Chapter 6 – Integumentary System (Full Lecture Outline)

Integumentary System – Big Picture

  • Integrative organ system that covers the entire body
    • Components: skin (cutaneous membrane) + accessory structures (nails, hair, sweat glands, sebaceous glands)
    • Serves as physical/physiological barrier, sensory canvas, and visual indicator of internal health
    • Field of medical study: Dermatology

6.1 Composition & Primary Layers

  • Epidermis
    • Keratinized, stratified squamous epithelium (avascular)
  • Dermis
    • Dense irregular connective tissue; houses vasculature & nerves
  • Subcutaneous Layer (Hypodermis) – technically NOT part of integument, but functionally tied
    • Areolar + adipose CT; anchors skin, insulates, stores energy, ideal injection site

Epidermal Micro-Anatomy (From deep → superficial)

  • Stratum basale
    • Single layer; cuboidal/low-columnar
    • Cell types & significance:
    • Keratinocyte stem cells → continuous renewal; synthesize keratin (adds toughness)
    • Melanocytes → produce melanin (eumelanin & pheomelanin) in melanosomes; UV shield for nuclear DNA
    • Tactile (Merkel) cells → mechanoreceptors; chemical release → activate sensory neurons
  • Stratum spinosum
    • "Spiny" appearance (desmosomes)
    • Non-dividing keratinocytes + epidermal dendritic (Langerhans) cells → initiate immune response
  • Stratum granulosum
    • 3–53\text{–}5 layers; onset of keratinization
    • Cells fill w/ keratin → organelles/nucleus disintegrate → cell death, but structure intact
  • Stratum lucidum ("clear layer")
    • 2–32\text{–}3 layers; only in thick skin (palms/soles)
    • Cells packed w/ eleidin (intermediate keratin product → translucency)
  • Stratum corneum
    • 20–3020\text{–}30 layers of dead, anucleate, fully keratinized cells; dry surface resists abrasion & infection
Variations in Epidermis
  • Thick Skin
    • All 5 strata, sweat glands, NO hair follicles/sebaceous glands
    • Locations: palms & soles
  • Thin Skin
    • Lacks stratum lucidum; contains hair, sebaceous & sweat glands; covers most of body
  • Color Determinants
    • Hemoglobin: oxygenated blood → pinkish; deoxygenated → bluish (cyanosis)
    • Melanin: hereditary baseline + UV exposure modulation; albinism = enzyme defect → no melanin
    • Carotene: dietary yellow-orange, stored in subcutaneous tissue/stratum corneum → can convert to Vitamin A
Skin Markings
  • Nevus (mole) – circumscribed melanocyte overgrowth (monitor for change)
  • Freckles – localized ↑melanocyte activity, not ↑cell number
  • Hemangiomas – benign vascular tumors; may fade or persist ("port-wine stain")
  • Friction ridges – epidermal-dermal folds forming fingerprints; ↑grip & tactile sensitivity
UV Radiation & Cosmetic Products
  • Sun emits UVA/UVB/UVC (UVC blocked by ozone)
  • Sunscreens – broad-spectrum formulas reflect/absorb UVA & UVB; effectiveness rated by SPF\text{SPF}
  • Sunless tanners – stain stratum corneum; NO UV protection

Dermis Details

  • Overall
    • Highly vascular, innervated, stretchy yet strong due to collagen
    • Contains: glands, hair follicles, nail roots, arrector pili, sensory receptors, immune cells
Papillary Layer
  • Superficial areolar CT; dermal papillae interlock w/ epidermal ridges → ↑surface area, nutrient diffusion, grip
Reticular Layer
  • Deeper ∼\sim80 % of dermis; dense irregular CT, bundles of collagen + elastic fibers
Lines of Cleavage (Tension Lines)
  • Orientation of collagen bundles → surgical relevance: incisions parallel to lines = minimal gaping, rapid healing; perpendicular = risk of scar/stretching
  • Stretch marks (striae) = torn collagen when skin overstretched (pregnancy, obesity)
Tattoos
  • Ink injected into dermis (cells don’t shed like epidermis) → permanence; laser removal shatters pigment; newer bio-inks designed to fade upon treatment

Subcutaneous (Hypodermis)

  • Areolar + adipose; pads/protects, stores triglycerides, insulates, extensive vasculature → rapid drug uptake; thickness patterned by sex hormones (females > males in breasts, hips; opposite in nuchal region)

Physiological Functions of Integument

  • Protection – mechanical, chemical, microbial, UV, temp extremes
  • Water homeostasis – water-resistant keratin & lipids; still allows perspiration & transpiration (slow evaporation)
  • Vitamin D synthesis – UV converts 7-dehydrocholesterol → vitamin D_3 → liver/kidney → calcitriol; raises intestinal Ca2+^{2+}/PO43−_4^{3-} absorption
  • Secretion & Absorption – sweat excretes salts, urea; skin selectively absorbs (lipid-soluble drugs) → basis of transdermal patches
  • Immune surveillance – dendritic cells intercept pathogens, present antigens
  • Thermoregulation – vasoconstriction (heat retention) vs vasodilation + merocrine sweating (heat loss)
  • Sensory reception – millions of cutaneous receptors transduce touch, pressure, vibration, pain, temp → CNS

