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 the external environment and performs multiple essential functions.

Layers and Structure of the Skin

Major Layers

• Epidermis: The outermost layer, composed mainly of keratinized stratified squamous epithelium. Provides protection and is avascular.

• Dermis: The deeper, thicker layer made of connective tissue. Contains blood vessels, nerves, hair follicles, and glands.

• Subcutaneous tissue (hypodermis): Lies beneath the dermis, consisting mainly of adipose tissue. Functions in insulation and energy storage.

Hair

Structure and Function

• Composed of dead keratinized cells.

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

• Functions:

  • Warns of insects on skin

  • Hair on head guards against physical trauma

  • Protects from heat loss

  • Shields skin from sunlight

• Nerves: Muscle movement (innervation/contraction) and sensation

Regions of Hair

• Shaft: Area above the scalp where keratinization is complete

• Root: Area within the scalp where keratinization is ongoing

Structure of a Hair

• Medulla: Central core of large cells and air spaces

• Cortex: Several layers of flattened cells surrounding the medulla

• Cuticle: Outermost layer consisting of overlapping cells

Hair pigments are produced by melanocytes in hair follicles. Combinations of different melanins create various hair colors. Gray/white hair results from decreased melanin production and air bubbles in the shaft.

Structure of a Hair Follicle

• Extends from the epidermal surface to the dermis

• Hair bulb: Expanded deep end of follicle, contains sensory nerve endings (hair follicle 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 capillaries that supply nutrients to growing hair

Types and Growth of Hair

• Vellus hair: Pale, fine body hair of children and adult females

• Terminal hair: Coarse, long hair of scalp, eyebrows, and after puberty, axillary and pubic regions

• True (frank) baldness: Genetically determined and sex-influenced; often due to follicular response to dihydrotestosterone (DHT)

Nutrition and hormones affect hair growth. Follicles cycle between active and regressive phases. Average growth is about 2.25 mm per week; about 90 scalp hairs are lost daily.

Nails

Structure and Function

• Scale-like modifications of the epidermis containing hard keratin

• Protect distal, dorsal surfaces of fingers and toes

• Major parts:

  • Nail matrix: Responsible for nail growth

  • Nail bed: Underlies the nail plate

  • Lunule: Thickened, white, crescent-shaped area at the nail base

  • Hyponychium: Area under the free edge; accumulates dirt

• Nails appear pink due to underlying capillaries

Clinical - Homeostatic Imbalance

• Nail appearance can indicate disease:

  • Yellow-tinged nails: May signal respiratory or thyroid disorders

  • Thickened, yellow nails: Can indicate fungal infection

  • Spoon nail (koilonychia): Concave nail; may signal iron deficiency

  • Beau's lines: Horizontal lines; may indicate severe illness

Cutaneous Glands

Types of Glands

• Sudoriferous (sweat) glands: All skin surfaces except nipples and parts of external genitalia

  • About 3 million per person

  • Two main types:

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

    2. Apocrine sweat glands: Confined to axillary and anogenital areas. Secrete viscous, milky/yellowish sweat with fatty substances and proteins. Begin functioning at puberty; may act as sexual scent glands.

• Modified apocrine glands:

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

Summary Table: Cutaneous Glands

Gland Type	Location	Secretion	Main Function
Eccrine Sweat	All over body, especially palms, soles, forehead	Watery sweat (99% water, salts, etc.)	Thermoregulation
Apocrine Sweat	Axillary, anogenital areas	Viscous, milky/yellowish sweat	May act as sexual scent gland
Ceruminous	External ear canal	Cerumen (earwax)	Traps debris, protects ear
Mammary	Breasts	Milk	Nourishment for infants
Sebaceous	All over body except palms/soles	Sebum (oily)	Lubricates skin/hair, bactericidal

Functions of the Skin

Overview

The skin is a multifunctional organ that acts as a barrier and performs several vital physiological roles.

• Protection

• Body temperature regulation

• Cutaneous sensations

• Metabolic functions

• Blood reservoir

• Excretion of wastes

1. Protection

• Chemical barrier: Skin secretions (acid mantle, defensins) and melanin protect against bacteria and UV radiation.

• Physical barrier: Keratinized cells and tight junctions prevent entry of pathogens and water loss.

• Biological barrier: Dendritic cells and macrophages provide immune defense.

2. Body Temperature Regulation

• Sweat glands produce about 500 mL/day of insensible perspiration at rest.

• Increased body temperature leads to dilation of dermal vessels and increased sweat production (up to 12 L/day).

• Cold environments cause dermal blood vessels to constrict, reducing heat loss.

3. Cutaneous Sensations

• Skin contains sensory receptors for touch, temperature, and pain.

• Free nerve endings sense painful stimuli.

4. Metabolic Functions

• Synthesizes vitamin D for calcium absorption.

• Chemicals from keratinocytes can disarm carcinogens and activate hormones.

• Collagenase aids in natural turnover of collagen.

5. Blood Reservoir

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

• Blood can be shunted to other organs as needed.

6. Excretion

• Excretes nitrogenous wastes, salt, and water through sweat.

Developmental and Clinical Aspects

Developmental Aspects

• Fetal: By end of 4th month, skin of fetus is developed; lanugo coat forms in 5th and 6th months.

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

• Aging: Skin replacement slows, becomes thinner, drier, and itchier. Decreased gland activity and elasticity lead to cold intolerance, wrinkles, and increased risk of cancer.

Ways to Delay Aging

• UV protection

• Good nutrition

• Hydration

• Good hygiene

Clinical - Homeostatic Imbalances

• Whiteheads: Blocked sebaceous glands

• Acne: Inflammation of sebaceous glands, often due to bacterial infection

• Other conditions: Nail changes, hair loss, and skin lesions can indicate systemic diseases

Additional info: For more details on homeostatic interrelationships, refer to standard Anatomy & Physiology textbooks or course resources.