The Integumentary System – Structure, Function, and Clinical Aspects

The Integumentary System

Overview

The integumentary system is composed of the skin and its associated structures, including hair, nails, and glands. It serves as the body's primary barrier against environmental hazards and plays a vital role in protection, sensation, thermoregulation, and metabolic functions.

Skin Structure and Layers

Main Layers of Skin

• Epidermis: Outermost layer, composed of stratified squamous epithelium. Provides protection and is involved in sensation.

• Dermis: Middle layer, contains connective tissue, blood vessels, nerves, and accessory structures such as hair follicles and glands.

• Subcutaneous Tissue (Hypodermis): Deepest layer, composed mainly of adipose tissue, providing insulation and cushioning.

Hair

Structure and Function

• Hair consists of dead keratinized cells.

• Found on most skin surfaces except palms, soles, lips, nipples, and parts of external genitalia.

• Functions:

  1. Warn of insects on skin

  2. Protect head against physical trauma

  3. Shield skin from heat loss

  4. Shield skin from sunlight

Structure of a Hair

• Pili: Flexible strands of dead, keratinized cells.

• Produced by hair follicles.

• Contains hard keratin, which is tougher and more durable than soft keratin found in skin.

• Cells do not flake off.

Parts of Hair

• Root: Area within scalp where keratinization is ongoing.

• Shaft: Area where keratinization is complete.

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

• Cortex: Several layers of flattened cells surrounding medulla.

• Cuticle: Outer layer consisting of overlapping layers of single cells.

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; considered a sensory touch receptor.

• Wall of follicle composed of:

  • Peripheral connective tissue sheath (from dermis)

  • Glassy membrane (thickened basal lamina)

  • Epithelial root sheath (from epidermis)

• Arrector pili: Small band of smooth muscle attached to follicle; responsible for "goose bumps".

• Hair papilla: Dermal tissue containing a knot of capillaries that supplies nutrients to growing hair.

Types and Growth of Hair

• Terminal hair: Coarse, long hair found on scalp and eyebrows; appears at puberty in axillary and pubic regions, face and neck of males.

• Follicles cycle between active and regressive phases; average growth rate is 2.25 mm/week; lose about 90 scalp hairs daily.

• Alopecia: Hair thinning in both sexes after age 40–65; terminal hairs are replaced by vellus hairs.

• True (frank) baldness: Genetically determined and sex-influenced; male pattern baldness caused by follicular response to DHT (dihydrotestosterone).

Nails

Structure and Function

• Scale-like modifications of epidermis containing hard keratin.

• Act as protective covers for distal, dorsal surfaces of fingers and toes.

• Consist of free edge, nail plate, and root.

• Nail bed is epidermis underlying keratinized nail plate.

• Nail matrix: Thickened portion of bed responsible for nail growth.

• Nail folds: Skin folds that overlap border of nail.

• Eponychium: Nail fold that projects onto surface of nail body (also called cuticle).

• Lunule: Thickened nail matrix, appears white.

• Hyponychium: Area under free edge of plate that accumulates dirt.

Clinical Relevance

• Nail appearance can help diagnose diseases:

  • Yellow nails: May indicate respiratory or thyroid gland disorder.

  • Thickened yellow nails can be due to fungal infection.

  • Beau's lines: Horizontal lines may indicate severe illnesses (e.g., uncontrolled diabetes, heart attack, cancer chemotherapy).

Cutaneous Glands

Sweat Glands (Sudoriferous Glands)

• All skin surfaces except nipples and parts of external genitalia contain sweat glands (about 3 million per person).

• Two main types:

  1. Eccrine (merocrine) sweat glands: Most numerous; abundant on palms, soles, forehead. Ducts connect to pores. Function in thermoregulation, regulated by sympathetic nervous system. Secrete sweat (99% water, salts, vitamin C, antibodies, dermcidin, metabolic wastes).

  2. Apocrine sweat glands: Secrete viscous milky or yellowish sweat containing fatty substances and proteins. Bacteria break down sweat, leading to body odor. Larger than eccrine glands, ducts empty into hair follicles. Begin functioning at puberty; may act as sexual scent glands.

• Modified apocrine glands:

  • Ceruminous glands: Lining of external ear canal; secrete cerumen (earwax).

  • Mammary glands: Secrete milk.

Sebaceous (Oil) Glands

• Widely distributed, except for thick skin of palms and soles.

• Most develop from hair follicles and secrete into hair follicles.

• Relatively inactive until puberty; stimulated by hormones (especially androgens).

