The Integumentary System: Structure and Function

The Integumentary System

Overview of the Integumentary System

The integumentary system is the body's largest organ system, providing protection and several vital functions. It consists of the skin and its accessory structures.

• Components: Skin, hair, nails, sweat (sudoriferous) glands, and sebaceous (oil) glands.

• Main Functions: Protection, sensation, temperature regulation, metabolic functions, blood reservoir, and excretion.

Structure of the Skin

Layers of the Skin

The skin is composed of two main layers and an associated subcutaneous layer:

• Epidermis: The superficial region, made of keratinized stratified squamous epithelial tissue.

• Dermis: Lies beneath the epidermis, composed of strong, flexible connective tissue.

• Hypodermis (Superficial Fascia): A subcutaneous layer deep to the skin, primarily made of adipose tissue. It absorbs shock, insulates, and anchors skin to underlying structures, mainly muscles. Additional info: The hypodermis is not technically part of the skin but is often discussed with it due to its close association.

Cells of the Epidermis

Major Cell Types

The epidermis contains several specialized cell types, each with distinct functions:

• Keratinocytes: Produce fibrous keratin, a protein that provides protective properties. These are the most abundant cells in the epidermis and are tightly connected by desmosomes. Millions of keratinocytes slough off daily.

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

• Dendritic (Langerhans) Cells: Star-shaped macrophages that patrol the deep epidermis and are key activators of the immune system.

• Tactile (Merkel) Cells: Sensory receptors that sense touch.

Layers of the Epidermis

Strata of the Epidermis

The epidermis is organized into four or five distinct layers (strata), depending on the region of the body:

• Thick skin (palms, soles): Five layers

• Thin skin (rest of body): Four layers (lacks stratum lucidum)

The layers, from deep to superficial, are:

1. Stratum basale (basal layer)

2. Stratum spinosum (prickly layer)

3. Stratum granulosum (granular layer)

4. Stratum lucidum (clear layer; only in thick skin)

5. Stratum corneum (horny layer)

Stratum Basale (Basal Layer)

The deepest epidermal layer, firmly attached to the dermis. It consists of a single row of stem cells that actively divide (mitosis), producing two daughter cells each cycle. One cell migrates toward the surface, dying as it moves upward, while the other remains as a stem cell. This layer is also called the stratum germinativum due to its mitotic activity. About 10-25% of the layer is composed of melanocytes.

Stratum Spinosum (Prickly Layer)

This layer is several cell layers thick and contains a weblike system of intermediate prekeratin filaments attached to desmosomes, allowing cells to resist tension and pulling. Keratinocytes in this layer appear spiky. Melanin granules and dendritic cells are abundant here.

Stratum Granulosum (Granular Layer)

Consists of four to six flattened cell layers. In this layer, cell appearance changes: cells flatten, nuclei and organelles disintegrate, and keratinization begins. Cells accumulate keratohyaline granules (help form keratin in upper layers) and lamellar granules (water-resistant glycolipid that slows water loss). Cells above this layer die due to lack of nutrients.

Stratum Lucidum (Clear Layer)

Found only in thick skin, this thin, translucent band consists of two to three rows of clear, 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. These dead cells protect deeper layers from the environment, prevent water loss, and act as a barrier against abrasion, penetration, and biological, chemical, and physical assaults.

Summary Table: Layers of the Epidermis

Layer	Main Features	Location
Stratum basale	Single row of stem cells, mitotically active, contains melanocytes	Deepest layer, attached to dermis
Stratum spinosum	Several layers, spiny keratinocytes, desmosomes, dendritic cells	Above stratum basale
Stratum granulosum	4-6 layers, keratinization begins, lamellar granules	Middle layer
Stratum lucidum	2-3 rows, clear, dead keratinocytes	Only in thick skin (palms, soles)
Stratum corneum	20-30 rows, dead, keratinized cells, protective barrier	Outermost layer

Key Terms and Concepts

• Keratin: A tough, fibrous protein that provides protection and waterproofing to the skin.

• Melanin: A pigment produced by melanocytes that protects against UV radiation.

• Desmosomes: Specialized cell junctions that tightly connect keratinocytes.

• Apoptosis: Programmed cell death; occurs as keratinocytes move toward the surface.

Example: Clinical Relevance

• Sunburn: Excessive UV exposure damages DNA in skin cells, leading to increased melanin production as a protective response.

• Calluses: Thickening of the stratum corneum due to repeated friction or pressure.