Chapter 6 – Integumentary System Comprehensive Notes

Composition & Macro-Organization of the Integument

• The integument (skin) is the body’s largest organ; structurally layered and functionally integrated with deeper fascia and musculoskeletal, immune, and endocrine systems.

• Principal layers (from superficial → deep):

  • Epidermis – keratinized stratified squamous epithelium.

  • Dermis – dense irregular CT, vascular and innervated.

  • Subcutaneous layer (hypodermis/superficial fascia) – areolar + adipose CT; technically not part of the integument but functionally contiguous.

Epidermis: Detailed Histology & Cell Biology

• General traits

  • Keratinized, stratified squamous epithelium; provides mechanical/chemical barrier.

  • Arranged in strata; first three strata contain metabolically active (living) keratinocytes.

• Strata (deep → superficial) with cellular/functional highlights

  • Stratum basale

  • Single layer, cuboidal–low columnar stem cells; rests on basement membrane.

  • Cell types

    • Keratinocytes – proliferating stem cells; synthesize intermediate-filament protein keratin → imparts tensile strength, water resistance.

    • Melanocytes – neural-crest derived; produce & package melanin within melanosomes, transfer to keratinocytes → UV shield for nuclear DNA; pigment variants: eumelanin (brown/black) vs. pheomelanin (tan/yellow/red).

    • Tactile (Merkel) cells – mechanoreceptors; compression releases chemical neurotransmitter → stimulates afferent nerve endings (fine touch, texture).

  • Stratum spinosum

  • “Spiny” appearance from desmosome-anchored intermediate filaments.

  • Several layers of polygonal, non-dividing keratinocytes displaced from basale.

  • Houses epidermal dendritic (Langerhans) cells – antigen-presenting cells initiating cutaneous immune responses.

  • Stratum granulosum

  • $3–5$ layers; onset of keratinization:

    • Keratohyalin granules aggregate keratin filaments.

    • Nucleus/organelles disintegrate → cell death; forms tough, dehydrated sac.

  • Stratum lucidum ("clear layer")

  • $2–3$ layers; present only in thick skin (palms/soles).

  • Cells packed with translucent protein eleidin (keratin intermediate).

  • Stratum corneum

  • $20–30$ layers of dead, anucleate, flattened keratinocytes (corneocytes).

  • Dry, lipid-coated surface resists abrasion, penetration, microbial growth.

• Keratinization summary & significance

  • Occurs from stratum basale → corneum (~$2$ weeks migration, $2$ additional weeks in corneum).

  • Produces durable, water-resistant barrier vital for terrestrial life.

• Thickness & regional variants

  • Thick skin: all $5$ strata, sweat glands, NO hair or sebaceous glands; sites of high friction (palms, soles).

  • Thin skin: lacks stratum lucidum; contains hair, sebaceous & sweat glands; covers most of body.

• Pigmentation & markings

  • Hemoglobin – iron-containing RBC protein; oxyhemoglobin imparts bright red hue in vascular dermis, visible through thin epidermis.

  • Melanin – UV-induced; genetically regulated; racial variation tied to melanin type & distribution, not melanocyte number.

  • Carotene – dietary (yellow-orange vegetables); accumulates in stratum corneum & subcutaneous fat; precursor to vitamin A.

  • Markings: nevus (mole), freckles, hemangiomas (capillary/cavernous), friction ridges (dermal–epidermal folds = fingerprints; enhance grip & tactile sensitivity).

Dermis: Connective-Tissue Matrix & Biomechanics

• Overall composition: CT proper rich in collagen type I, elastic & reticular fibers; houses accessory structures, vessels, nerves, immune cells.

• Papillary layer

  • Superficial $~1/5$ of dermis; areolar CT.

  • Dermal papillae interlock with epidermal ridges → increase surface area for nutrient diffusion, stabilize junction, form fingerprints.

  • Capillary loops and sensory Meissner (tactile) corpuscles.

• Reticular layer

  • Deep $~4/5$; dense irregular CT; thick collagen bundles → strength, elasticity.

  • Contains hair follicles, sebaceous & sweat glands, Pacinian (lamellated) corpuscles.

• Lines of cleavage (Langer’s lines)

  • Orientation of predominant collagen/elastic bundles; surgical incisions parallel → minimal gaping, faster healing; perpendicular → risk of widened scar.

  • Over-stretching breaks collagen → striae (stretch marks).

Subcutaneous Layer (Hypodermis)

• Areolar + adipose CT; thickness distribution modulated by sex hormones (↑ in females breasts/hips, males neck/abdomen).

• Functions: mechanical cushion, energy storage (triglycerides), thermal insulation, passageway for vessels/nerves.

• Highly vascular → preferred site for subcutaneous drug injections (e.g., insulin, epinephrine) enabling rapid systemic uptake.

Integrated Functions of the Integument

• Protection – physical, chemical, biological barrier against trauma, pathogens, UV, chemicals.

• Water homeostasis – prevents excessive water loss (insensible perspiration $~500$ mL/day) & water gain; lipid coating + tight junctions.

