Integumentary System

Skin Layers - Our Body's Overcoat

• Epidermis: Avascular, stratified squamous epithelium.
  • Layers (deep→superficial): Basale, Spinosum, Granulosum, Lucidum (thick skin), Corneum. 📌 Mnemonic: "British Spies Go Like Clockwork".
  • Cells: Keratinocytes (majority), Melanocytes (pigment), Langerhans (immune), Merkel (sensory).
• Dermis: Vascular connective tissue; "true skin".
  • Papillary Layer: Superficial, loose CT, dermal papillae.
  • Reticular Layer: Deep, dense irregular CT; collagen, elastin.
  • Structures: Vessels, nerves, hair follicles, glands.
• Hypodermis (Subcutis): Adipose & loose CT. Insulation, shock absorption, anchors skin.

⭐ The dermo-epidermal junction is secured by hemidesmosomes in stratum basale cells anchoring it to the basement membrane (basal lamina).

Epidermal Crew - Cellular Cast

• Keratinocytes: (~90%); produce keratin (structural protein); arranged in layers.
• Melanocytes: Stratum basale; neural crest origin; melanin synthesis (UV shield). Ratio to basal keratinocytes ~1:10.
• Langerhans Cells: Stratum spinosum; bone marrow-derived (dendritic); antigen presentation; Birbeck granules (tennis racket).
• Merkel Cells: Stratum basale; mechanoreceptors (light touch); neuroendocrine function.

⭐ Langerhans cells, found mainly in stratum spinosum, possess Birbeck granules (tennis-racket appearance) and are key for immune surveillance in the epidermis (antigen presentation).

Skin's Gadgets - Hairy, Naily, Glandy

• Hair:
  • Components: Follicle, bulb (matrix cells), dermal papilla, arrector pili (smooth muscle).
  • Growth Cycle: Anagen (growth, 85-90%, 2-6 yrs), Catagen (involution, ~2 wks), Telogen (rest, ~3 mths). 📌 ACT.
  • Types: Lanugo (fetal), vellus (fine), terminal (coarse).
• Nails:
  • Structure: Nail plate (hard keratin), nail bed, matrix (growth), eponychium (cuticle), hyponychium.
  • Growth: Fingernails ~0.1 mm/day; toenails slower.
• Glands:
  • Sebaceous: Holocrine (sebum); pilosebaceous unit; absent palms/soles.
  • Sweat (Sudoriferous):
    • Eccrine: Merocrine; thermoregulation; widespread (palms, soles, forehead).
    • Apocrine: Merocrine (misnomer); axillae, anogenital; puberty; odor (bacterial action).
  • Specialized: Ceruminous (ear), Mammary (modified apocrine).

⭐ Sebaceous glands are typically absent on palms and soles, which is why these areas are less prone to acne.

Skin's Superpowers - Jack of All Trades

• Protection: Keratin (mechanical barrier); acid mantle (pH 4.5-5.5), defensins (chemical); Langerhans cells (immune surveillance).
• Thermoregulation: Sweating (evaporative cooling); cutaneous blood flow regulation (vasodilation/constriction).
• Sensation: Detects touch (Meissner's), pressure (Pacinian), pain (nociceptors), temperature.
• Metabolic: Vitamin D3 synthesis (UVB on 7-dehydrocholesterol); energy storage (subcutaneous fat).
• Excretion: Minor role in eliminating urea, salts.
• Wound Repair: Facilitates healing processes.

⭐ Skin is the body's largest organ, contributing approximately 15-16% to total body weight.

Clinical Hits - When Skin Cries Out

• Psoriasis: Auspitz sign (pinpoint bleeding on scale removal), Koebner phenomenon (lesions at trauma sites).
• Pemphigus Vulgaris: Nikolsky's sign positive (gentle rubbing separates epidermis). Flaccid bullae.
• SLE: Malar (butterfly) rash, photosensitivity.
• Dermatomyositis: Heliotrope rash (eyelids), Gottron's papules (knuckles).
• SJS/TEN: Drug-induced epidermal necrosis. SJS <10% BSA, TEN >30% BSA.

⭐ Leser-Trélat sign (sudden appearance of multiple seborrheic keratoses) can indicate internal malignancy, often GI adenocarcinoma.

High‑Yield Points - ⚡ Biggest Takeaways

• Epidermal layers: Basale, Spinosum, Granulosum, Lucidum (thick skin), Corneum. Keratinocytes predominate.
• Melanocytes (neural crest) make melanin; Langerhans cells (bone marrow) are APCs.
• Dermis: Papillary layer (Meissner's corpuscles) and Reticular layer (Pacinian corpuscles, Lines of Langer).
• Skin appendages: Sebaceous (holocrine), Eccrine (thermoregulation), Apocrine glands.
• Vitamin D synthesis starts in skin (UV-B on 7-dehydrocholesterol).
• Burns: Rule of Nines for BSA; depth (1st-4th degree) indicates severity.
• Key sensory receptors: Meissner's (touch), Pacinian (vibration), Merkel cells (texture).