Chapter 5: The Integumentary System – Structure and Function

The Integumentary System

Overview of the Integumentary System

The integumentary system is the body's largest organ system, serving as the primary barrier between internal tissues and the external environment. It consists of several components that work together to protect, regulate, and sense the body.

• Skin: The main protective covering of the body.

• Hair: Provides protection and sensory input.

• Nails: Protect the distal phalanges and aid in manipulation.

• Sweat glands: Involved in thermoregulation and excretion.

• Sebaceous (oil) glands: Secrete sebum to lubricate and waterproof the skin and hair.

Example: The skin acts as a barrier to pathogens and helps regulate body temperature through sweat production.

Structure of Skin

Distinct Regions of the Skin

The skin is composed of two main layers, each with unique structural and functional properties.

• Epidermis: The superficial region made of epithelial tissue; it is avascular (lacks blood vessels).

• Dermis: The deeper region made of fibrous connective tissue; it is vascular (contains blood vessels).

• Subcutaneous tissue (hypodermis): Lies deep to the skin, not technically part of the skin but shares some functions. It consists mostly of adipose tissue, which absorbs shock and insulates underlying structures.

Example: The dermis contains nerve endings, blood vessels, and glands, supporting the epidermis and contributing to skin function.

Epidermis

Introduction to the Epidermis

The epidermis is the outermost layer of the skin, providing a protective barrier against environmental hazards. It is composed of stratified squamous epithelial cells and is responsible for the production of keratin.

• Avascular: Receives nutrients via diffusion from the underlying dermis.

• Cell types: Includes keratinocytes, melanocytes, dendritic (Langerhans) cells, and tactile (Merkel) cells.

• Functions: Protection, sensation, and prevention of water loss.

Example: Keratinocytes in the epidermis produce keratin, a protein that strengthens the skin and makes it water-resistant.

Layers of the Epidermis

The epidermis is organized into distinct layers, each with specialized cells and functions. The number of layers varies depending on the location (four layers in thin skin, five in thick skin such as the palms and soles).

• Stratum basale: Deepest layer; single row of actively dividing stem cells. Contains melanocytes and Merkel cells.

• Stratum spinosum: Several layers of keratinocytes connected by desmosomes; contains dendritic cells.

• Stratum granulosum: 1–5 layers of flattened cells; contains lamellar granules (waterproofing) and keratohyaline granules (keratin formation).

• Stratum lucidum: Present only in thick skin; thin, translucent layer of dead keratinocytes.

• Stratum corneum: Most superficial layer; 20–30 layers of dead, flat, keratinized cells that are continuously shed and replaced.

Example: The stratum corneum provides the main barrier to water loss and environmental damage.

Major Cell Types in the Epidermis

• Keratinocytes: Produce keratin, a tough, protective protein.

• Melanocytes: Produce melanin, which protects against UV radiation.

• Dendritic (Langerhans) cells: Immune cells that help protect against pathogens.

• Tactile (Merkel) cells: Sensory receptors for touch.

Example: Melanocytes transfer melanin to keratinocytes, shielding their DNA from UV damage.

Summary Table: Layers of the Epidermis

Layer	Main Features	Location
Stratum basale	Single row of stem cells, melanocytes, Merkel cells	Deepest
Stratum spinosum	Keratinocytes, dendritic cells, desmosomes	Above basale
Stratum granulosum	Lamellar granules, keratohyaline granules	Middle
Stratum lucidum	Clear, dead keratinocytes	Only in thick skin
Stratum corneum	Dead, keratinized cells	Most superficial

Additional info: The hypodermis (subcutaneous layer) is not technically part of the skin but is often discussed with the integumentary system due to its supportive and insulating functions.