Structure and Function of Skin

Epidermis - Top Layer Tales

• Outermost, avascular protective layer. Turnover: approx. 28-30 days.
• Layers (superficial to deep): 📌 Come Let's Get Sun Burnt
  • Stratum Corneum: Anucleated, keratin-filled dead cells (squames); barrier.
  • Stratum Lucidum: Clear, homogenous layer; only in thick skin (palms, soles).
  • Stratum Granulosum: 2-3 layers; keratohyalin granules, lamellar granules (waterproofing).
  • Stratum Spinosum: Prickle cells (desmosomes); Langerhans cells present.
  • Stratum Basale (Germinativum): Single layer of cuboidal/columnar cells; mitotic keratinocytes, melanocytes, Merkel cells.
• Principal Cells:
  • Keratinocytes (~85%): Produce keratin; form the layers.
  • Melanocytes (Basale): Pigment (melanin); UV protection.
  • Langerhans cells (Spinosum): Immune defense; antigen presentation.
  • Merkel cells (Basale): Touch sensation.

⭐ Langerhans cells are bone marrow-derived, antigen-presenting cells found predominantly in the stratum spinosum. oka

Dermis - Deep Support System

• Connective tissue layer deep to epidermis; provides structural support, elasticity, tensile strength.
• Main Components:
  • Cells: Fibroblasts (produce matrix), mast cells, macrophages.
  • Fibers: Collagen (Type I > Type III for strength), elastin (elasticity).
  • Ground Substance: Proteoglycans, hyaluronic acid (hydration, volume).
• Two Layers:
  • Papillary Dermis: Superficial, loose CT; dermal papillae (finger-like projections into epidermis); Meissner's corpuscles (light touch).
  • Reticular Dermis: Deeper, dense irregular CT; thicker collagen bundles; Pacinian corpuscles (pressure/vibration), Ruffini corpuscles (stretch).
• Houses: Blood vessels, nerves, lymphatics, skin appendages (hair follicles, sweat & sebaceous glands).

⭐ The dermis is primarily composed of Type I collagen, providing tensile strength, with Type III collagen also present, especially in the papillary dermis.

Skin Appendages - Hairy & Sweaty Business

• Hair:
  • Types: Lanugo, vellus, terminal.
  • Cycle: Anagen (growth, ~3 years), Catagen (involution, ~3 weeks), Telogen (rest, ~3 months). 📌 ACT.
  • Arrector pili: Smooth muscle; goosebumps.
• Sebaceous Glands:
  • Holocrine; sebum.
  • With hair follicle (pilosebaceous unit).
  • Absent: palms, soles.
• Sweat Glands:
  • Eccrine: Thermoregulation; opens to skin; cholinergic.
  • Apocrine: Axillae, anogenital; opens to follicle; adrenergic; odor (bacterial).
• Nails:
  • Hard keratin.
  • Matrix: Growth.
  • Fingernail growth: ~3 mm/month.

⭐ Eccrine sweat glands are crucial for thermoregulation and open directly onto the skin surface, whereas apocrine glands, associated with hair follicles, are found in axillary and anogenital regions.

Subcutis & Functions - Cushion & Capes

• Subcutis (Hypodermis): The "Cushion"
  • Deepest skin layer: Loose connective tissue, adipose cells (fat).
  • Features: Panniculus adiposus, large blood vessels, nerves.
  • Functions: Energy storage, thermal insulation, shock absorption, skin mobility, endocrine (leptin).
• Overall Skin Functions (📌 CAPES & more):
  • Cushioning & Protection:
    • Mechanical barrier (subcutis, dermis).
    • UV (melanin), chemical, microbial defense.

    ⭐ The subcutis (hypodermis) acts as a crucial thermal insulator and shock absorber due to its rich adipose tissue content.

  • Absorption: Limited (e.g., topical drugs).
  • Sensation: Touch, pressure, pain, temperature.
  • Excretion: Sweat (urea, salts, H₂O).
  • Synthesis: Vitamin D ($7-dehydrocholesterol + UVB \rightarrow Previtamin D3 \rightarrow Vitamin D3 (Cholecalciferol)$).
  • Temperature regulation: Vasculature, sweat, fat.
  • Immune defense: Langerhans cells, lymphocytes.

High‑Yield Points - ⚡ Biggest Takeaways

• Epidermis: Stratified squamous epithelium; main cells: Keratinocytes, Melanocytes, Langerhans cells, Merkel cells.
• Dermis: Connective tissue with collagen (strength), elastin (elasticity); contains vessels, nerves, appendages.
• Hypodermis: Adipose tissue for insulation, energy storage, and shock absorption.
• Epidermal Layers (CLGSB): Stratum Corneum, Lucidum (thick skin only), Granulosum, Spinosum, Basale.
• Key Functions: Protection (barrier), thermoregulation, sensation, Vitamin D synthesis, immune response.
• Skin Appendages: Hair follicles, sebaceous glands (sebum), sweat glands (eccrine/apocrine), and nails.