Chapter 5: The Integumentary System – Structure and Function

Integumentary System Overview

The integumentary system is the body's largest organ system, serving as a protective barrier and playing key roles in sensation, temperature regulation, and more. It consists of the skin and its accessory structures.

• Sweat (sudoriferous) glands: Produce sweat for thermoregulation and excretion.

• Oil (sebaceous) glands: Secrete sebum to lubricate and waterproof the skin and hair.

• Subcutaneous tissue: Also known as the hypodermis, provides insulation and energy storage.

Layers of the Skin

The skin is composed of three main layers, each with distinct structure and function:

• Epidermis: The outermost, protective layer made of keratinized stratified squamous epithelium.

• Dermis: The middle layer, composed of connective tissue, housing blood vessels, nerves, and accessory structures.

• Hypodermis (Superficial fascia): The deepest layer, consisting mainly of adipose tissue and some areolar connective tissue. It anchors the skin to underlying structures, absorbs shock, and insulates the body.

Additional info: The hypodermis is not technically part of the skin but is often discussed with it due to its close association and functional importance.

Epidermis

The epidermis is a thin, avascular layer composed mostly of keratinized stratified squamous epithelial cells. It provides a waterproof barrier and creates our skin tone.

Cell Types of the Epidermis

• Keratinocytes: Produce the fibrous protein keratin, which gives skin its protective properties. These cells are the most abundant in the epidermis and are continuously produced and shed.

• Melanocytes: Spider-shaped cells in the deepest epidermis that produce the pigment melanin. Melanin is packaged into melanosomes and transferred to keratinocytes, protecting DNA from UV damage.

• Dendritic (Langerhans) cells: Star-shaped macrophages that patrol the deep epidermis, ingesting foreign substances and activating the immune response.

• Tactile epithelial (Merkel) cells: Sensory receptors for touch, located at the epidermal-dermal junction.

Layers of the Epidermis

The epidermis is organized into distinct layers (strata), from deep to superficial:

• Stratum basale (deepest)

• Stratum spinosum

• Stratum granulosum

• Stratum lucidum (only in thick skin)

• Stratum corneum (most superficial)

Thick skin (palms, fingertips, soles) contains all five layers, while thin skin (rest of the body) lacks the stratum lucidum.

Stratum Basale (Basal Layer)

The stratum basale is the deepest epidermal layer, firmly attached to the dermis. It consists of a single row of stem cells that are highly mitotic, producing two daughter cells at a time. One cell migrates toward the surface, taking 25–45 days to reach it, while the other remains as a stem cell.

• Function: Continuous regeneration of the epidermis.

• Example: Rapid healing of minor skin abrasions due to active cell division in this layer.

Stratum Spinosum (Prickly Layer)

This layer is several cell layers thick and contains a weblike system of intermediate pre-keratin filaments attached to desmosomes, allowing cells to resist tension and pulling. Keratinocytes in this layer appear spiky (prickle cells). Melanosomes and dendritic cells are also abundant here.

Stratum Granulosum (Granular Layer)

Consisting of 4–6 cell layers, the stratum granulosum is where keratinization begins. Cells flatten, nuclei and organelles disintegrate, and keratohyaline granules accumulate to help form keratin fibers. Lamellar granules release a water-resistant glycolipid, slowing water loss. Cells above this layer die due to lack of nutrients.

Stratum Lucidum (Clear Layer)

Present only in thick skin, this thin, translucent band consists of two to three rows of flat, dead keratinocytes. It is superficial to the stratum granulosum.

Stratum Corneum (Horny Layer)

The outermost layer, consisting of 20–30 rows of flat, anucleated, keratinized dead cells. It accounts for three-quarters of the epidermal thickness and provides protection against abrasion, penetration, and water loss.

• Function: Physical and chemical barrier to the environment.

Summary Table: Layers of the Epidermis

Layer	Main Features	Location
Stratum basale	Single row of mitotic stem cells; melanocytes and Merkel cells present	Deepest layer, attached to dermis
Stratum spinosum	Several layers; keratinocytes with pre-keratin filaments; dendritic cells	Above stratum basale
Stratum granulosum	4–6 layers; keratinization begins; lamellar granules	Middle layer
Stratum lucidum	2–3 rows of clear, dead keratinocytes	Only in thick skin (palms, soles)
Stratum corneum	20–30 layers of dead, keratinized cells; barrier function	Most superficial

Additional info: The process of keratinization and the presence of specialized cells in each layer are essential for the skin's protective and sensory functions.