Integumentary System

Epidermis: The superficial, avascular protective shield composed of keratinized stratified squamous epithelium. Receives nutrients strictly via diffusion from dermal capillaries.
Dermis: The deeper, highly vascularized structural layer rich in dense connective tissue, blood vessels, and nerve endings.
Hypodermis (Subcutaneous Layer): Not part of the skin proper, but anchors skin to muscle. Packed with adipose tissue for shock absorption and thermal insulation.

Key Epidermal Cell Types:
- Keratinocytes: Chief skin cells producing keratin for physical durability; fully renewed every $25\text{ to }45\,\text{days}$ .
- Melanocytes: Deep-layer spider-shaped cells producing melanin melanosomes to shield cell DNA from harmful UV rays.
- Dendritic (Langerhans) Cells: Immune macrophages patrolling for invading pathogens.
- Tactile (Merkel) Cells: Touch sensory receptors paired with sensory nerve endings.
Epidermal Strata (Deep to Superficial):
1. Stratum Basale: A single row of rapidly dividing stem cells continuous with new keratinocyte production.
2. Stratum Spinosum: Prickly layer connected by desmosomes with thick pre-keratin filaments and abundant dendritic cells.
3. Stratum Granulosum: $1\text{ to }5\,\text{layers}$ where cells flatten, lose nuclei, and release lamellar glycolipids (waterproofing) and keratohyaline granules.
4. Stratum Lucidum: Clear, translucent band of dead keratinocytes found strictly in thick skin (palms, soles).
5. Stratum Corneum: Horny top layer ( $75\%$ of total epidermal thickness) comprising $20\text{ to }30\,\text{rows}$ of dead, anucleate, keratin-filled sacs shedding continuously as dander.

Papillary Layer: Upper $20\%$ of dermis composed of loose areolar connective tissue. Features dermal papillae (capillary loops, pain/touch receptors) forming friction ridges for enhanced grip and unique fingerprint impressions.
Reticular Layer: Deeper $80\%$ of dermis formed from dense irregular connective tissue rich in collagen and elastic fibers.
Skin Markings:
- Cleavage Lines: Less dense collagen separations; surgical cuts made parallel to these lines heal faster with minimal gaping.
- Flexure Lines: Deep skin creases at joints where dermis binds to underlying structures.

Pigments:
- Melanin: Ranges from reddish-yellow to brownish-black; sun exposure triggers production (tanning) to absorb solar UV radiation.
- Carotene: Yellow-orange plant pigment accumulating in the stratum corneum and hypodermis fat; converts into vitamin A.
- Hemoglobin: Oxygenated red blood pigment in dermal capillaries causing the pinkish hue of fair skin.
Diagnostic Indicators:
- Cyanosis: Blue tint indicating low oxygenation.
- Pallor: Pale appearance from anemia, low blood pressure, or stress.
- Erythema: Redness from fever, inflammation, or allergy.
- Jaundice: Yellowing from liver dysfunction and bilirubin buildup.
- Ecchymoses: Bruising caused by blood leaking from damaged dermal vessels.

Hair (Pili): Flexible strands of dead, hard-keratinized cells consisting of a root and shaft. Shaft shape dictates texture (flat = curly, oval = wavy, round = straight).
Nails: Scale-like modifications of hard keratin over digits; driven by active cell division in the nail matrix.
Sweat (Sudoriferous) Glands:
- Eccrine Glands: Abundant on palms, soles, and forehead; produce hypotonic sweat ( $99\%$ water, salts, dermcidin) for evaporative thermoregulation.
- Apocrine Glands: Confined to axillary/anogenital regions; produce protein- and lipid-rich secretion decomposed by skin flora into body odor. Includes ceruminous (earwax) and mammary (milk) glands.
Sebaceous (Oil) Glands: Secrete lipid-rich sebum into hair follicles to lubricate skin/hair, prevent water loss, and kill bacteria.

Protection: Chemical barrier (low pH acid mantle, dermcidin, melanin), physical barrier (keratin, glycolipids), and biological barrier (dendritic cells, dermal macrophages).
Temperature Regulation: Cools via sensible/insensible perspiration ( $0.5\,\text{L/day}$ baseline) and conserves heat via dermal vessel constriction.
Metabolic & Storage Functions: Synthesizes vitamin D precursor (cholecalciferol), serves as a blood reservoir (holding up to $5\%$ of total blood volume), and excretes nitrogenous wastes in sweat.

Skin Cancers:
- Basal Cell Carcinoma: Most common ( $80\%$ ), least malignant; proliferation of stratum basale cells cured by excision in $99\%$ of cases.
- Squamous Cell Carcinoma: Originates in stratum spinosum keratinocytes; scaly papule on scalp, ears, or hands with rapid growth potential.
- Melanoma: Cancer of melanocytes; highly metastatic and chemotherapy-resistant. Detected using the ABCD Rule (Asymmetry, Border irregularity, Color variation, Diameter > $6\,\text{mm}$ ).
Burn Assessment & Severity:
- Rule of Nines: Divides TBSA into $9\%$ segments to estimate fluid loss and guide fluid replacement.
- First-Degree: Epidermal damage only; redness, swelling, pain; heals in $2\text{ to }3\,\text{days}$ .
- Second-Degree: Epidermal and upper dermal damage; causes fluid-filled blisters; heals in $3\text{ to }4\,\text{weeks}$ .
- Third-Degree: Full-thickness destruction extending to hypodermis; non-painful due to destroyed nerve endings; requires debridement and skin autografts.

Development: Epidermis derives from ectoderm; dermis/hypodermis derive from mesoderm.
Aging Effects: Epidermal cell renewal slows down; skin becomes thin and dry as sebaceous activity drops. Elasticity loss and collagen degradation produce wrinkles and cold intolerance.