The Integumentary System: Structure and Function

The Integumentary System

Overview of the Skin and Hypodermis

The integumentary system consists of the skin and its derivatives, including hair, nails, and glands. The skin is the largest organ of the body, accounting for approximately 7% of total body weight and varying in thickness from 1.5 to 4.4 mm. It is composed of two main layers: the epidermis (superficial) and the dermis (deep). Beneath the dermis lies the hypodermis (subcutaneous tissue), which is not technically part of the skin but shares some of its properties.

Functions of the Skin and Hypodermis

• Protection: Cushions organs and protects against mechanical injury, chemicals, water loss, and ultraviolet (UV) radiation.

• Body Temperature Regulation: Capillary networks and sweat glands help regulate heat loss.

• Excretion: Sweat glands excrete urea, salts, and water.

• Vitamin D Production: Epidermal cells use UV radiation to synthesize vitamin D, essential for calcium metabolism.

• Sensory Reception: Contains sensory receptors for touch, pain, and temperature.

Structure of the Skin

Epidermis

The epidermis is a keratinized stratified squamous epithelium composed of four main cell types:

• Keratinocytes: Most abundant; produce keratin, a tough fibrous protein. Located mainly in the stratum spinosum.

• Melanocytes: Located in the basal layer; synthesize and secrete the pigment melanin.

• Tactile Epithelial Cells: Found in the basal layer; associated with sensory nerve endings.

• Dendritic Cells: Located in the stratum spinosum; part of the immune system, acting as macrophage-like cells.

Layers of the Epidermis

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

• Stratum Basale (Germinativum): Deepest layer; a single row of actively mitotic stem cells. Contains melanocytes and tactile epithelial cells.

• Stratum Spinosum: Several layers of keratinocytes unified by desmosomes; contains dendritic cells.

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

• Stratum Lucidum: Present only in thick skin (palms, soles); a few rows of flat, dead keratinocytes.

• Stratum Corneum: Most superficial; 20–30 layers of dead, keratinized cells providing protection.

Dermis

Structure and Layers of the Dermis

The dermis is a strong, flexible connective tissue layer richly supplied with blood vessels and nerves. It is divided into two layers:

• Papillary Dermis: Superficial 20% of the dermis; composed of areolar connective tissue. Contains dermal papillae, which increase surface area for exchange of gases, nutrients, and wastes. Dermal papillae lie atop dermal ridges, forming epidermal ridges (fingerprints) that enhance gripping ability.

• Reticular Dermis: Deeper 80% of the dermis; composed of dense irregular connective tissue. Contains thick bundles of collagen fibers for strength and resilience, as well as cleavage lines (separations between collagen bundles) and flexure lines (deep skin creases at joints).

Vascular and Nerve Supply

• Dermal Plexus: Deep network of blood vessels between the hypodermis and dermis.

• Subpapillary Plexus: More superficial network just below the dermal papillae.

• These vascular plexuses play a key role in temperature regulation and nutrient delivery.

Hypodermis (Subcutaneous Layer)

The hypodermis lies deep to the dermis and is composed of areolar and adipose connective tissue. It anchors the skin to underlying structures (such as muscle), insulates the body, and serves as an energy reservoir. The distribution of adipose tissue varies between males and females.

Appendages of the Skin

Nails

Nails are scalelike modifications of the epidermis made of hard keratin. They consist of the free edge, nail plate, root, nail folds, and the eponychium (cuticle). The nail matrix is responsible for nail growth.

Hair and Hair Follicles

Hair is a flexible strand of dead, keratinized cells. It consists of:

• Root: Embedded in the skin.

• Shaft: Projects above the skin surface.

Hair has three concentric layers:

• Medulla: Central core.

• Cortex: Surrounds the medulla.

• Cuticle: Outermost layer.

Hair follicles extend from the epidermis into the dermis. The hair bulb is the deep, expanded end of the follicle, surrounded by a root plexus (sensory nerve endings). The wall of the follicle consists of a peripheral connective tissue sheath, a glassy membrane, and an epithelial root sheath. The arrector pili muscle (smooth muscle) attaches to the follicle and causes hair to stand erect when contracted.

Glands of the Skin

• Sebaceous (Oil) Glands: Occur over most of the body except palms and soles. Secrete sebum (an oily substance) via holocrine secretion. Sebum lubricates and waterproofs the skin and hair, and has antibacterial properties.

• Sudoriferous (Sweat) Glands: Widely distributed. Two main types:

  • Eccrine Glands: Most numerous; produce true sweat (99% water, some salts, metabolic wastes).

  • Apocrine Glands: Confined to axillary, anal, and genital areas; produce a thicker, musky sweat. Modified apocrine glands include ceruminous (earwax) and mammary (milk) glands.

Clinical Considerations

Burns

Burns are classified by severity:

• First-degree: Only the upper epidermis is damaged; redness and pain, no blisters.

• Second-degree: Upper part of the dermis is also damaged; blisters appear, minimal scarring.

• Third-degree: Full thickness of skin is consumed; area appears white, red, or blackened.

The rule of nines is used to estimate the extent of burns in adults.

Skin Cancer

Three main types of skin cancer:

• Basal Cell Carcinoma: Least malignant and most common; arises from cells in the stratum basale.

• Squamous Cell Carcinoma: Arises from keratinocytes of the stratum spinosum; can metastasize if not removed.

• Melanoma: Cancer of melanocytes; most dangerous due to rapid metastasis.

Summary Table: Layers of the Epidermis

Layer	Main Features	Key Cell Types
Stratum Corneum	20–30 layers of dead, keratinized cells; protection	Keratinocytes (dead)
Stratum Lucidum	Only in thick skin; few rows of flat, dead cells	Keratinocytes (dead)
Stratum Granulosum	1–5 layers; keratohyaline and lamellar granules	Keratinocytes
Stratum Spinosum	Several layers; desmosomes; immune defense	Keratinocytes, Dendritic cells
Stratum Basale	Deepest; mitotic stem cells; pigment production	Keratinocytes, Melanocytes, Tactile cells