Chapter 5- Module 4 The Integumentary System Notes

Structure of the Skin and Hypodermis

• Skin (Integument): Covers the body with a surface area of $1.2\text{ to }2.2\,\text{m}^2$ and weighs $9\text{ to }11\,\text{lbs}$.

• Main Regions:

  • Epidermis: Outermost protective shield composed of keratinized stratified squamous epithelium; unvascularized (supplied by diffusion from the dermis).

  • Dermis: Underlies the epidermis and forms the bulk of the skin; composed of vascularized fibrous connective tissue.

  • Cutaneous Membrane: The combination of the epidermis and dermis.

• Hypodermis (Superficial Fascia):

  • Subcutaneous layer deep to the skin (not part of the skin proper).

  • Composed mainly of adipose tissue for shock absorption, thermal insulation, and anchoring the skin to underlying structures (primarily muscle).

  • Accumulates in region-specific areas (female thighs and breasts; male anterior abdomen).

Layers and Cells of the Epidermis

• Cell Types:

  • Keratinocytes: Main cell type connected by desmosomes; produces protective keratin (type I acidic and type II basic); originates in the stratum basale and renews every $25\text{ to }45\,\text{days}$. Sheds off as dander at a rate of $\sim 50,000\,\text{cells/min}$.

  • Melanocytes: Constitute $10\%\text{ to }25\%$ of cells in the deepest epidermal layer; synthesize melanin packaged in melanosomes to protect keratinocyte nuclei from UV damage.

  • Dendritic (Langerhans) Cells: Bone marrow-derived phagocytes that present foreign antigens to activate the immune system.

  • Tactile (Merkel) Cells: Located at the epidermal-dermal junction paired with sensory nerve endings to function as touch receptors.

• Epidermal Layers (Deep to Superficial):

  • Stratum Basale (Stratum Germinativum): Deepest single row of actively mitotic stem cells firmly attached to the dermis.

  • Stratum Spinosum: Several cell layers thick; contains keratinocytes with intermediate pre-keratin filaments anchored to desmosomes, along with melanosomes and dendritic cells.

  • Stratum Granulosum: $4\text{ to }6$ layers of flattened keratinocytes undergoing organelle disintegration; contains keratohyaline granules (form keratin) and lamellar granules (release water-resistant glycolipids).

  • Stratum Lucidum: Thin, translucent band of dead keratinocytes present only in thick skin (palms, fingertips, soles).

  • Stratum Corneum: Superficial barrier consisting of $20\text{ to }30$ layers of dead, anucleate keratinized cells protecting against mechanical abrasion, water loss, and chemical assaults.

Structure and Layers of the Dermis

• Dermis Overview: Strong, flexible connective tissue layer containing fibroblasts, macrophages, mast cells, nerve fibers, blood/lymphatic vessels, hair follicles, and cutaneous glands.

• Papillary Layer:

  • Superficial layer composed of loose areolar connective tissue with collagen and elastic fibers.

  • Features dermal papillae containing capillary loops, Meissner's corpuscles (touch receptors), or free nerve endings (pain receptors).

  • In thick skin, papillae lie atop dermal ridges to form friction ridges (fingerprints).

• Reticular Layer:

  • Accounts for $\sim 80\%$ of dermal thickness; composed of dense irregular fibrous connective tissue.

  • Nourished by the cutaneous plexus situated between the reticular dermis and hypodermis.

  • Contains collagen fiber bundles that form invisible cleavage lines; surgical cuts parallel to cleavage lines gap less and heal faster.

  • Contains flexure lines (dermal folds at joints where skin is tightly bound to deep structures).

Appendages of the Skin

• Hairs and Hair Follicles:

  • Composed of dead, hard keratinized cells. Features a shaft (complete keratinization) and root (ongoing keratinization).

  • Hair cross-section consists of an inner medulla, middle cortex, and outer single-layer cuticle.

  • Structure includes a hair bulb, hair follicle receptor (root hair plexus), hair matrix (mitotic layer), hair papilla (dermal vascular supply), and arrector pili muscle (smooth muscle responsible for pull and goose bumps).

  • Hair types: Vellus hair (fine body hair) and Terminal hair (coarse scalp, eyebrow, axillary, and pubic hair). Cycles between active ($6\text{ to }10\,\text{years}$) and regressive ($2\text{ to }3\,\text{months}$) phases; average growth is $2.25\,\text{mm}$ per week with $\sim 90$ scalp hairs lost daily.

