M

Integumentary System – Comprehensive Study Notes

Introduction

  • Largest organ: skin (integument) forms integumentary system with hair, nails, glands, sensory receptors.
  • Key functions
    • Helps maintain body temperature (thermoregulation)
    • Converts inactive vitamin D to active form (calcitriol)
    • Provides extensive sensory information
    • Contributes to homeostasis of every body system

System-Wide Homeostatic Contributions

  • Skeletal: skin activates vitamin D → absorption of \text{Ca}^{2+}, \text{PO}_4^{3-} for bone maintenance.
  • Muscular: supplies circulating \text{Ca}^{2+} needed for contraction.
  • Lymphatic/Immune
    • First-line mechanical & chemical barrier
    • Intraepidermal macrophages (Langerhans) & dermal macrophages initiate immune responses.
  • Respiratory: nasal hair filters dust; pain stimuli can alter breathing rate.
  • Nervous: cutaneous receptors relay touch, pressure, thermal, pain to CNS.
  • Endocrine: keratinocytes transform vitamin D → calcitriol (hormone).
  • Cardiovascular: dermal vasoconstriction/dilation modulates cutaneous blood flow.
  • Digestive & Urinary: shared vitamin D activation; sweat aids minor excretion.
  • Reproductive
    • Genital & areolar receptors contribute to sexual arousal; suckling triggers neuroendocrine milk-ejection; mammary glands are modified sweat glands; abdominal skin stretches during pregnancy.

Gross Structure of the Skin

  • Two principal layers
    • Epidermis (superficial, epithelial)
    • Dermis (deep, connective tissue)
  • Hypodermis (subcutaneous layer)
    • Areolar + adipose connective tissue
    • Anchors skin to fascia & organs; not part of skin proper.

Detailed Components (Cross-Section)

  • Surface specializations: epidermal ridges, sweat pores.
  • Epidermis contains strata; dermis subdivided into papillary & reticular regions.
  • Accessory structures penetrate dermis/hypodermis: hair follicles, arrector pili muscles, sebaceous & sweat glands, sensory corpuscles (Meissner, Pacinian), free nerve endings, vascular plexuses.

Epidermal Cell Types

  1. Keratinocytes (~90\%)
    • Produce keratin + lamellar granules (lipid sealant).
  2. Melanocytes (~8\%) in stratum basale
    • Synthesize melanin → UV shielding of nuclei.
  3. Intraepidermal macrophages (Langerhans)
    • Immune surveillance; easily damaged by UV.
  4. Tactile epithelial (Merkel) cells
    • Contact disks of sensory neurons → light touch.

Thin vs Thick Skin

  • Thin (hairy)
    • Covers most body, epidermal thickness 0.10–0.15\,\text{mm}, lacks stratum lucidum; contains hair, sebaceous glands, sparse eccrine units.
  • Thick (hairless)
    • Palms, palmar digits, soles; epidermis 0.6–4.5\,\text{mm} (thick stratum corneum & lucidum); epidermal ridges prominent; no hair or sebaceous glands; numerous eccrine glands & sensory receptors.

Epidermal Strata (Deep → Superficial)

  • Stratum basale
    • Single row cuboidal/columnar keratinocytes with tonofilaments; continual mitosis; houses melanocytes & Merkel cells.
  • Stratum spinosum
    • 8–10 layers of spiny keratinocytes; Langerhans & melanocyte processes present.
  • Stratum granulosum
    • 3–5 layers flattened cells; keratohyalin converts tonofilaments → keratin; lamellar granules exocytose lipids.
  • Stratum lucidum (thick skin only)
    • 4–6 rows of clear, dead, anucleate keratinocytes packed with keratin.
  • Stratum corneum
    • 15–50+ layers of dead, flat, keratin-filled cells; desquamated & replaced via keratinization cycle (~4–6 weeks).

Keratinization & Epidermal Growth

  • Basale mitosis → upward migration; differentiation accompanied by keratin accumulation & organelle loss.
  • Surface cells continuously shed; cycle speed influenced by injury (epidermal growth factor accelerates).

