The Integumentary System

Overview

The integumentary system consists of the skin and its accessory structures. It is the largest organ system in the body and serves multiple protective, regulatory, and sensory functions.

• Skin is composed of different tissues joined together to perform specific functions.

• It is the largest organ of the body.

• Accessory structures include hair, nails, and glands.

Structure of the Skin

Main Layers of the Skin

• Epidermis: The superficial, epithelial layer composed primarily of keratinized stratified squamous epithelium.

• Dermis: The deeper, connective tissue layer that provides structural support and houses blood vessels, nerves, and accessory structures.

• Hypodermis (subcutaneous layer): Not technically part of the skin; consists mainly of adipose tissue and provides insulation and energy storage.

Cells of the Epidermis

• Keratinocytes: Most abundant; produce keratin, a fibrous protein that provides strength and waterproofing. Connected by desmosomes.

• Melanocytes: Found in the deepest layer; produce melanin pigment, which protects against UV radiation.

• Epidermal dendritic cells (Langerhans cells): Act as macrophages, providing immune defense.

• Tactile (Merkel) cells: Sensory receptors for touch, located in the deepest layer.

Layers of the Epidermis

1. Stratum Basale: Deepest, single layer of stem cells attached to the dermis. Contains mitotically active keratinocytes, melanocytes, and occasional Merkel cells.

2. Stratum Spinosum: 8-10 layers of cells with desmosomes; provides strength and flexibility. Contains keratinocytes and dendritic cells.

3. Stratum Granulosum: 3-5 layers of keratinocytes undergoing keratinization and apoptosis. Contains keratohyalin granules and lamellated granules.

4. Stratum Lucidum: Thin, clear layer found only in thick skin (palms, soles). Provides extra protection.

5. Stratum Corneum: Most superficial; 20-30 layers of dead, keratinized cells. Provides a tough, protective barrier and forms calluses with friction.

Dermis

Structure and Function

• Composed of strong, flexible connective tissue.

• Contains nerves, blood vessels, lymphatic vessels, and accessory structures (hair follicles, glands).

• Divided into two layers:

Structural Basis of Skin Color

• Melanin: Brown-black pigment produced by melanocytes; protects against UV damage.

• Carotene: Yellow-orange pigment from diet; accumulates in stratum corneum.

• Hemoglobin: Red pigment in blood; gives skin a pinkish hue, especially in light-skinned individuals.

Alterations in skin color can indicate disease (e.g., cyanosis, erythema, jaundice).

Skin Glands

Sebaceous (Oil) Glands

• Attached to hair follicles; secrete sebum (oil) to lubricate and waterproof skin and hair.

• Found all over body except thick skin (palms, soles).

Sweat (Sudoriferous) Glands

• Eccrine glands: Widely distributed; secrete watery sweat onto skin surface for thermoregulation.

• Apocrine glands: Found in axillary and anogenital areas; secrete thicker sweat into hair follicles, begin functioning at puberty.

Hair and Nails

• Hair: Consists of dead keratinized cells; functions in protection, sensation, and thermoregulation.

• Nails: Protect distal phalanges and aid in grasping/manipulation.

Functions of the Integumentary System

1. Protection: Physical, chemical, and biological barriers against pathogens, UV radiation, and dehydration.

2. Body Temperature Regulation: Sweat secretion and dermal blood flow regulate heat loss.

3. Cutaneous Sensation: Sensory receptors detect touch, pressure, pain, and temperature.

4. Metabolic Functions: Synthesis of vitamin D for calcium absorption.

5. Excretion: Minor role in eliminating wastes via sweat.

6. Blood Reservoir: Skin can hold up to 5% of the body's blood supply.

Clinical Aspects

Burns

• First-degree burns: Affect only the epidermis.

• Second-degree burns: Involve epidermis and upper dermis.

• Third-degree burns: Destroy entire thickness of skin.

• Major threat: dehydration due to fluid loss.

• Rule of 9's used to estimate burn area.

Skin Cancer

• ABCDE rule for melanoma detection: Asymmetry, Border irregularity, Color variation, Diameter >6mm, Evolving shape/size.