Structure and Function of Skin

Epidermis - Outer Bodyguard

• Outermost, avascular, stratified squamous epithelium; primary barrier (pathogens, UV, water).
• Layers (Deep to Superficial): 📌 Mnemonic: "Britney Spears Glows Like Candy"
  • S. Basale (Germinativum): Single cuboidal layer, stem cells (mitotic); melanocytes, Merkel cells.
  • S. Spinosum: Polyhedral "prickle" cells (desmosomes); Langerhans cells.
  • S. Granulosum: Flattened cells; keratohyalin granules; lamellar bodies (lipid barrier).
  • S. Lucidum: Clear, thin; only thick skin (palms, soles); eleidin.
  • S. Corneum: Anucleated corneocytes (keratin-filled); desquamation.
• Key Cells:
  • Keratinocytes (~85%): Main cells; produce keratin (strength, barrier).
  • Melanocytes (Neural crest): Basal; melanin (UV block); 1 per 10 basal keratinocytes.
  • Langerhans Cells (Bone marrow): Spinosum; APCs (immune defense).
  • Merkel Cells (Neural crest): Basal; mechanoreceptors (light touch).
• Epidermal turnover: Approx. 4 weeks (basale to corneum).

⭐ Birbeck granules (tennis-racket shaped) are characteristic of Langerhans cells.

Dermis & Dermo-Epidermal Junction - Supportive Matrix

• Dermis: Connective tissue; support, elasticity, strength.
  • Papillary Dermis: Superficial. Loose CT. Type III & I collagen. Fine elastic fibers. Dermal papillae. Capillaries, Meissner's (touch).
  • Reticular Dermis: Deeper. Dense irregular CT. Predominantly Type I collagen. Coarse elastic fibers. Vessels, appendages, Pacinian (pressure), Ruffini (stretch).
  • Cells: Fibroblasts (synthesize matrix), mast cells, macrophages.
  • Ground Substance: GAGs (e.g., hyaluronic acid), proteoglycans.
• Dermo-Epidermal Junction (DEJ): Basement Membrane Zone (BMZ).
  • Layers (Epidermis to Dermis):
    • Hemidesmosomes (basal cells).
    • Lamina Lucida (laminin, BP antigens).
    • Lamina Densa (Type IV collagen).
    • Sublamina Densa (Anchoring Fibrils - Type VII collagen).
  • 📌 DEJ Layers: Heavy Lions Drink Soda (Hemidesmosomes, Lucida, Densa, Sublamina densa).
  • Functions: Adhesion, barrier, mechanical support.

  ⭐ Defects in Type VII collagen (anchoring fibrils) cause Dystrophic Epidermolysis Bullosa.

Skin Appendages & Subcutis - Skin's Gadgets

• Eccrine Glands: Thermoregulation (sweat); entire body, esp. palms, soles, axillae; cholinergic sympathetic innervation.
• Apocrine Glands: Scent (pheromones); axillae, anogenital, areolae; adrenergic sympathetic innervation; decapitation secretion. Active from puberty.
• Sebaceous Glands: Secrete sebum (lubrication); holocrine secretion; face, scalp (most); androgen-sensitive. Part of pilosebaceous unit.
• Hair Follicles:
  • Structure: Infundibulum, isthmus, inferior segment (bulb, papilla).
  • Cycle: Anagen (growth, ~85-90%), Catagen (involution, ~1-2%), Telogen (rest, ~10-15%). 📌 ACT.
  • Arrector pili muscle: Smooth muscle, causes piloerection ("goosebumps").
• Nails: Nail plate, bed, matrix (growth). Fingernails grow ~3 mm/month.
• Subcutis (Hypodermis): Adipose tissue lobules, connective tissue septa. Functions: Insulation, energy storage, shock absorption.

⭐ Apocrine glands utilize decapitation secretion (apical cytoplasm loss), sebaceous glands use holocrine secretion (entire cell lysis), and eccrine glands employ merocrine secretion (exocytosis).

Overall Skin Functions - Body's Versatile Shield

• Protection (Multi-layered Defense):
  • Physical: Against trauma, friction (keratin).
  • Chemical: Resists mild acids, alkalis.
  • Microbial: Acid mantle, defensins, Langerhans cells.
  • UV Radiation: Melanin absorption.
  • Dehydration: Impermeable stratum corneum, lipids.
• Sensation (Sensory Input): Detects touch, pressure, pain, temperature via specialized receptors.
• Thermoregulation (Temperature Balance):
  • Sweating (evaporative cooling).
  • Dermal vasodilation/vasoconstriction.
  • Subcutaneous fat insulation.
• Metabolic Activity:
  • Vitamin D synthesis (UVB on 7-dehydrocholesterol).
  • Energy storage (subcutaneous fat).
• Immune Response: Antigen presentation (Langerhans cells), cytokine production (keratinocytes).
• Excretion (Limited): Small amounts of urea, salts, water via sweat.

⭐ Langerhans cells in the epidermis are crucial antigen-presenting cells, initiating skin's immune defense.

High‑Yield Points - ⚡ Biggest Takeaways

• Epidermis: Stratum corneum (keratin barrier), spinosum (Langerhans cells, desmosomes), basale (melanocytes, stem cells).
• Dermo-Epidermal Junction: Hemidesmosomes and Type VII collagen anchor epidermis; defects cause blistering.
• Dermis: Rich in Type I collagen; supports appendages, blood vessels, and nerves.
• Skin Appendages: Eccrine (sweat/thermoregulation), apocrine (odor), sebaceous glands (sebum), hair follicles.
• Specialized Cells: Melanocytes (UV defense), Langerhans cells (immune response), Merkel cells (touch sensation).
• Core Functions: Protection (barrier), thermoregulation, Vitamin D synthesis, sensation, immune defense.