Integumentary System – Exam Review Notes

Integumentary System Overview

• Largest body system; ~16\% body weight, surface area 1.5{-}2\,m^2

• Components
  • Cutaneous membrane: epidermis (epithelium) + dermis (CT)
  • Accessory structures: hair, glands, nails (originate in dermis, project through epidermis)
  • Subcutaneous (hypodermis): loose CT/adipose, vascular, sensory

• Key functions: protection, excretion, thermoregulation, melanin/keratin production, \text{vitamin D}_3 synthesis, lipid storage, sensation, immune coordination

Epidermis

• Stratified squamous, avascular; nourished by dermal diffusion

• Principal cell: keratinocyte (keratin-rich)

• Skin types
  • Thin: 4 strata; most of body
  • Thick: 5 strata; palms/soles (adds stratum lucidum)

• Strata (deep → superficial)

  1. Stratum basale: stem (basal) cells, epidermal ridges; tactile discs, melanocytes

  2. Stratum spinosum: 8–10 keratinocyte layers, dendritic (Langerhans) cells

  3. Stratum granulosum: 3–5 layers, produce keratin & keratohyalin → cells die

  4. Stratum lucidum: clear, only in thick skin

  5. Stratum corneum: 15–30 keratinized layers; water-resistant; turnover 7–10 d, shed ~2 wk

• Water loss
  • Insensible: diffusion/evaporation (~500\,mL day)
  • Sensible: sweat glands

• Epidermal Growth Factor (EGF): accelerates basal mitosis, keratin, repair, secretion

Dermis

• Between epidermis & hypodermis; supports/accessory origins

• Papillary layer: areolar CT, capillaries, nerves; dermal papillae; site of dermatitis

• Reticular layer: dense irregular CT, collagen + elastic fibers; strength/elasticity (turgor)

• Tension (cleavage) lines: collagen fiber orientation; parallel cuts heal better

• Blood supply: cutaneous plexus (deep), subpapillary plexus (superficial); bruises = vessel damage

• Innervation: controls blood flow, glands; receptors—Meissner (light touch, papillae) & Pacinian (pressure, reticular)

Subcutaneous (Hypodermis)

• Adipose-rich loose CT; anchors skin, insulates, energy store

• Contains large vessels; common for hypodermic injections

• Fat distribution modulated by sex hormones

Skin Color

• Pigments
  • Melanin: red-yellow or brown-black; melanocytes → melanosomes; UV protection
  • Carotene: orange-yellow, dietary; vitamin A precursor

• Hemoglobin: oxygenated → red; deoxygenated → dark red (cyanosis)

• Clinical hues: jaundice (bilirubin), ↑MSH/ACTH (bronzing), vitiligo (melanocyte loss)

Vitamin D_3

• Epidermal cholecalciferol produced via UV; liver & kidney convert to calcitriol ⇒ Ca^{2+}/PO_4^{3-} absorption

• Deficiency → rickets (weak, bowed bones)

Hair

• Covers body except palms, soles, lips, parts of genitals

• Functions: insulation, UV/particle guard, sensation (root plexus)

• Follicle: extends to dermis; arrector pili (smooth muscle) causes goose bumps

• Hair structure: medulla (soft keratin) – cortex & cuticle (hard keratin)

• Follicle layers: internal root sheath, external root sheath, glassy membrane

• Growth cycle: active growth → club hair (rest) → shedding, new growth

• Types: vellus (fine) vs terminal (coarse, pigmented); color from melanocytes, genetic

Sebaceous & Sweat Glands

• Sebaceous: holocrine; sebum to follicles or skin (sebaceous follicles) → lubricate, antibacterial

• Sweat glands
  • Apocrine: axillae, nipples, pubis; merocrine secretion into follicles; odorous; myoepithelial
  • Eccrine (merocrine): widespread, esp. palms/soles; watery 99\% secretion → thermoregulation, excretion, protection

• Other glands: mammary (milk), ceruminous (earwax)

• Control: ANS (sebaceous/apocrine globally); eccrine locally; sensible perspiration key for thermoregulation

Nails

• Keratinized plates protecting distal digits; diagnostic indicators

• Structures: nail body over nail bed; lateral nail grooves/folds; hyponychium under free edge; root beneath proximal fold; eponychium (cuticle); lunula (pale crescent)

• Growth from nail root (matrix) near phalanx

Repair & Healing

• Injury → bleeding, inflammation (mast cells), scab formation

• Macrophages clean; fibroblasts/endothelial cells form granulation tissue

• Remodeling: clot dissolves, capillaries decline, collagen scar forms; overproduction → keloid

Aging Effects

• Thinner epidermis/dermis, ↓dendritic cells, ↓vitamin D_3, ↓melanocytes & gland activity, reduced dermal blood flow & elasticity, hair-follicle function declines, altered fat/hair distribution, slower repair