The Integumentary System: Structure, Function, and Clinical Aspects

The Integumentary System

Overview

The integumentary system is the body's largest organ system, primarily consisting of the skin and its derivatives. It serves as a protective barrier and plays vital roles in sensation, thermoregulation, and metabolic functions.

Main Components

• Skin

• Hair

• Nails

• Glands (sebaceous and sweat glands)

• Nerve endings

Functions of the Integumentary System

• Protection: Acts as a physical and chemical barrier to abrasion, invasion, dehydration, and ultraviolet (UV) light.

• Body Temperature Regulation: Produces sweat, which reduces body temperature through evaporative cooling.

• Sensation: Nerve endings detect touch, temperature, pressure, and pain.

• Excretion: Perspiration serves to eliminate bodily wastes.

• Metabolic Functions: Synthesis of vitamin D occurs in the epidermis upon exposure to UV light.

• Blood Reservoir: The dermis is extensively vascular and stores large volumes of blood until needed elsewhere.

Structure of the Skin

The skin consists of two distinct regions:

• Epidermis: Superficial, avascular, composed of epithelial tissue.

• Dermis: Deeper, vascular, composed of connective tissue.

The subcutaneous layer (hypodermis) lies just deep to the skin, anchoring it to underlying muscles, storing fat, and containing nerve endings sensitive to pressure. It is not considered part of the skin but shares some functions.

Epidermis

Cells of the Epidermis

• Keratinoctyes: Produce the fibrous protein keratin, which gives the skin its waterproof, protective properties. Make up most of the epidermis. Surface keratinocytes are dead and millions fall off every day.

• Melanocytes: Located in the deepest epidermis. Produce the pigment melanin, the primary contributor to skin color. Melanin is packaged into melanosomes, which are transferred to keratinocytes and help protect their nuclei from UV damage.

• Epidermal Dendritic (Langerhans) Cells: Patrol the deep epidermis as part of the immune system.

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

Layers of the Epidermis

• Stratum Basale (Basal Layer): Deepest layer, consists of a single layer of stem cells that move towards the surface to replace dead cells. Contains about 10-25% melanocytes, actively producing new cells.

• Stratum Spinosum (Prickly Layer): Several cell layers thick, made of spiny-looking prickle cells, beginning to produce keratin. Also find scattered dendritic cells.

• Stratum Granulosum (Granular Layer): Four to six layers of flattened cells. Cells begin to accumulate keratin granules.

• Stratum Lucidum (Clear Layer): Only present in thick skin (palms, soles). Thin, translucent band of dead keratinocytes.

• Stratum Corneum (Horny Layer): Accounts for three-quarters of epidermal thickness. Consists of dead, flattened, keratinized cells that provide a durable overcoat for the body.

Dermis

Structure and Cells

• Composed of strong, flexible connective tissue.

• Cells include fibroblasts, macrophages, occasional mast cells, and white blood cells.

• Contains nerves, blood vessels, and lymphatic vessels.

• Contains hair follicles, oil glands, and sweat glands.

Layers of the Dermis

• Papillary Layer: Made of areolar connective tissue with loose collagen and elastic fibers, and blood vessels. Contains dermal papillae, which house capillaries, nerve endings, and touch receptors. In thick skin, dermal papillae lie on top of dermal ridges, giving rise to fingerprints.

• Reticular Layer: Makes up 80% of dermal thickness. Consists of dense, irregular connective tissue, providing strength and resiliency. Contains collagen fibers that bind water, keeping skin hydrated. Cutaneous plexus (network of blood vessels) lies between this layer and the hypodermis. Extracellular matrix contains pockets of adipose cells. Cleavage lines in the reticular layer are caused by collagen fibers running parallel to the skin surface, important for surgical incisions.

Skin Color

Pigments Contributing to Skin Color

• Melanin: Only pigment made in skin, produced by melanocytes in the stratum basale. Packaged into melanosomes and sent to keratinocytes to shield DNA from sunlight. Two forms: reddish-yellow to brownish-black. All humans have similar numbers of melanocytes; differences in skin color are due to amount and form of melanin produced. Freckles and pigmented moles are local accumulations of melanin.

