Overview
Skin is the largest organ of the human body by surface area (~1.5–2 m²) and mass (~15% of body weight). It forms an unbroken stratified epithelial barrier that physically separates the internal milieu from the external environment. The organ comprises three principal layers: the epidermis (avascular stratified squamous epithelium), the dermis (fibrovascular connective tissue), and the hypodermis (subcutaneous adipose and loose connective tissue).
Core physiological functions include:
- Permeability barrier: The stratum corneum lipid matrix (ceramides, cholesterol, fatty acids) restricts transepidermal water loss (TEWL) to <5 g/m²/h and prevents entry of environmental xenobiotics.
- Thermoregulation: Eccrine sweat glands (evaporative cooling) and cutaneous vasodilation/vasoconstriction modulate heat exchange to maintain core temperature.
- UV protection: Epidermal melanocytes synthesize melanin pigments (eumelanin, pheomelanin) that absorb and scatter UVA/UVB radiation, limiting DNA damage in deeper cell layers.
- Vitamin D synthesis: UVB photons convert 7-dehydrocholesterol in the epidermis to pre-vitamin D3, which is thermally isomerized to vitamin D3 (cholecalciferol). Vitamin D3 undergoes 25-hydroxylation in the liver (forming 25-OH-D3 / calcifediol) and then 1α-hydroxylation in the kidney (forming 1,25-(OH)₂-D3 / calcitriol), the biologically active form regulating calcium homeostasis.
- Sensory reception: Specialized mechanoreceptors (Meissner corpuscles — light touch; Pacinian — vibration/pressure; Merkel discs — fine spatial discrimination; Ruffini endings — skin stretch) and free nerve endings (pain, temperature, itch) transduce mechanical and nociceptive stimuli.
- Innate immunity: Keratinocyte-derived antimicrobial peptides (AMPs), Langerhans cell antigen surveillance, and pattern-recognition receptor signaling constitute the skin's first immunological line of defense.
- Wound healing: Sequential hemostasis, inflammation, proliferation, and remodeling phases restore barrier integrity after injury.
Clinical relevance: Skin diseases affect >30% of the world population at any time and include globally prevalent conditions (atopic dermatitis, psoriasis, acne) and life-threatening malignancies (melanoma, SCC). The barrier function makes the skin both a therapeutic target and a delivery route for topical and transdermal drugs.
Anatomy
Epidermis Layers
| Layer (superficial → deep) | Composition / Key Features | Notes |
|---|---|---|
| Stratum corneum | 15–20 layers of dead, anucleate corneocytes (terminally differentiated keratinocytes) embedded in a lipid matrix of ceramides, cholesterol, and fatty acids — the "brick and mortar" model; ~10–20 µm thick in non-palmar/plantar skin | Principal permeability barrier; TEWL <5 g/m²/h; site of action of most topical emollients and barrier-repair therapies |
| Stratum lucidum | Clear, densely packed cells; present only in thick skin (palms, soles) | Thin transitional zone between granulosum and corneum in glabrous skin |
| Stratum granulosum | 2–3 layers; keratohyalin granules (contain profilaggrin/filaggrin and loricrin); lamellar bodies (Odland bodies) release ceramide-rich lipid into intercellular space → forms the lipid barrier; tight junctions (claudin-1, claudin-4, ZO-1) seal paracellular route | Filaggrin (FLG) proteolysis generates natural moisturizing factor (NMF) amino acids in corneum; FLG loss-of-function mutations (R501X, 2282del4) are the strongest genetic risk factor for atopic dermatitis |
| Stratum spinosum | Several layers of polyhedral keratinocytes connected by desmosomes (desmoglein 1/3, desmoplakin); lamellar body synthesis begins; keratins K1/K10 expressed | Site of Langerhans cells; desmoglein-1 is target of pemphigus foliaceus autoantibodies |
| Stratum basale | Single layer of columnar/cuboidal keratinocytes anchored to basement membrane by hemidesmosomes (BP180, BP230, α6β4 integrin); contains proliferating keratinocyte stem cells, melanocytes (~1:10 ratio), and Merkel cells | Keratinocyte transit time from basale to corneum ~28 days (accelerated to ~4 days in psoriasis) |
Dermis
| Sub-layer | Composition | Key Structures |
|---|---|---|
| Papillary dermis | Thin layer of loose collagen I fibers, capillary loops (dermal papillae projecting into epidermis), fibroblasts | Meissner corpuscles (fingertip light touch); close nutritional supply to avascular epidermis |
| Reticular dermis | Thick bundles of collagen I and III, elastin network, fibroblasts, macrophages, mast cells | Hair follicles, eccrine sweat glands, apocrine glands, sebaceous glands, sensory nerve bundles (Pacinian corpuscles in deep dermis/hypodermis), Ruffini endings |
Hypodermis (Subcutaneous Layer)
Loose connective tissue and adipose lobules separated by fibrous septa. Functions include thermal insulation, mechanical cushioning, energy reservoir (lipolysis substrate), and anchoring point for the dermis to underlying fascia. Contains large blood vessels and lymphatics that supply the overlying skin.
