Skin Anatomy and Physiology: Layers, Functions, and Clinical Relevance

Three-Layer Architecture

1. Epidermis (Avascular, Stratified Squamous Epithelium)

• Stratum basale: Stem cells, melanocytes (melanin), Merkel cells (touch).
• Stratum spinosum: Keratinocytes + desmosomes; Langerhans cells (immune surveillance).
• Stratum granulosum: Keratohyalin granules, lamellar bodies (lipids for barrier).
• Stratum lucidum: Clear layer – thick skin only (palms/soles).
• Stratum corneum: 15–30 layers of anucleate corneocytes + lipid matrix (ceramides, cholesterol, FAs) – "bricks and mortar" barrier.

Turnover: ~28 days (slows with age).

2. Dermis (Connective Tissue)

• Papillary dermis: Loose CT, dermal papillae (fingerprints), capillaries, Meissner's corpuscles (light touch).
• Reticular dermis: Dense irregular CT – collagen I/III (tensile strength), elastin (recoil), fibroblasts, hair follicles, glands, Ruffini/Pacinian corpuscles.

3. Hypodermis (Subcutaneous Tissue)

• Adipose lobules (insulation, energy, contour), fibrous septa, vessels, nerves.

Physiological Functions

Function	Mechanism
Barrier	Stratum corneum lipids + tight junctions + antimicrobial peptides + acidic pH (acid mantle)
Sensation	Mechanoreceptors (Merkel, Meissner, Ruffini, Pacinian), thermoreceptors, nociceptors (free nerve endings)
Thermoregulation	Vasodilation/constriction, sweating (eccrine), piloerection, countercurrent exchange
Excretion	Sweat (water, electrolytes, urea, trace metals)
Absorption	Topical drugs (lipophilic, <500 Da), toxins, some nutrients
Vitamin D synthesis	UVB → 7-dehydrocholesterol → pre-D3 → D3 (liver/kidney activation)
Sebum production	Sebaceous glands (holocrine) → lipids, squalene, wax esters – lubrication, antimicrobial, pheromones
Immunological	Langerhans cells, dermal DCs, T-cells, mast cells, antimicrobial peptides (cathelicidin, defensins)
Microbiome	Commensals (Staph epidermidis, Cutibacterium, Corynebacterium) – colonization resistance, immune education

Clinical Correlates

• Barrier defect → atopic dermatitis, ichthyosis.
• Melanocyte dysfunction → vitiligo, melasma, melanoma.
• Collagen/elastin loss → aging, solar elastosis, Ehlers-Danlos.
• Sebaceous dysregulation → acne, seborrhea.
• Microbiome dysbiosis → AD, acne, rosacea, malodor.

Understanding skin structure-function guides rational skincare, disease management, and cosmetic intervention.