• Secrete sebum (oily, holocrine secretion) that softens hair and skin and has bactericidal properties.

Table: Summary of Cutaneous Glands

Gland Type	Location	Secretion	Function
Eccrine (Merocrine) Sweat	All skin except nipples/genitalia	Sweat (99% water, salts, etc.)	Thermoregulation
Apocrine Sweat	Axillary/anogenital regions	Viscous sweat (proteins, fatty substances)	Body odor, sexual scent
Ceruminous	Ear canal	Cerumen (earwax)	Protects ear
Mammary	Breasts	Milk	Nourishment
Sebaceous (Oil)	Most skin, except palms/soles	Sebum (oil)	Lubricates skin/hair, bactericidal

Functions of Skin

Protection

• Skin is the first and foremost barrier against environmental hazards.

• Main functions include:

  1. Protection

  2. Body temperature regulation

  3. Cutaneous sensations

  4. Metabolic functions

  5. Blood reservoir

  6. Excretion

Barriers Provided by Skin

• Chemical barrier:

  • Sweat contains antimicrobial proteins.

  • Cells secrete antimicrobial defensin.

  • Acid mantle (low pH) retards bacterial multiplication.

  • Melanin provides a chemical barrier against UV radiation damage.

• Physical barrier:

  • Flat, dead, keratinized cells of stratum corneum block most water and water-soluble substances.

  • Some chemicals have limited penetration (e.g., organic solvents, heavy metals).

• Biological barrier:

  • Epidermis contains phagocytic cells (dendritic cells) that engulf foreign antigens and activate immune response.

  • Dermis contains macrophages that activate immune system.

  • DNA can absorb harmful UV radiation, converting it to harmless heat.

Body Temperature Regulation

• Under normal conditions, sweat glands produce about 500 ml/day of unnoticeable sweat.

• If body temperature rises, dermal vessels dilate and increase sweat gland activity, producing up to 3 gallons of noticeable sweat (sensible perspiration) to cool the body.

• In cold environments, dermal blood vessels constrict, reducing heat loss.

Cutaneous Sensations

• Sensory receptors are part of the nervous system.

• Epidermis contains receptors for touch, temperature, and pain.

• Free nerve endings sense painful stimuli.

Metabolic Functions

• Skin synthesizes vitamin D, needed for calcium absorption in the intestine.

• Chemicals from keratinocytes can disarm some carcinogens.

• Keratinoctyes can activate some hormones (e.g., convert cortisone into hydrocortisone).

• Collagenase produced by skin aids in natural turnover of collagen to prevent wrinkles.

Blood Reservoir

• Skin can hold up to 5% of the body's total blood volume.

• Blood can be shunted from skin to other organs as needed.

Excretion

• Skin excretes limited amounts of nitrogenous wastes (ammonia, urea, uric acid).

• Sweating causes loss of salt and water.

Developmental Aspects of the Integumentary System

Fetal Development

• By end of 4th month, skin of fetus is developed.

• Lanugo coat: Delicate hairs in 5th and 6th month.

• Vernix caseosa: Sebaceous gland secretion that protects fetal skin in amniotic fluid.

Infancy to Adulthood

• Skin thickens and accumulates more subcutaneous fat; sweat and sebaceous gland activity increases, leading to acne.

• Optimal appearance during 20s and 30s.

• After age 30, cumulative environmental assaults begin to show; scaling and dermatitis become more common.

Aging Skin

• Epidermal replacement slows; skin becomes thin, dry, and itchy.

• Decreased sebaceous gland activity.

• Subcutaneous fat and elasticity decrease due to reduced numbers of melanocytes and dendritic cells.

• Increased risk of cancer.

• Hair thinning.

Ways to Delay Aging

• Healthy diet

• Regular exercise

• Sun protection

• Avoid smoking

Clinical Connections

• Acne: Infectious inflammation of sebaceous glands, resulting in pimples; associated with Propionibacterium acnes infection.

• Whiteheads: Blocked sebaceous glands.

• Blackheads: Oxidized secretion in blocked sebaceous glands.

• Cradle cap: Overactive sebaceous glands in infants, leading to scaly scalp lesions.

Homeostatic Interrelationships

The integumentary system interacts with other body systems to maintain homeostasis, including immune defense, temperature regulation, and metabolic functions. (See textbook page 168 for details.)

Additional info: Academic context and explanations have been expanded for clarity and completeness. Table 5.1 has been recreated based on standard textbook content.