Integumentary System Overview

Introduction to the Integumentary System

The integumentary system is the body's largest and most accessible organ system, commonly referred to as the skin or integument. It serves as the first line of defense against environmental hazards and plays a vital role in homeostasis.

• Components: The system consists of the cutaneous membrane (skin) and accessory structures (hair, nails, glands).

• Surface Area: Covers 1.5–2 m2 and accounts for about 16% of total body weight.

• Functions: Protection, sensation, temperature regulation, excretion, vitamin D synthesis, and lipid storage.

Structure of the Skin

Cutaneous Membrane

• Epidermis: Outermost layer, composed of stratified squamous epithelium.

• Dermis: Deeper layer, consisting of the papillary (areolar connective tissue) and reticular (dense irregular connective tissue) layers.

Accessory Structures

• Hairs

• Nails

• Exocrine glands: Sebaceous (oil) and sweat glands

• Sensory receptors and arrector pili muscles

• Cutaneous plexus: Network of blood vessels

Subcutaneous Layer (Hypodermis)

• Not technically part of the skin; separates skin from deeper structures.

• Dominated by adipose tissue; important for energy storage and insulation.

Functions of the Integumentary System

• Protection: Against impact, abrasion, fluid loss, and chemical attack.

• Excretion: Salts, water, and organic wastes via glands.

• Temperature Regulation: Insulation and evaporative cooling.

• Melanin Production: Protects against UV radiation.

• Keratin Production: Provides water resistance and protection.

• Vitamin D3 Synthesis: Essential for calcium metabolism.

• Lipid Storage: In dermis and subcutaneous layer.

• Sensation: Touch, pressure, pain, and temperature detection.

Epidermis: Structure and Function

Layers of the Epidermis (from deep to superficial)

1. Stratum basale: Attached to basement membrane; contains basal (stem) cells and Merkel cells (touch receptors).

2. Stratum spinosum: 8–10 layers of keratinocytes; contains dendritic (Langerhans) cells for immune defense.

3. Stratum granulosum: 3–5 layers; cells produce keratin and keratohyalin, become flatter and die.

4. Stratum lucidum: Only in thick skin (palms, soles); clear, densely packed dead cells.

5. Stratum corneum: 15–30 layers of dead, keratinized cells; water-resistant, but not waterproof.

• Thin skin: Covers most of the body; has four layers (lacks stratum lucidum).

• Thick skin: Found on palms and soles; has all five layers.

Cellular Dynamics

• Cells migrate from the stratum basale to the stratum corneum in 7–10 days.

• Cells in the stratum corneum remain for about 2 weeks before being shed.

Determinants of Skin Color

Pigments and Circulation

• Carotene: Orange-yellow pigment from diet.

• Melanin: Brown, yellow-brown, or black pigment produced by melanocytes in the stratum basale; packaged in melanosomes and transferred to keratinocytes.

• Hemoglobin: Red pigment in blood; increased blood flow causes redness, decreased flow causes paleness or cyanosis (bluish color).

Factors Affecting Skin Color

• Genetic differences in melanin production (not number of melanocytes).

• Exposure to UV radiation increases melanin production.

• Thickness and degree of keratinization.

Skin Cancer

• Basal cell carcinoma: Most common, originates in stratum basale, rarely metastasizes.

• Malignant melanoma: Cancer of melanocytes, highly metastatic and dangerous.

Dermis and Subcutaneous Layer

Dermis Structure

• Papillary layer: Areolar tissue with capillaries, lymphatics, and sensory neurons.

• Reticular layer: Dense irregular connective tissue with collagen and elastic fibers; contains blood vessels, nerves, and accessory organs.

Subcutaneous Layer (Hypodermis)

• Separates skin from deeper tissues; dominated by adipose tissue.

• Site of energy storage; distribution varies by sex and age.

Sensory Receptors

• Free nerve endings: Detect touch and pressure.

• Tactile (Meissner) corpuscles: Detect light touch, pressure, and vibration.

• Lamellated (Pacinian) corpuscles: Detect deep pressure and vibration.

• Ruffini corpuscles: Detect pressure and skin stretching.

Cleavage (Tension) Lines

• Formed by the arrangement of collagen and elastic fibers.

• Clinically significant for surgery and wound healing: cuts parallel to lines heal better with less scarring.