• Vitamin D synthesis

  1. UV B converts 7-dehydrocholesterol in epidermis → vitamin D_3 (cholecalciferol).

  2. Hepatic hydroxylation → $25$-hydroxy-D$_3$.

  3. Renal hydroxylation → calcitriol (active form) which elevates intestinal $\text{Ca}^{2+}$ & $\text{PO}_4^{3-}$ absorption → skeletal health.

• Secretion & absorption – sweat excretes salts, urea; sebum lubricates/bactericidal; transdermal drug delivery (nicotine, hormone patches) exploits selective permeability.

• Immune surveillance – epidermal & dermal dendritic cells present antigens → activate T-helper cells.

• Thermoregulation – vasoconstriction (heat conservation) vs. vasodilation (heat dissipation); merocrine sweating (evaporative cooling).

• Sensory reception – diverse mechanoreceptors (Merkel discs, Meissner, Pacinian, Ruffini endings), nociceptors, thermoreceptors.

Appendages Derived from Epidermis

Nails

• Plates of densely packed, hard keratin; protect distal digits, aid grasping.

• Key structures: nail plate (root, body, free edge), nail bed, matrix (growth zone), lunula, eponychium (cuticle), hyponychium.

Hair (Pili)

• Anatomy

  • Zones: bulb (living matrix around vascular papilla), root (within follicle), shaft (external).

  • Follicle: outer CT root sheath (from dermis) + inner epithelial root sheath.

  • Layers of hair: medulla (soft keratin), cortex (hard keratin), cuticle (shingle-like cells).

  • Arrector pili – smooth muscle; sympathetic stimulation → piloerection ("goosebumps"), assists sebum spread.

• Functions

  • Protection (scalp UV cushion, nasal/eyelash filtering, ear canal debris).

  • Heat retention (scalp in cold climates).

  • Sensory – root hair plexus detects light displacement.

  • Visual/sexual identification (eyebrows, beard, pubic hair).

• Color – melanocyte activity at matrix; aging → ↓ melanin → gray; complete loss → white (air bubbles + keratin only).

• Growth cycle

  • Anagen (active proliferation) $≈$ $18$ mo–$7$ yr.

  • Catagen (regression) $~3$ weeks.

  • Telogen (resting/shedding) $~3$ mo; then bulb re-enters anagen; normal loss $50–100$ hairs/day.

Cutaneous Exocrine Glands

• Merocrine (eccrine) sweat glands

  • Simple coiled tubular; widespread (≈ $3–4$ million); duct opens to pore.

  • Secretion: $99\%$ water, electrolytes (Na+, Cl\−), metabolites, urea; pH $4–6$ acidic mantle inhibits microbes.

  • Primary role: thermoregulation via evaporative cooling.

• Apocrine sweat glands

  • Axillae, areolae, pubic, perianal regions; duct empties into hair follicles.

  • Viscous secretion (proteins + lipids) → bacterial metabolism → body odor; activate at puberty (androgen responsive).

• Sebaceous glands

  • Holocrine; secrete sebum (lipids, cell debris) into hair follicles; functions: lubrication, waterproofing, antimicrobial.

  • Androgen-stimulated; surge at puberty → acne vulgaris.

• Ceruminous glands

  • Modified apocrine in external acoustic meatus; secrete cerumen (earwax) = sebum + exfoliated cells; traps debris, waterproofs tympanic membrane.

• Mammary glands

  • Modified apocrine; alveolar glands produce milk (water, proteins, lipids, lactose, IgA) under prolactin/oxytocin control.

Tissue Repair & Wound Healing

• Regeneration – replacement with same cell type → restores function (e.g., epidermis).

• Fibrosis – CT scar replaces tissue when regeneration limited (dermis, severe burns) → ↓ function.

• Sequential stages

  1. Hemostasis & inflammation – vessels bleed; clot forms (fibrin mesh) trapping leukocytes.

  2. Clot & cleansing – neutrophils/macrophages phagocytose debris/pathogens; clot = temporary barrier.

  3. Proliferative (granulation) phase – angiogenesis produces granulation tissue (vascularized, soft CT); fibroblasts lay collagen.

  4. Maturation/remodeling – epithelial regeneration below scab; CT fibrosis/scar contraction over weeks–months.

Aging & Pathology of the Integument

• Intrinsic aging changes

  • ↓ stem cell activity → slower repair, thinning epidermis.

  • ↓ collagen, elastin → wrinkles, sagging.

  • ↓ immune Langerhans cells → ↑ infection, malignancy risk.

  • Hair follicle atrophy → alopecia or finer hairs; ↓ melanocytes → gray hair.

• Extrinsic modifiers

  • UV radiation & smoking – generate reactive oxygen species, DNA mutations; accelerate photo-aging, elastosis, carcinogenesis.

• Skin cancer – most prevalent cancer; high incidence on UV-exposed head/neck; fair-skinned at highest risk (less melanin protection).

Clinical & Applied Connections

• Lines of cleavage guide surgical incisions, forensic wound analysis.

• Transdermal drug delivery exploits lipid solubility & epidermal permeability gradient (e.g., nitroglycerin, fentanyl).

• Burn management: thickness of skin layers informs first/second/third-degree classification and fluid resuscitation calculations (Rule of Nines).

• Vitamin D deficiency links integument to skeletal disorders (rickets, osteomalacia) and immune modulation.

• Awareness of nevus evolution (ABCDE rule) crucial for early melanoma detection.