• Nails: Scalelike epidermal modifications containing hard keratin; growth originates at the nail matrix.

• Sweat (Sudoriferous) Glands:

  • Eccrine (Merocrine) Glands: Most abundant (palms, soles, forehead); produce hypotonic sweat ($99\%$ water, salts, vitamin C, antibodies, dermcidin, metabolic wastes) for thermoregulation.

  • Apocrine Glands: Axillary and anogenital regions; empty into hair follicles; secrete viscous sweat containing fatty substances and proteins (odorless until broken down by skin bacteria); activate at puberty. Include ceruminous glands (earwax) and mammary glands (milk).

• Sebaceous (Oil) Glands:

  • Branched alveolar holocrine glands associated with hair follicles (absent in thick skin of palms and soles).

  • Stimulated by androgen hormones at puberty to secrete sebum (bactericidal lipid mixture) to lubricate hair and skin.

Functions of the Integumentary System

• Protection:

  • Chemical Barrier: Low pH secretions, dermcidin, defensins, and UV-absorbing melanin.

  • Physical Barrier: Keratin and extracellular glycolipids block water loss and water-soluble substances.

  • Biological Barrier: Dendritic cells and dermal macrophages present antigens to immune cells; DNA absorbs UV and converts it to heat.

• Body Temperature Regulation:

  • Unnoticed insensible perspiration ($\sim 500\,\text{mL/day}$) occurs at ambient temperatures below $31\text{ to }32\,^\circ\text{C}$.

  • Dermal blood vessel dilation and sweat secretion (sensible perspiration) cool the body when heat rises; vessel constriction conserves heat in cold environments.

• Cutaneous Sensation: Sensory nerve endings and specialized corpuscles detect temperature, touch (mechanoreceptors), and pain.

• Metabolic Functions: Synthesis of vitamin D precursor via sunlight, chemical conversion of carcinogens, and production of collagenase.

• Blood Reservoir: Holds up to $5\%$ of the body's total blood volume.

• Excretion: Eliminates nitrogenous wastes and salts via sweat.

Skin Pigmentation and Homeostatic Imbalances

• Skin Pigments:

  • Melanin: Synthesized from tyrosine via tyrosinase into eumelanin (brown/black, effective UV blocker) or pheomelanin (red/yellow, UV-permeable).

  • Carotene: Yellow-orange pigment accumulating in the stratum corneum and hypodermis; converts to vitamin A.

  • Hemoglobin: Produces a pinkish hue in fair skin.

• Clinical Diagnostic Signs:

  • Cyanosis: Blue tint from poor blood oxygenation.

  • Erythema: Redness from inflammation, fever, or allergy.

  • Pallor: Paleness from anemia or low blood pressure.

  • Jaundice: Yellowing from excess bilirubin/liver dysfunction.

  • Bronzing: Metallic shade from steroid deficiency in Addison's disease.

  • Bruises: Hematomas beneath the skin.

• Skin Cancers:

  • Basal Cell Carcinoma: Most common and least malignant; stratum basale cells invade dermis/hypodermis ($99\%$ cure rate with excision).

  • Squamous Cell Carcinoma: Arises from stratum spinosum keratinocytes; forms scaly papule with metastatic potential.

  • Melanoma: Cancer of melanocytes; highly metastatic and chemoresistant. Diagnosed via the ABCD rule: Asymmetry, Border irregularity, Color variation, and Diameter > $6\,\text{mm}$.

• Burns and Severity:

  • Threats include fluid loss, dehydration, electrolyte imbalance, circulatory shock, and renal failure. Assessed using the Rule of Nines.

  • First-Degree: Epidermal damage only; localized redness, swelling, and pain.

  • Second-Degree: Epidermal and upper dermal damage; blisters form.

  • Third-Degree: Full-thickness burn destroying nerve endings (painless); requires skin grafts.

  • Critical Thresholds: >$25\%$ body with second-degree, >$10\%$ body with third-degree, or third-degree burns on the face, hands, or feet.

• Developmental Aspects:

  • Fetal skin features a lanugo coat (delicate hair) and vernix caseosa (sebaceous secretion).

  • Aging leads to epidermal thinning, reduced elasticity and subcutaneous fat, decreased glandular activity, and hair loss.