Burns and Skin Grafts

Classification of Burns

• First-degree (superficial): Only epidermis affected; redness and mild pain.

• Second-degree (partial-thickness): Epidermis and part of dermis; blistering, pain, swelling.

• Third-degree (full-thickness): Destroys epidermis, dermis, and may extend into subcutaneous layer; less pain due to nerve damage, requires skin grafting.

Rule of Nines

Used to estimate the percentage of body surface area affected by burns.

Types of Skin Grafts

• Split-thickness: Epidermis and superficial dermis.

• Full-thickness: Epidermis and all dermal layers.

• Autograft: Patient's own skin (best choice, no rejection).

• Allograft: Donor skin (cadaver).

• Xenograft: Animal skin.

Accessory Structures of the Skin

Hair

• Types: Terminal (coarse, pigmented) and vellus (fine, unpigmented).

• Structure: Shaft (visible), root (embedded), follicle (surrounds root), bulb (base), papilla (connective tissue, blood supply), matrix (growth zone).

• Layers: Medulla (soft keratin), cortex (hard keratin), cuticle (hard keratin).

• Growth cycle: Active phase (2–5 years), resting phase, shedding (club hair).

• Arrector pili muscle: Causes hair to stand erect (goosebumps).

Exocrine Glands

• Sebaceous glands: Holocrine glands producing sebum (oily, antimicrobial, lubricates hair and skin).

• Sweat glands:

  • Apocrine: Found in axillae, nipples, pubic region; produce odorous secretion; influenced by hormones.

  • Merocrine (eccrine): Widely distributed; secrete watery, electrolyte-rich fluid for thermoregulation and excretion.

• Myoepithelial cells: Assist in discharge of glandular secretions.

Nails

• Structure: Nail body (visible), nail bed (underlying epidermis), nail root (production site), lunula (pale crescent), eponychium (cuticle), hyponychium (under free edge).

• Function: Protects tips of fingers and toes, aids in manipulation.

• Clinical relevance: Nail appearance can indicate systemic disease (e.g., pitting in psoriasis, concave nails in anemia).

Age-Related Changes in the Integument

• Fewer melanocytes: Increased risk of sunburn, paler skin.

• Drier, thinner epidermis: More prone to injury and infection.

• Reduced vitamin D3 production: Leads to muscle weakness and brittle bones.

• Diminished immune response: Fewer dendritic cells.

• Thinning dermis: Wrinkling and sagging.

• Decreased perspiration: Greater risk of overheating.

• Reduced blood supply: Poor temperature regulation.

• Fewer active follicles: Thinner, grayer hair.

• Slower skin repair: Delayed wound healing.

Endocrine Interactions and Vitamin D3 Synthesis

• Glucocorticoids: Reduce epidermal barrier effectiveness.

• Thyroid hormones: Maintain blood flow to skin.

• Sex hormones: Increase epidermal thickness, accelerate repair.

• Growth factors (e.g., EGF): Stimulate cell division, keratin production, and repair.

• Growth hormone: Stimulates fibroblast activity and collagen production.

Vitamin D3 Synthesis

• UV radiation converts a steroid in the skin to cholecalciferol (vitamin D3).

• Liver and kidneys convert cholecalciferol to calcitriol, which is essential for calcium and phosphate absorption in the intestine.

• Deficiency leads to rickets in children and decreased bone density in adults.

Skin Repair and Regeneration

Phases of Skin Repair

1. Inflammation phase: Bleeding and mast cell activation; inflammation occurs.

2. Migration phase: Blood clot (scab) forms; basal cells migrate; macrophages remove debris; granulation tissue forms.

3. Proliferation phase: Epidermal cells proliferate; fibroblasts produce collagen; clot dissolves.

4. Scarring phase: Scab is shed; scar tissue forms (fibrous, noncellular).

• Keloid: Raised, thickened scar tissue extending beyond original injury site.

Image Context

Scanning Electron Micrograph of Skin

The provided image is a high-magnification scanning electron micrograph showing the interface between the epidermis (upper, brownish layers) and the dermis (lower, greenish layer). The wavy boundary represents the dermal papillae interdigitating with epidermal ridges, which increases surface area for attachment and nutrient exchange.

• Significance: This structure strengthens the connection between epidermis and dermis and is essential for skin integrity.

• Clinical relevance: Loss of this interface can lead to blistering and skin fragility.