Chapter 5: The Integumentary System – Structure, Function, and Clinical Aspects

The Integumentary System

Overview of Skin Structure

The skin is the largest organ of the body, providing a protective barrier and performing multiple essential functions. It consists of three main layers: the epidermis, dermis, and hypodermis (subcutaneous layer). Each layer has distinct structural and functional characteristics.

• Epidermis: The superficial region composed of epithelial tissue; avascular.

• Dermis: Underlies the epidermis; mostly fibrous connective tissue; vascular.

• Hypodermis: Subcutaneous layer deep to skin; not part of skin but shares some functions; mostly adipose tissue for shock absorption and insulation.

Cells of the Epidermis

The epidermis is primarily made of keratinized stratified squamous epithelium and contains four main cell types, each with specialized functions:

• Keratinocytes: Produce keratin, a protein that provides protective properties; tightly connected by desmosomes; major cell type of the epidermis.

• Melanocytes: Located in the deepest epidermis; produce melanin pigment, which protects keratinocyte nuclei from UV damage.

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

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

Layers of the Epidermis

The epidermis is organized into four or five distinct layers (strata), depending on skin thickness. Thick skin (hands, feet) contains five layers; thin skin contains four.

• Stratum basale: Deepest layer; single row of stem cells; site of mitosis.

• Stratum spinosum: Several layers of keratinocytes unified by desmosomes; contains pre-keratin filaments.

• Stratum granulosum: One to five layers of flattened cells; organelles deteriorate; keratinization begins.

• Stratum lucidum: Only in thick skin; thin, translucent band.

• Stratum corneum: Most superficial layer; 20–30 layers of dead cells; provides durable protection.

The Dermis

Structure and Function

The dermis is a strong, flexible connective tissue layer containing fibroblasts, macrophages, mast cells, and white blood cells. It houses nerves, blood vessels, lymphatic vessels, hair follicles, oil glands, and sweat glands.

• Papillary layer: Superficial region; contains dermal papillae with capillary loops, free nerve endings, and touch receptors (Meissner’s corpuscles).

• Reticular layer: Deep layer; makes up ~80% of dermal thickness; dense fibrous connective tissue with elastic and collagen fibers for strength, resiliency, and hydration.

Dermal Papillae and Friction Ridges

Dermal papillae project into the epidermis, forming friction ridges in thick skin. These ridges enhance gripping ability and contribute to fingerprints.

Cleavage and Flexure Lines

Collagen fibers in the reticular layer create cleavage (tension) lines, which are important for surgical incisions. Flexure lines are dermal folds at or near joints, visible on hands and fingers.

Damage to the Dermis

Extreme stretching can cause dermal tears, resulting in striae (stretch marks). Acute trauma may cause blisters, which are fluid-filled pockets separating epidermal and dermal layers.

Skin Color

Pigments Contributing to Skin Color

Three pigments contribute to skin color:

• Melanin: Produced by melanocytes; shields DNA from UV; amount and form determine skin color.

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

• Hemoglobin: Pinkish hue in fair skin due to transparency and low melanin.

Clinical Color Changes

• Cyanosis: Blue skin color due to low oxygenation.

• Pallor: Pale color from anemia, low blood pressure, fear, or anger.

• Erythema: Redness from fever, hypertension, inflammation, or allergy.

• Jaundice: Yellow cast from liver disorders.

• Bruises: Black-and-blue marks from clotted blood beneath skin.

• Hyperpigmentation: Dark areas may indicate insulin resistance.

Hair

Structure and Function

Hair consists of dead, keratinized cells and is absent from palms, soles, lips, nipples, and parts of external genitalia. Functions include protection, sensory input, and regulation of heat loss.

• Shaft: Extends above scalp; keratinization complete.

• Root: Within scalp; keratinization ongoing.

• Medulla: Central core of large cells and air spaces.

• Cortex: Layers of flattened cells surrounding medulla.

• Cuticle: Outermost layer of overlapping single cells.

Hair Follicle and Growth

Hair follicles extend from the epidermal surface to the dermis. The hair bulb contains the matrix (site of cell division), root hair plexus (sensory nerve endings), and hair papilla (nutrient supply).

• Vellus hair: Fine, pale body hair.

• Terminal hair: Coarse, long hair on scalp, eyebrows, and at puberty in axillary/pubic regions.

• Hair growth: Influenced by nutrition and hormones; follicles cycle between active and regressive phases.

• Alopecia: Hair thinning after age 40.

• True baldness: Genetically determined; male pattern baldness due to DHT.

Nails

Structure and Clinical Significance

Nails are scale-like modifications of the epidermis containing hard keratin. They protect the distal, dorsal surface of fingers and toes and consist of the free edge, nail plate, and root. The nail matrix is responsible for growth.

• Yellow-tinged nails: May indicate respiratory or thyroid disorders.

• Thickened yellow nails: Often due to fungal infection.

• Koilonchya (spoon nail): Outward concavity; may signal iron deficiency.

• Beau’s lines: Horizontal lines; may indicate severe illness.

Glands of the Skin

Sweat Glands

Sweat glands (sudoriferous glands) are distributed throughout the skin except for nipples and parts of external genitalia. There are two main types:

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

• Apocrine glands: Confined to axillary and anogenital areas; secrete viscous sweat containing fatty substances and proteins; responsible for body odor; begin functioning at puberty.

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

Sebaceous (Oil) Glands

Sebaceous glands are widely distributed except for thick skin. They secrete sebum, an oily holocrine secretion with bactericidal properties, which softens hair and skin. Overactivity can lead to seborrhea (cradle cap) in infants and acne in adolescents.

Functions of Skin

Major Functions

• Protection: Physical, chemical, and biological barrier.

• Body temperature regulation: Via sweat glands and blood flow.

• Cutaneous sensations: Touch, pain, temperature.

• Metabolic functions: Synthesis of vitamin D.

• Blood reservoir: Stores blood in dermal vessels.

• Excretion: Removal of wastes via sweat.

Skin Cancer

Types and Risk Factors

Most skin tumors are benign, but overexposure to UV radiation and frequent skin irritation increase cancer risk. Three major types:

• Basal cell carcinoma

• Squamous cell carcinoma

• Melanoma: Cancer of melanocytes; highly metastatic and resistant to chemotherapy. Early detection is critical (ABCD rule: Asymmetry, Border irregularity, Color, Diameter).

Burns

Classification and Clinical Management

Burns are tissue damage caused by heat, electricity, radiation, or chemicals. The immediate threat is dehydration and electrolyte imbalance, which can lead to renal shutdown and circulatory shock. The Rule of Nines is used to estimate fluid loss.

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

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

• Third-degree: Entire thickness of skin; gray-white, cherry red, or blackened; no pain (nerve endings destroyed); skin grafting usually necessary.

Skin Development and Aging

Fetal to Adult Skin

• Fetal: Skin developed by end of 4th month; lanugo coat (delicate hairs) in 5th/6th month; vernix caseosa (sebaceous secretion) protects skin.

• Infancy to adulthood: Skin thickens, accumulates more subcutaneous fat; gland activity increases, leading to acne.

• Aging: Epidermal replacement slows; skin becomes thin, dry, and itchy; subcutaneous fat and elasticity decrease; increased risk of cancer; hair thinning.

• Prevention: UV protection, good nutrition, hydration, hygiene.