Skin Layers: Epidermis, Dermis & Hypodermis — Structure & Function

The skin (integumentary system) is the body's largest organ, organized into three primary layers with distinct structure and function.

1. Epidermis (Outermost; Avascular)

• Thickness: 0.05-1.5 mm (thinnest: eyelids; thickest: palms/soles).
• Cell types:
  • Keratinocytes (90-95%): produce keratin, form barrier; differentiate as they move outward.
  • Melanocytes (5-10%): produce melanin; transfer to keratinocytes via melanosomes.
  • Langerhans cells: antigen-presenting immune cells (dendritic).
  • Merkel cells: mechanosensation (light touch).
• Strata (deep to superficial):
  1. Stratum basale (germinativum): stem cells, mitosis, melanocytes, Merkel cells.
  2. Stratum spinosum: keratinocyte adhesion (desmosomes); Langerhans cells.
  3. Stratum granulosum: keratohyalin granules, lamellar bodies (lipid secretion).
  4. Stratum lucidum: clear, dead keratinocytes; only thick skin (palms/soles).
  5. Stratum corneum: 15-30 layers of corneocytes (dead, flattened, keratin-filled) in lipid matrix — "brick and mortar" barrier.
• Functions: Barrier (water retention, pathogen/chemical exclusion), UV protection (melanin), immune surveillance, sensory (Merkel), vitamin D synthesis (UVB → 7-dehydrocholesterol).

2. Dermis (Middle; Vascularized, Innervated)

• Composition: Dense irregular connective tissue (collagen I & III, elastin, proteoglycans, fibroblasts).
• Layers:
  • Papillary dermis (superficial 20%): loose CT, dermal papillae (fingerprints), capillary loops, Meissner's corpuscles (light touch).
  • Reticular dermis (deep 80%): dense CT, thick collagen bundles, sweat glands, hair follicles, sebaceous glands, Ruffini endings (stretch), Pacinian corpuscles (vibration/deep pressure).
• Appendages:
  • Hair follicles + arrector pili + sebaceous glands (pilosebaceous unit).
  • Eccrine sweat glands (thermoregulation; everywhere).
  • Apocrine sweat glands (axilla, groin; odor).
• Functions: Structural support, elasticity, thermoregulation (vasodilation/constriction, sweating), sensation, immune cell trafficking, wound healing scaffold.

3. Hypodermis / Subcutaneous Tissue (Innermost)

• Composition: Loose areolar CT + adipose tissue (adipocytes in lobules separated by fibrous septa).
• Thickness: Highly variable (2 mm to >10 cm); sex-specific distribution (android vs. gynoid).
• Functions: Energy storage, thermal insulation, mechanical cushioning, body contour, vascular/neural conduit, hormone secretion (leptin, adiponectin, resistin).

Structures NOT Part of Skin Layers

• Hair (epidermal derivative, resides in follicle)
• Nails (epidermal derivative, nail unit)
• Sweat glands (dermal appendages)
• Sebaceous glands (dermal appendages)
• Muscles (skeletal: deep to hypodermis; smooth: arrector pili, vascular)
• Bones (deep to muscle)

Clinical Relevance

• Burn depth: 1st (epidermis), 2nd (dermis), 3rd (full thickness), 4th (deep to skin).
• Skin cancer origin: BCC/SCC (keratinocytes), melanoma (melanocytes), Merkel cell.
• Pressure injury staging: depth of tissue loss.
• Drug delivery: transdermal (stratum corneum barrier), intradermal, subcutaneous.