Accessory Structures

Nails
  • Hard derivatives of stratum corneum → protect distal digits, aid grip
  • Anatomy: free edge, nail body, nail root (collectively = nail plate); rests on nail bed (living epidermis)
  • Growth zone: nail matrix; pale lunula overlies thick matrix; eponychium (cuticle) seals proximal fold; hyponychium secures free edge
  • Disorders: brittle nails, ingrown nails, fungal onychomycosis, yellow-nail syndrome, spoon nails, Beau’s lines (systemic stress), benign vertical ridging
Hair
  • Almost ubiquitous; absent palms/soles/lips/parts of genitals
  • Types:
    • Lanugo – fetal, fine, unpigmented
    • Vellus – primary body hair (children & females)
    • Terminal – coarse, pigmented (scalp, brows, lashes; androgen-dependent regions)
  • Structural Zones: bulb (living cells + papilla), root, shaft (external)
  • Internal layers: medulla (soft keratin), cortex (hard), cuticle (single squamous layer)
  • Follicle: epithelial root sheath + connective tissue sheath; associated arrector pili (smooth muscle → goosebumps)
  • Functions: UV & trauma protection (scalp), particle filtration (nostrils, ear canal, lashes), heat retention, sensory detection (root plexus), visual & social signaling
  • Color: melanin variants; gray/white = ↓melanin production/air bubbles
  • Growth cycle: anagen (active, years), catagen (regression, weeks), telogen (resting/shedding, months) → normal loss ≈10–100\approx 10\text{–}100 hairs/day
  • Pathologies: alopecia areata (autoimmune), diffuse loss (stress, hormones), male-pattern baldness (genetic androgen sensitivity), hirsutism (excessive androgen-linked growth)
Cutaneous Exocrine Glands
  • Merocrine (Eccrine) Sweat Glands
    • Most numerous; simple coiled; ducts open at sweat pores
    • Secretion: sweat (∼99% water+1% solutes)(\sim 99\% \text{ water} + 1\% \text{ solutes}) → evaporative cooling, excretion, antibacterial film (acidic)
  • Apocrine Sweat Glands
    • Localized to axillae, areolae, pubic & anal regions; ducts empty into hair follicles
    • Viscous, protein-/lipid-rich sweat → bacterial digestion → odor; active at puberty (hormonal control)
  • Sebaceous (Oil) Glands
    • Holocrine; secrete sebum into hair follicles → lubricates skin/hair, bactericidal; androgen-stimulated (acne onset at puberty)
  • Ceruminous Glands
    • Modified apocrine in external acoustic meatus; produce cerumen (earwax) → traps debris, lubricates tympanic membrane
  • Mammary Glands
    • Modified apocrine in breasts; activated in pregnancy/lactation → synthesize breast milk (nutrition + immunity)
Clinical: Acne
  • Plugged sebaceous ducts; hormones ↑sebum, keratin plugs; bacterial metabolism → inflammation; therapies: benzoyl peroxide, salicylic acid, antibiotics, retinoids; untreated → scarring

Repair & Regeneration

  • Regeneration – replacement with same cell type → full function
  • Fibrosis – replacement with scar tissue (dense collagen) → partial functional loss
  • Wound-healing stages
    1. Hemostasis: vessels severed → bleed; platelets form clot (temporary barrier)
    2. Inflammation: leukocytes clean debris/pathogens
    3. Proliferation: angiogenesis + granulation tissue (vascular CT) fills gap; fibroblasts synthesize collagen
    4. Maturation: epithelium regenerates; underlying CT scar remodels
Psoriasis (Autoimmune Example)
  • T-cells attack keratinocytes → accelerated turnover → thick, scaly plaques; itching/pain; treatments: corticosteroids, phototherapy, immunomodulators
Burns
  • Primary mortality factors: fluid loss, infection, thermoregulation failure
  • 1st° – epidermis only; redness, pain; cool water therapy
  • 2nd° – epidermis + part dermis; blistering; minimal scarring possible
  • 3rd° – full-thickness (epidermis, dermis, possibly hypodermis); hospitalization, debridement/skin grafts, high caloric demand; severity assessed via Rule of Nines (% body surface area)

Developmental Biology

  • Week 7: ectoderm → periderm + basal layer (future epidermis)
  • Week 11: mesoderm → mesenchyme → dermis
  • Week 21: stratum corneum & friction ridges evident; vernix caseosa (periderm + sebum) coats fetus
  • Week 32: nails complete; Weeks 9–12: hair buds; Week 20: sweat & sebaceous primordia form (palms/soles first)

Aging & Photo-Damage

  • ↓ stem cell activity → slow repair, thin epidermis; ↓ collagen/elastin → wrinkles; ↓ dendritic cells → infection risk
  • Hair follicles shrink → thinning;
  • Extrinsic aging factors: UV radiation & smoking → DNA damage, ROS production, collagen breakdown → accelerated wrinkles + oncogenesis (skin cancers prevalent on head/neck; fair skin higher risk)
Cosmetic Intervention: Botox
  • Botulinum toxin A blocks ACh release → temporary paralysis of facial muscles; smooths expression lines; effect lasts months, requires repeat injections

Study Prompts / Connections

  • Correlate keratinization with barrier function & desquamation rate (≈ 4 weeks cycle)
  • Link vitamin D pathway to skeletal health (calcitriol → Ca2+^{2+} homeostasis)
  • Recall lines of cleavage in surgical planning & C-sections
  • Compare holocrine vs merocrine vs apocrine secretion modes (cellular sacrifice vs exocytosis)
  • Discuss thermoregulation interplay: vasomotor tone + sweat evaporation (physics of latent heat)