Specialized Cell Types
| Cell Type | Location / Abundance | Key Functions & Markers |
|---|---|---|
| Keratinocyte | ~95% of epidermis; all layers in various differentiation stages | Principal permeability barrier cells; produce a repertoire of 14 cytokeratins (K5/K14 in basale; K1/K10 in spinosum/granulosum); antimicrobial peptide secretion (β-defensins, S100 proteins); TLR expression for innate pathogen sensing |
| Melanocyte | ~5% of basal layer; also in hair follicle bulb; neural crest-derived | Synthesize eumelanin (brown-black) and pheomelanin (red-yellow) in membrane-bound organelles called melanosomes; melanosomes transferred via dendritic extensions to surrounding keratinocytes (~1 melanocyte : 36 keratinocytes = "epidermal melanin unit"); melanin caps keratinocyte nuclei on sun-facing side (supranuclear cap) to shield DNA; UV exposure upregulates melanogenesis via α-MSH/MC1R/MITF/tyrosinase axis |
| Langerhans cell | Stratum spinosum; bone marrow-derived dendritic cell | CD1a⁺ CD207⁺ (langerin); foreign antigen capture via Birbeck granules; present antigens via MHC class II to naïve T cells in draining lymph nodes after emigration; sentinel for cutaneous immune surveillance; tolerogenic in steady-state, immunogenic on activation |
| Merkel cell | Stratum basale, especially finger pads and lips; neuroendocrine origin | Mechanoreceptor forming Merkel disc complex with associated Aβ afferent nerve ending; slow-adapting type I (SAI) mechanoreceptor encoding fine spatial detail and sustained pressure; express CK20 (marker); Merkel cell carcinoma — aggressive neuroendocrine skin cancer linked to Merkel cell polyomavirus |
| Mast cell | Dermis (perivascular); bone marrow-derived; tissue-resident | Immune sentinel; IgE cross-linking of FcεRI by antigen → degranulation → histamine, tryptase, prostaglandin D2 release → urticaria, anaphylaxis; also activated by complement (C3a, C5a), substance P, and TLR ligands; roles in wound healing, fibrosis, and tumor immunity |
| Dermal fibroblast | Dermis throughout | Produce collagen I, III, elastin, fibronectin; ECM remodeling via MMPs; activated to myofibroblasts in wound healing and fibrotic disease (scleroderma) |
| Dermal dendritic cell | Dermis; separate from Langerhans cells | CD11c⁺; antigen presentation; cross-presentation to CD8⁺ T cells; patrol dermis for pathogens |
Barrier Function
Permeability Barrier
The stratum corneum permeability barrier follows the "brick and mortar" model: corneocytes (bricks) are embedded in a matrix of extracellular lipids (mortar) comprising ceramides (~50% by mole fraction), cholesterol (~25%), and long-chain free fatty acids (~15%). These lipids are secreted as lamellar bodies from the stratum granulosum and organize into crystalline bilayer stacks in the intercellular space, creating an effective hydrophobic barrier against water loss and xenobiotic entry.
Key molecular components of barrier integrity:
- Filaggrin (FLG): Aggregates keratin filaments during cornification; proteolytic degradation products form natural moisturizing factor (NMF) — a hygroscopic mixture of amino acids, PCA, urocanic acid — that hydrates the corneum. Loss-of-function FLG mutations are the strongest genetic risk factor for atopic dermatitis and ichthyosis vulgaris.
