resuscitation monitored by urine output (0.5 mL/kg/hour in adults; 1 mL/kg/hour in children); invasive haemodynamic monitoring in complex cases. Underresuscitation causes acute kidney injury and multi-organ failure; overresuscitation causes abdominal compartment syndrome, pulmonary oedema, and limb compartment syndrome (“fluid creep”).
Key Dermatological Conditions — Pathophysiology Rooted in Skin Biology
Every major inflammatory skin disease can be understood mechanistically through the lens of integumentary anatomy and physiology — as a specific disruption of barrier function, immune cell behaviour, epidermal turnover, or vascular regulation. The conditions below represent the core dermatology curriculum for medical, nursing, and biomedical science students, and their pathogenesis directly illustrates the principles covered throughout this guide.
Atopic Dermatitis (Eczema) — the Barrier-Immune Axis
Atopic dermatitis (AD) affects ~15–20% of children and ~5–10% of adults in Western countries — the most prevalent inflammatory skin disease globally. Pathogenesis: primary barrier dysfunction (most commonly from loss-of-function mutations in FLG encoding filaggrin — the dominant structural protein of the stratum corneum — found in ~40% of Europeans with AD) → skin dryness, increased TEWL, impaired acid mantle → antigen penetration → Langerhans cell and ILC2 activation → Th2-skewed inflammation (IL-4, IL-13, IL-31 — the itch cytokine — and IL-33, TSLP) → mast cell activation, eosinophil recruitment → intense pruritus driving scratching → further barrier damage (itch-scratch cycle). S. aureus colonises >90% of AD lesional skin, releasing virulence factors (TSST-1, delta toxin) that further activate mast cells and Th2 cells. Treatment: emollients (barrier restoration), topical corticosteroids, topical calcineurin inhibitors; dupilumab (anti-IL-4Rα biologic blocking IL-4/IL-13 signalling) is highly effective for moderate-severe AD.
Psoriasis — Accelerated Epidermal Turnover
Affects ~2–3% of the global population. Pathogenesis: a Th17-driven chronic inflammatory cycle initiated by environmental triggers in genetically predisposed individuals (HLA-Cw6 is the strongest genetic association). Activated dendritic cells produce IL-12 and IL-23 → Th1 and Th17 cell activation → keratinocytes stimulated by TNF-α, IL-17A, IL-22 → accelerated epidermal turnover (transit time reduced from 40–56 days to 3–4 days → parakeratosis — nucleated cells retained in the stratum corneum, producing the characteristic silvery scale over a red, well-demarcated plaque). VEGF overexpression drives dilated, tortuous capillaries visible through thin scale (Auspitz sign — pinpoint bleeding on scale removal). Nail pitting and onycholysis from psoriatic involvement of the nail matrix. Treatment: topical (emollients, corticosteroids, calcipotriol); phototherapy (NB-UVB, PUVA); systemic (methotrexate, cyclosporine, acitretin); biologics (anti-TNF — adalimumab; anti-IL-12/23 — ustekinumab; anti-IL-17 — secukinumab, ixekizumab; anti-IL-23 — risankizumab, guselkumab — the newest and most effective class).
Basal Cell Carcinoma (BCC) — Hedgehog Pathway
The most common human cancer (~80% of skin cancers). Arises from basal keratinocyte progenitors or hair follicle bulge stem cells; driven by: Hedgehog (Hh) pathway activation (PTCH1 loss-of-function mutations, most UV-induced — permitting constitutive SMO activation → Gli transcription factors → cell proliferation); TP53 mutations; UV-induced mutational signature. Almost never metastasises but causes significant local destruction. Subtypes: nodular (most common — pearly papule with telangiectasia and central ulceration); superficial; morphoeic/sclerosing (aggressive local invasion). Treatment: surgical excision; Mohs micrographic surgery (staged excision with intraoperative margin assessment, highest cure rate); vismodegib, sonidegib (SMO inhibitors — approved for locally advanced/metastatic BCC).
Acne — Four Factors in the Pilosebaceous Unit
Affects ~85% of adolescents and ~25% of adults. Four interacting factors: (1) Increased sebum production (driven by androgens stimulating sebaceous glands — dihydrotestosterone acting on the sebocyte androgen receptor; DHEA-S the predominant androgen in women); (2) Abnormal follicular keratinisation (ductal hypercornification → corneocyte accumulation → microcomedone formation → closed comedone/whitehead → open comedone/blackhead); (3) Cutibacterium acnes (gram-positive anaerobe producing lipases that cleave sebum triglycerides → free fatty acids → follicular inflammation; expressing PAMPs that activate TLR-2 on keratinocytes and sebocytes → IL-1, TNF-α, IL-8 → neutrophil recruitment → pustule, nodule, cyst formation); (4) Inflammation driven by the above cascade. Treatment (stepwise): topical retinoids (normalise follicular keratinisation); benzoyl peroxide (antimicrobial); topical antibiotics (clindamycin); oral antibiotics (doxycycline); hormonal (combined OCP, spironolactone); isotretinoin (systemic retinoid — reduces sebum production by ~90%, most effective agent for severe acne).