• Carotene: Yellow to orange pigment, most obvious in palms and soles. Can be converted to vitamin A for vision and epidermal health.

• Hemoglobin: Pinkish hue of fair skin is due to lower levels of melanin, allowing color of oxygenated hemoglobin in blood to show through.

Skin color is influenced by the relative amounts of these three pigments and by genetics and environmental factors.

Hair

Structure and Function

• Consists of dead, keratinized cells.

• Functions: Warns of insects on skin, guards scalp against physical trauma, heat loss, and sunlight.

• Regions: Shaft (above skin, keratinization complete), Root (within skin, keratinization ongoing).

Structure of a Hair Follicle

• Extends from epidermal surface to dermis.

• Hair bulb: Expanded area at deep end of follicle.

• Hair follicle receptor (root hair plexus): Sensory nerve endings that wrap around bulb, acting as touch receptors.

• Arrector pili muscle: Smooth muscle attached to follicle, responsible for "goose bumps."

• Hair papilla: Dermal tissue containing capillaries that supply nutrients to growing hair.

Nails

• Protective cover for distal, dorsal surface of fingers and toes.

• Composed of hard keratin.

• Structure: Free edge, nail plate, root, nail bed, nail matrix (responsible for growth), nail folds, eponychium (cuticle), hyponychium (area under free edge).

• Nails appear pink due to underlying capillaries; thickened nail matrix appears as a white crescent (lunule).

Glands of the Skin

Sebaceous (Oil) Glands

• Widely distributed, usually associated with hair follicles.

• Secrete sebum (an oily substance) that lubricates skin and hair, prevents water loss, and has antibacterial properties.

Sudoriferous (Sweat) Glands

• Two main types:

• Eccrine (Merocrine) Sweat Glands: Most numerous, ducts connect to pores, function in thermoregulation, secrete sweat (99% water, salts, vitamin C, antibodies, dermcidin, metabolic wastes).

• Apocrine Sweat Glands: Confined to axillary and anogenital areas, secrete viscous milky or yellowish sweat containing fatty substances and proteins, larger than eccrine glands, ducts empty into hair follicles.

• Modified apocrine glands include ceruminous glands (earwax) and mammary glands (milk).

Skin Cancer

• Most skin tumors are benign (do not spread), but some are malignant (cancerous).

• Risk factors include overexposure to UV radiation and frequent irritation of the skin.

• Some skin tumors contain enzymes that can repair damaged DNA.

Major Types of Skin Cancer

1. Basal Cell Carcinoma: Most common, least malignant. Stratum basale cells proliferate and invade dermis and hypodermis. Cured by surgical excision in 99% of cases.

2. Squamous Cell Carcinoma: Second most common, can metastasize. Involves keratinocytes of stratum spinosum. Usually a scaly, reddened papule on scalp, ears, lower lip, or hands. Good prognosis if treated by radiation therapy or removed surgically.

3. Melanoma: Cancer of melanocytes, most dangerous due to high metastatic rate and resistance to chemotherapy. Treated by wide surgical excision and immunotherapy. Key to survival is early detection using the ABCD rule:

   • Asymmetry: Two sides of the pigmented area do not match.

   • Border irregularity: Exhibits indentations.

   • Color: Contains several colors (black, brown, tan, sometimes red or blue).

   • Diameter: Larger than 6 mm (size of a pencil eraser).

Burns

• Damage caused by denaturation of proteins, which destroys cells.

• Immediate threat is dehydration and electrolyte imbalance, leading to renal shutdown and circulatory shock.

• The "Rule of Nines" is used to estimate the volume of fluid loss by dividing the body into 11 sections, each representing 9% of body surface area.

Classification of Burns

• First-degree: Localized redness, edema (swelling), and pain.

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

• Third-degree: Entire thickness of skin involved; skin color is gray-white, cherry red, or blackened. No edema or pain (nerve endings destroyed). Skin grafting usually required.

Critical Burns

• More than 25% of body has second-degree burns.

• More than 10% of body has third-degree burns.

• Face, hands, or feet bear third-degree burns.

Treatment of Burns

• Debridement (removal) of burned skin.

• Temporary covering.

• Skin grafts.