Dermis

  • Connective tissue with collagen + elastic fibers; thickness varies.
  • Papillary region (≈1/5)
    • Areolar CT; dermal papillae form epidermal ridges (fingerprints); capillaries, Meissner corpuscles, free nerve endings; highly vascular.
  • Reticular region (≈4/5)
    • Dense irregular CT; thick collagen bundles + coarse elastin; houses hair follicles, sebaceous & sudoriferous glands, Pacinian corpuscles, blood vessels; provides extensibility & elasticity.

Skin Pigments

  • Melanin (eumelanin – brown-black; pheomelanin – yellow-red)
    • Same melanocyte density across races; synthesis rate & vesicle dispersion vary.
  • Hemoglobin: oxygenated Hb imparts red hue.
  • Carotene: precursor of vitamin A; accumulates in stratum corneum & adipocytes producing yellow-orange tint.

Hypodermis (SubQ Layer)

  • Areolar + adipose; stores fat, insulates, cushions, houses major vessels (cutaneous plexus) & Pacinian corpuscles.

Hair

  • Distribution: absent on palms, palmar digits, soles, plantar digits, parts of external genitalia.
  • Composition: dead keratinized cells bound by proteins.
  • Anatomy
    • Shaft (above surface) & root (dermis) with concentric layers: medulla (pigment/air), cortex (bulk), cuticle (shingle-like).
    • Follicle: external & internal epithelial root sheaths + dermal root sheath; base expands to bulb containing vascular papilla & germinal matrix.
    • Associated structures: arrector pili (smooth muscle → piloerection), hair root plexus (touch receptor), sebaceous glands.
  • Growth Cycle
    • Anagen (growth) ≈ 2–6 yrs
    • Catagen (regression) ≈ 2–3 wks
    • Telogen (rest) ≈ 3 mos; cycle length/location/hormones determine length.
  • Hair Types
    • Lanugo (fetal), Vellus (peach-fuzz), Terminal (long, coarse, pigmented).
  • Color: determined by melanin type/amount; gray = ↓melanin + air; white = lack of melanin + air bubbles.

Cutaneous Glands

  1. Sebaceous (oil)
    • Holocrine secretion of sebum (triglycerides, cholesterol, proteins, salts);
    • Functions: lubricates hair/skin, prevents dehydration, bacteriostatic; opens into follicles; absent on palms/soles; activated at puberty.
  2. Sudoriferous (sweat)
    • Eccrine (merocrine)
      • Widely distributed (forehead, palms, soles densest);
      • Secretory coil in dermis; duct to surface pore;
      • Watery sweat: \text{H}_2\text{O}, ions, urea, uric acid, ammonia, glucose, lactic acid;
      • Thermoregulation, waste removal, emotional sweating (palms, soles first).
    • Apocrine (actually merocrine mechanism)
      • Axillae, groin, areolae, beard region; ducts into hair follicle; starts at puberty;
      • Viscous secretion: eccrine components + lipids/proteins → odor via bacteria; emotional & sexual stimuli.
  3. Ceruminous (ear wax)
    • Modified apocrine in external auditory canal; secretion + sebum forms cerumen; barrier against water/bacteria/insects.

Nails

  • Plates of hard keratinized epidermal cells.
  • Structures
    • Nail body (plate) with lunula (whitish, thickened matrix)
    • Free edge (extends past digit)
    • Nail root (under skin)
    • Eponychium (cuticle) = stratum corneum fold
    • Hyponychium secures edge to fingertip
    • Nail bed (skin beneath plate)
    • Matrix: deep epithelium whose mitosis forms nail.
  • Growth ~ 1\,\text{mm/week} fingers (slower toes); influenced by age, nutrition, health.

Major Physiological Functions

  • Thermoregulation
    • Eccrine sweating (evaporative cooling)
    • Vasodilation/vasoconstriction of dermal vessels.
  • Blood Reservoir: dermal vessels hold 8–10\% of total resting blood volume.
  • Protection
    • Keratin barrier, lipid sealant, acidic sweat, sebum, melanin UV shield, macrophages.
  • Sensation: tactile (Meissner, Merkel, hair plexus), pressure/vibration (Pacinian), thermal, nociceptive.
  • Excretion & Absorption
    • Sweat excretes water, salts, urea, ammonia, uric acid.
    • Absorbs lipid-soluble drugs/toxins; basis of transdermal delivery (nitroglycerin, nicotine, estrogen).
  • Vitamin D Synthesis
    • UV-B converts 7-dehydrocholesterol → cholecalciferol; liver & kidneys yield calcitriol \bigl(\text{1,25-dihydroxycholecalciferol}\bigr) → enhances intestinal \text{Ca}^{2+} absorption.