- Tight junctions: Claudin-1, claudin-4, occludin, and ZO-1 in the stratum granulosum form a paracellular seal providing a secondary barrier "below" the corneum, preventing penetration of even small hydrophilic molecules.
- Loricrin and involucrin: Major cornified envelope proteins cross-linked by transglutaminase 1 (TGM1) to form the insoluble protein scaffold of the corneocyte.
- TEWL measurement: Transepidermal water loss <5 g/m²/h is normal; elevated TEWL (>10–15 g/m²/h) indicates barrier disruption (as seen in atopic dermatitis, burns, peeling skin syndrome).
Inside-out vs. outside-in sensitization: FLG barrier defects allow environmental allergens (house dust mite Der p1, peanut Ara h proteins) to penetrate and prime Th2 sensitization — the "outside-in hypothesis" for the atopic march (eczema → food allergy → asthma).
Antimicrobial Barrier
The skin surface is maintained at pH 4.5–5.5 (acid mantle) by secretions from sebaceous and eccrine glands, inhibiting many pathogens. Keratinocyte-derived antimicrobial peptides (AMPs) include:
- β-defensins: hBD-1 (constitutive), hBD-2 and hBD-3 (inducible by IL-1β, TNF-α, bacteria); broad-spectrum activity vs. gram-positive, gram-negative bacteria, fungi, and enveloped viruses.
- Cathelicidin LL-37 (encoded by CAMP): Stored in neutrophil granules and secreted by keratinocytes; direct membrane disruption of bacteria; also chemotactic, angiogenic, and mast cell-activating.
- Dermcidin: Secreted by eccrine sweat glands; forms membrane-spanning channels in bacterial membranes.
- S100 proteins (psoriasin / S100A7): Zinc-sequestering AMPs particularly active against E. coli.
Innate Immunity
Skin is not merely a passive barrier but an active immune organ that constitutively surveys for pathogens and danger signals. The skin immune system (SIS) comprises resident and circulating innate and adaptive immune cells in both the epidermis and dermis.
Pattern Recognition
Keratinocytes express TLR1, TLR2, TLR3, TLR4, TLR5, and TLR9, as well as cytosolic PRRs (RIG-I, MDA5, NLRP3). Upon PAMP recognition, keratinocytes rapidly produce IL-1α, IL-18, TNF-α, IL-8 (CXCL8), CCL2, and type I interferons to initiate the inflammatory cascade and recruit neutrophils, monocytes, and NK cells.
Skin Microbiome
The skin hosts ~10¹² microorganisms across ~1000 species. Staphylococcus epidermidis is the dominant commensal and actively competes with pathogen S. aureus through niche exclusion and production of bacteriocins (e.g., lantibiotics). S. epidermidis also stimulates keratinocyte AMP production and educates the local T cell compartment toward tolerance of commensals while maintaining reactivity to pathogens. In atopic dermatitis, microbiome dysbiosis — enrichment with S. aureus at the expense of diverse commensals — correlates with flare severity.
Langerhans Cell Emigration
Upon activation by contact sensitizers or pathogens, Langerhans cells downregulate E-cadherin (releasing them from keratinocyte adhesion) and upregulate CCR7, driving migration via dermal lymphatics to the paracortex of regional lymph nodes, where they present peptide-MHC II complexes to naïve CD4⁺ T cells. This is the cellular basis of contact hypersensitivity and allergen sensitization.