Melanoma — Malignant Melanocyte Transformation
Accounts for only ~1% of skin cancers but ~75% of skin cancer deaths, because of its capacity for early lymphatic and haematogenous metastasis. Key driver mutations: BRAF V600E (~50%, found in melanocytes in UV-damaged skin); NRAS (~20%); NF1 (~15%); KIT (acral and mucosal melanoma). UV-induced mutations in TERT promoter (+124 or +146 bp position) are among the most frequent early driver events. Risk factors: fair skin (phototypes I–II), family history, multiple naevi (>50), atypical mole syndrome, prior melanoma, immunosuppression, xeroderma pigmentosum. ABCDE criteria (Asymmetry, Border irregularity, Colour variation, Diameter >6mm, Evolution) guide clinical suspicion. Staging (AJCC 8th edition) based on Breslow thickness (depth of invasion in mm), ulceration, mitotic rate, lymph node involvement, and distant metastasis. Treatment for metastatic disease has been revolutionised: BRAF/MEK inhibitors (dabrafenib + trametinib for BRAF V600-mutant disease — response rate ~70%, median PFS 12 months); anti-PD-1 checkpoint immunotherapy (pembrolizumab, nivolumab — durable responses in ~40%); combination ipilimumab (anti-CTLA-4) + nivolumab.
Pemphigus and Bullous Pemphigoid — DEJ and Desmosome Autoimmunity
Pemphigus vulgaris: IgG autoantibodies against desmoglein-3 (and desmoglein-1 in extensive disease) → acantholysis (loss of keratinocyte intercellular adhesion) → flaccid blisters that rupture easily, leaving painful erosions predominantly on mucous membranes and skin; Nikolsky sign positive (lateral shear pressure causes epidermal slippage). Life-threatening without treatment. Treated with high-dose corticosteroids, rituximab (anti-CD20, depleting B cells), azathioprine. Bullous pemphigoid (BP): IgG autoantibodies against BP180 (type XVII collagen, a transmembrane hemidesmosomal protein) and BP230 → complement activation at the DEJ → neutrophil/eosinophil recruitment → subepidermal cleavage → tense blisters on urticarial base; pruritus often precedes blistering; more common in the elderly (mean age ~75). Nikolsky sign negative (blisters are deep, at the DEJ, and therefore tense). Treated with topical superpotent corticosteroids, systemic corticosteroids, doxycycline, methotrexate.
The Skin Microbiome in Health and Disease — a Rapidly Evolving Field
- Healthy skin microbiome characteristics: High diversity; Staphylococcus epidermidis and Cutibacterium acnes (formerly Propionibacterium acnes) predominate on different body sites; fungi (Malassezia species) are dominant on sebaceous regions; composition varies by body site, host genetics, age, sex, and environmental exposures.
- Atopic dermatitis: Reduced biodiversity; Staphylococcus aureus overgrowth (found in 90% of lesional skin, <10% of healthy control skin); S. aureus exotoxins (TSST-1, exfoliatin, delta-toxin) directly activate mast cells and Th2 cells; targeting S. aureus (topical antimicrobials, dilute bleach baths, anti-staphylococcal antibiotics) can improve disease activity independently of anti-inflammatory treatment.
- Acne: Not simply C. acnes overgrowth (total C. acnes counts may not differ from controls) but an imbalance between C. acnes phylotypes — phylotype IA1 associated with comedonal acne; phylotype II associated with healthy skin. Antibiotic resistance in C. acnes is increasing; sparing topical antibiotic use (combining with BPO to reduce resistance) is now recommended.
- Psoriasis: Reduced Firmicutes; increased Actinobacteria; altered Proteobacteria — distinct from AD microbiome changes, reflecting the different inflammatory pathways (Th17 vs Th2).
- Therapeutic implications: Microbiome-modifying approaches in development include: topical application of S. epidermidis strains to compete with S. aureus in AD; probiotic supplementation; phage therapy targeting S. aureus; transplantation of healthy skin microbiome. Clinical trials ongoing for AD and acne.
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