Wound Healing

  • Epidermal (superficial) healing: migration of basale cells to bridge gap, mitosis & stratification.
  • Deep wound (into dermis/SubQ)
    1. Inflammatory phase: clot, vasodilation, phagocytes.
    2. Migratory: epithelial cells migrate; fibroblasts synthesize scar tissue; granulation.
    3. Proliferative: extensive fibroblast activity, angiogenesis.
    4. Maturation: collagen remodeled; scar (fibrosis) → maybe hypertrophic or keloid.

Developmental Origins

  • Epidermis, hair, nails, glands derive from ectoderm.
    • Timeline illustrations: buds at wk 12–18; melanocytes migrate; epidermal layers stratify.
  • Dermis arises from mesoderm → mesenchyme; dermal papillae & fibers differentiate ~wk 11 onward.

Aging Changes

  • Collagen fiber ↓ number & stiffness; elastic fibers lose elasticity → wrinkles.
  • ↓ Sebum & sweat → dryness, overheating risk.
  • ↓ Melanocytes → grey hair, uneven pigmentation.
  • Thinner SubQ fat → colder, pressure sores.
  • Brittle nails.

Clinical Connections

Pigment Disorders

  • Albinism: congenital tyrosinase deficiency → no melanin in skin/hair/eyes; vision defects & photosensitivity.
  • Vitiligo: autoimmune destruction of melanocytes → patchy depigmentation; genetic predisposition.

Skin Cancer

  • UV exposure = primary risk.
  • Types
    1. Basal cell carcinoma (stratum basale) – common, rarely metastasize.
    2. Squamous cell carcinoma (stratum spinosum) – may metastasize.
    3. Malignant melanoma (melanocytes) – rapid metastasis, high mortality.
  • ABCDE screening
    • Asymmetry, Border irregularity, Color variation, Diameter > 0.25\,\text{in} (≈6\,\text{mm}), Evolving.

Burns

  • Damage denatures proteins via heat, electricity, radiation, chemicals.
  • Degrees
    • First: epidermis; redness, mild pain.
    • Second: epidermis + dermis; blisters.
    • Third: full thickness; charred, numb (nerve loss).
  • Rule of nines estimates %TBSA for fluid/triage.

Pressure Ulcers

  • Prolonged ischemia → epithelial & underlying tissue necrosis; elderly & immobile at risk.

Ethical & Practical Implications

  • UV avoidance & sunscreen crucial for cancer prevention.
  • Early melanoma detection saves lives → educate on ABCDE.
  • Management of large burns requires fluid resuscitation, infection control; ethical allocation of grafts/resources.
  • Pressure-ulcer prevention protocols essential in long-term care.
  • Transdermal drug systems provide non-invasive therapy; need regulation for overdose/toxicology.

Key Numbers & Equations

  • Skin blood reservoir: 8\text{–}10\% of total volume at rest.
  • Thin skin thickness: 0.10\text{–}0.15\,\text{mm}; Thick skin: 0.6\text{–}4.5\,\text{mm}.
  • Hair growth cycle: Anagen 2\text{–}6\,\text{yr}, Catagen 2\text{–}3\,\text{wk}, Telogen ~3\,\text{mo}.
  • Melanoma diameter concern: >6\,\text{mm} (1/4 inch).

Conceptual Connections & Real-World Relevance

  • Vitamin D deficiency linked to osteoporosis; importance of moderate sun & dietary supplementation.
  • Skin as drug-delivery route (nicotine patches) leverages lipid permeability of stratum corneum.
  • Tattoos deposit ink in dermis; permanence tied to macrophage entrapment.
  • Cosmetic dermatology (laser resurfacing, fillers) targets collagen/elastin degradation in aging dermis.
  • Genetic conditions (e.g., Ehlers-Danlos) affecting collagen reflect dermal structure significance.