Pathology
| Disease | Mechanism / Pathway | Key Therapies |
|---|---|---|
| Atopic dermatitis (eczema) | FLG mutation → barrier disruption → allergen penetration → Th2/ILC2 activation → IL-4 and IL-13 upregulation → reduced barrier gene expression (feedback loop); IL-31 mediates itch; S. aureus colonization exacerbates inflammation via superantigen T cell activation | Topical corticosteroids; tacrolimus/pimecrolimus; dupilumab (anti-IL-4Rα, blocks IL-4 + IL-13); tralokinumab (anti-IL-13); JAK inhibitors (upadacitinib, abrocitinib) |
| Psoriasis | Th17/Th1 axis: IL-23 (from DCs/macrophages) → Th17 → IL-17A/F + IL-22 → keratinocyte hyperproliferation (transit time 4 days vs. 28 days normal) → plaques; genetic loci: HLA-C*06:02, IL-23R, IL-12B | Methotrexate, cyclosporine; biologics: anti-TNF (adalimumab), anti-IL-17A (secukinumab, ixekizumab), anti-IL-17RA (bimekizumab), anti-IL-23/p19 (guselkumab, risankizumab, tildrakizumab) |
| Melanoma | UV-induced BRAF V600E (~40–50%), NRAS Q61R/K, or NF1 mutations → constitutive MAPK signaling → unchecked melanocyte proliferation; also UV-induced p53 mutations; mismatch repair defects in desmoplastic melanoma | BRAF + MEK inhibitors (vemurafenib + cobimetinib; dabrafenib + trametinib); PD-1 checkpoint blockade (pembrolizumab, nivolumab); anti-CTLA-4 (ipilimumab); combination immunotherapy |
| Basal cell carcinoma (BCC) | Chronic UV exposure → PTCH1 loss-of-function or SMO gain-of-function → constitutive Hedgehog/GLI signaling → basal keratinocyte proliferation; most common cancer worldwide | Surgery/Mohs; vismodegib and sonidegib (Hedgehog pathway inhibitors) for advanced/metastatic BCC; PD-1 immunotherapy (cemiplimab) for Hedgehog-refractory |
| Squamous cell carcinoma (SCC) | UV (particularly UVB) → cyclobutane pyrimidine dimers → C→T transitions in TP53 → loss of p53 tumor suppressor → clonal expansion → actinic keratosis (precursor) → in-situ SCC (Bowen's disease) → invasive SCC | Surgery; radiation; cemiplimab (PD-1); for unresectable/metastatic; field therapy for actinic keratoses (5-FU, imiquimod, photodynamic therapy) |
| Wound healing — stages | 1. Hemostasis (0–hours): vasoconstriction + platelet plug + fibrin clot; platelet-derived PDGF and TGF-β initiate healing. 2. Inflammation (hours–days): neutrophil then macrophage influx; M1 macrophages debride; M2 macrophages release TGF-β, VEGF, IL-10. 3. Proliferation (days–weeks): fibroblast migration and collagen III deposition; angiogenesis (VEGF); re-epithelialization from wound margins (keratinocyte migration). 4. Remodeling (weeks–1 year): collagen III → collagen I; MMP/TIMP balance; myofibroblast contraction; scar maturation to ~80% original tensile strength | Moist wound healing; growth factor therapies (becaplermin/PDGF-BB for diabetic ulcers); skin substitutes; negative pressure wound therapy |
Connections
- part-ofIntegumentary system — principal organ of the integumentary system, together with hair follicles, nails, and cutaneous glands
- interfaces-withImmune system — Langerhans cells, dermal DCs, mast cells, T cells, NK cells form the skin immune system; primary site of contact hypersensitivity and allergen sensitization
- producesVitamin D — UVB-driven photosynthesis of pre-vitamin D3 in epidermis is the principal endogenous source; liver and kidney complete activation
- structural-basisCollagen — dermis is primarily collagen I and III; barrier tensile strength, wound healing scaffolding, and aging changes depend on collagen homeostasis
- containsMelanocytes — synthesize melanin for UV photoprotection; dysregulation leads to melanoma
- innervated-byNervous system — dense sensory innervation (free nerve endings, Meissner, Pacinian, Merkel, Ruffini receptors); sympathetic innervation of eccrine glands and cutaneous vasculature
References
- Candi E, Schmidt R, Melino G. The cornified envelope: a model of cell death in the skin. Nat Rev Mol Cell Biol. 2005;6:328–340.
- Williams MR, Gallo RL. Evidence that Human Skin Microbiome Dysbiosis Promotes Atopic Dermatitis. J Invest Dermatol. 2015;135:2960–2964.
- Proksch E, Brandner JM, Jensen JM. The skin: an indispensable barrier. Exp Dermatol. 2008;17(12):1063–1072.
- Gallo RL, Hooper LV. Epithelial antimicrobial defence of the skin and intestine. Nat Rev Immunol. 2012;12:503–516.
- Nestle FO, Di Meglio P, Qin JZ, Nickoloff BJ. Skin immune sentinels in health and disease. Nat Rev Immunol. 2009;9:679–691.