The Integumentary System

Overview

The integumentary system is the body's largest organ system, serving as a protective barrier and playing vital roles in sensation, thermoregulation, and metabolic processes. It consists of the skin and its derivatives, including sweat glands, oil glands, hair, and nails.

• Skin: The primary organ, composed of two main layers (epidermis and dermis) and an underlying subcutaneous layer.

• Sudoriferous (sweat) glands: Involved in thermoregulation and excretion.

• Sebaceous (oil) glands: Secrete sebum for lubrication and protection.

• Hairs and Nails: Protective and sensory structures.

• Subcutaneous tissue (hypodermis): Anchors skin, provides insulation and shock absorption.

Skin Structure

Layers of the Skin

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

• Epidermis: Outermost, avascular, composed of keratinized stratified squamous epithelium.

• Dermis: Underlies the epidermis, made of dense connective tissue, vascular, contains nerves and appendages.

• Subcutaneous tissue (Hypodermis): Not part of the skin proper, but supports skin function; mainly adipose tissue.

Cells of the Epidermis

• Keratinocytes: Produce keratin, provide protection, most abundant.

• Melanocytes: Produce melanin pigment, protect against UV radiation.

• Dendritic (Langerhans) cells: Immune defense, patrol deep epidermis.

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

Layers (Strata) of the Epidermis

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

• Stratum basale: Single row of mitotic stem cells; contains melanocytes and tactile cells.

• Stratum spinosum: Several layers of keratinocytes; contains melanosomes and dendritic cells.

• Stratum granulosum: Four to six layers; keratinization begins, cells flatten, organelles disintegrate.

• Stratum lucidum: Only in thick skin; thin, clear layer of dead keratinocytes.

• Stratum corneum: 20–30 layers of dead, keratinized cells; provides barrier function.

The Dermis

Structure and Function

The dermis is a strong, flexible connective tissue layer housing blood vessels, nerves, hair follicles, and glands. It is divided into two layers:

• Papillary dermis: Superficial, areolar connective tissue; contains dermal papillae with capillaries, nerve endings, and tactile corpuscles.

• Reticular dermis: Deep, dense irregular connective tissue; provides strength, elasticity, and houses most skin appendages.

Papillary Dermis and Friction Ridges

• Dermal papillae: Fingerlike projections into the epidermis, increase surface area for exchange and grip.

• Friction ridges: In thick skin, dermal papillae overlie dermal ridges, forming fingerprints that enhance grip and tactile sensation.

Reticular Dermis and Skin Lines

• Cleavage (tension) lines: Formed by collagen fiber orientation; important for surgical incisions.

• Flexure lines: Dermal folds at joints, allow skin to accommodate movement without damage.

Determinants of Skin Color

Pigments

• Melanin: Only skin-made pigment; shields DNA from UV damage; more sun exposure increases melanin production.

• Carotene: Yellow-orange pigment, accumulates in stratum corneum and hypodermis; can be converted to vitamin A.

• Hemoglobin: Red pigment in blood; gives fair skin a pinkish hue due to low melanin levels.

Freckles and moles are local accumulations of melanin.

Hair

Structure and Function

Hair (pili) consists of dead, keratinized cells and serves protective and sensory roles.

• Regions: Root (within scalp, active keratinization) and shaft (above scalp, keratinization complete).

• Parts of hair shaft: Medulla (core), cortex (middle), cuticle (outer layer).

• Pigmentation: Determined by melanins; red hair contains pheomelanin; gray/white hair results from decreased melanin and air bubbles.

Hair Follicle Structure

• Hair follicle: Extends from epidermis to dermis; contains hair bulb, root hair plexus (sensory), and matrix (growth zone).

• Arrector pili: Smooth muscle causing "goose bumps."

Types and Growth of Hair

• Vellus hair: Fine, pale body hair of children and adult females.

• Terminal hair: Coarse, long hair (scalp, eyebrows, axillary, pubic regions, face/neck in males).

• Growth cycles alternate between active and resting phases; nutrition and hormones influence growth.

Nails

Structure and Function

• Nails: Scale-like modifications of the epidermis, contain hard keratin, protect distal phalanges.

• Parts: Root (embedded), nail plate/body (visible), free edge, nail bed (under plate), nail matrix (growth zone).

• Nail folds: Overlap nail borders; cuticle (eponychium) and hyponychium (under free edge).

• Lunule: White crescent at base, thickened matrix.

Abnormal nail color or shape may indicate disease.

Skin Appendages: Glands

Sudoriferous (Sweat) Glands

• Eccrine (merocrine) glands: Most numerous, abundant on palms, soles, forehead; function in thermoregulation; secrete watery sweat (99% water, salts, wastes, dermcidin).

• Apocrine glands: Axillary and anogenital areas; secrete viscous, milky sweat into hair follicles; function may include sexual scent.

• Modified apocrine glands: Ceruminous (earwax) and mammary (milk) glands.

Sebaceous (Oil) Glands

• Widely distributed, except on palms and soles; secrete sebum (oily, bactericidal, softens hair/skin).

• Most develop from hair follicles; activity increases at puberty (stimulated by androgens).

Functions of the Skin

Protection

• Chemical barriers: Sweat (antimicrobial), sebum, acid mantle, melanin (UV protection).

• Physical barriers: Keratinized cells and glycolipids block water and many substances.

• Biological barriers: Dendritic cells (epidermis) and macrophages (dermis) activate immune responses.

Body Temperature Regulation

• Insensible perspiration (500 mL/day at rest); sensible perspiration (up to 12 L/day during heat).

• Vasodilation increases heat loss; vasoconstriction conserves heat.

Cutaneous Sensations

• Touch: Meissner's corpuscles, tactile epithelial cells.

• Pressure: Pacinian (lamellar) corpuscles.

• Pain: Free nerve endings.

• Hair movement: Hair follicle receptors.

Metabolic Functions

• Synthesizes vitamin D for calcium absorption.

• Keratinocytes activate hormones and disarm carcinogens.

• Produces collagenase for collagen turnover.

Blood Reservoir and Excretion

• Holds up to 5% of blood volume; can constrict to redirect blood.

• Excretes nitrogenous wastes (ammonia, urea, uric acid) and salts via sweat.

Clinical Aspects: Skin Cancer and Burns

Skin Cancer

• Basal cell carcinoma: Most common, least malignant; arises from stratum basale; high cure rate with excision.

• Squamous cell carcinoma: Arises from keratinocytes of stratum spinosum; can metastasize; good prognosis if treated early.

• Melanoma: Cancer of melanocytes; most dangerous, highly metastatic, resistant to chemotherapy; early detection critical (ABCD rule: Asymmetry, Border, Color, Diameter).

Burns

• First-degree: Epidermal damage only; redness, pain, swelling.

• Second-degree: Epidermal and upper dermal damage; blisters form.

• Third-degree: Full-thickness burn; skin gray-white, cherry red, or blackened; not painful (nerve endings destroyed); requires grafting.

Burns are critical if >25% of body has second-degree burns, >10% has third-degree burns, or face/hands/feet are involved. Treatment includes debridement, antibiotics, temporary coverings, and skin grafts.

Summary Table: Cutaneous Glands