The Integumentary System

Overview

The integumentary system is the body's largest organ system, providing essential protection and sensory functions. It consists of the skin, sweat (sudoriferous) glands, oil (sebaceous) glands, hair, nails, and subcutaneous tissue. Together, these components maintain homeostasis and defend against environmental threats.

• Skin: Primary barrier and sensory organ

• Sweat glands: Regulate temperature and excrete wastes

• Oil glands: Lubricate and protect skin and hair

• Hair: Provides protection and sensory input

• Nails: Protect fingertips and enhance sensation

• Subcutaneous tissue: Anchors skin, absorbs shock, insulates

Skin Structure

Layers of the Skin

The skin is composed of two main layers: the epidermis and dermis. Beneath these lies the subcutaneous tissue (hypodermis), which, while not technically part of the skin, is functionally important.

• Epidermis: Outermost layer, made of epithelial tissue, avascular

• Dermis: Underlies epidermis, dense connective tissue, vascular

• Subcutaneous tissue (Hypodermis): Mostly adipose tissue, anchors skin, provides insulation and shock absorption

The Epidermis

Cell Types

The epidermis is a keratinized stratified squamous epithelium, containing four main cell types:

• Keratinocytes: Produce keratin, the protein that gives skin its strength and waterproofing

• Melanocytes: Produce melanin pigment, protecting against UV radiation

• Dendritic (Langerhans) cells: Immune cells that patrol for pathogens

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

Layers of the Epidermis

The epidermis is organized into distinct strata, each with specialized functions:

• Stratum basale: Deepest layer, mitotically active stem cells, contains melanocytes

• Stratum spinosum: Several layers thick, contains pre-keratin filaments, dendritic cells

• Stratum granulosum: Cells flatten, keratinization begins, glycolipids for water resistance

• Stratum lucidum: Only in thick skin (palms, soles), clear layer of dead keratinocytes

• Stratum corneum: Outermost, 20–30 layers of dead, keratinized cells, barrier function

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, lymphatics, hair follicles, oil, and sweat glands.

• Papillary dermis: Superficial, areolar connective tissue, contains dermal papillae with capillaries and sensory receptors

• Reticular dermis: Deep, dense irregular connective tissue, provides strength, elasticity, and hydration

Dermal Modifications

• Friction ridges: Enhance grip and touch, form fingerprints

• Cleavage (tension) lines: Parallel collagen fibers, important for surgical incisions

• Flexure lines: Dermal folds at joints, visible on hands and feet

• Striae (stretch marks): Dermal tears from extreme stretching

Skin Color

Pigments

Three pigments contribute to skin color:

• Melanin: Produced by melanocytes, shields DNA from UV, varies in amount and form

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

• Hemoglobin: Red pigment in blood, gives pinkish hue to fair skin

Alterations in skin color can indicate disease (cyanosis, pallor, erythema, jaundice, bruises).

Hair

Structure and Function

Hair is composed of dead, keratinized cells and serves protective and sensory functions. It is absent from palms, soles, lips, nipples, and parts of external genitalia.

• Root: Within scalp, keratinization ongoing

• Shaft: Above scalp, keratinization complete

• Medulla: Central core

• Cortex: Surrounds medulla

• Cuticle: Outermost layer

Hair Follicle

• Follicle wall: Peripheral connective tissue sheath, glassy membrane, epithelial root sheath

• Hair bulb: Expanded deep end, contains hair matrix (actively dividing cells)

• Arrector pili: Smooth muscle causing "goose bumps"

Types and Growth

• Vellus hair: Fine, pale body hair

• Terminal hair: Coarse, long hair (scalp, eyebrows, axillary, pubic regions)

• Growth cycles: Active and resting phases, affected by nutrition and hormones

Nails

Structure and Function

Nails are scale-like modifications of the epidermis containing hard keratin. They protect the distal phalanges and aid in sensation.

• Root: Embedded in skin

• Nail plate/body: Visible attached portion

• Free edge: Extends beyond finger

• Nail bed: Epidermis under nail plate

• Nail matrix: Growth area

• Nail folds: Skin overlapping nail border

• Cuticle (eponychium): Projects onto nail body

• Hyponychium: Under free edge, collects dirt

Skin Appendages: Glands

Sweat Glands

• Eccrine (merocrine) sweat glands: Most numerous, thermoregulation, secrete watery sweat

• Apocrine sweat glands: Axillary/anogenital areas, secrete viscous sweat, function as scent glands

• Modified apocrine glands: Ceruminous (earwax), mammary (milk)

Oil (Sebaceous) Glands

• Holocrine glands, secrete sebum into hair follicles

• Lubricate skin and hair, bactericidal properties

• Inactive until puberty, stimulated by androgens

Summary Table: Cutaneous Glands

Functions of Skin

Protection

• Chemical barriers: Sweat, sebum, acid mantle, melanin

• Physical barriers: Keratinized cells, glycolipids

• Biological barriers: Dendritic cells, macrophages

Body Temperature Regulation

• Insensible perspiration: 500 ml/day

• Sensible perspiration: Up to 12 L/day during heat

• Vasoconstriction in cold, vasodilation in heat

Cutaneous Sensations

• Touch: Meissner's corpuscles, Merkel cells

• Pressure: Pacinian corpuscles

• Hair movement: Hair follicle receptors

• Pain: Free nerve endings

Metabolic Functions

• Synthesizes vitamin D for calcium absorption

• Disarms carcinogens, activates hormones

• Makes collagenase for collagen turnover

Blood Reservoir

• Holds up to 5% of blood volume

• Can shunt blood to other organs

Excretion

• Excretes nitrogenous wastes (ammonia, urea, uric acid)

• Loss of salt and water via sweat

Skin Disorders: Cancer and Burns

Skin Cancer

• Basal cell carcinoma: Most common, least malignant, arises from stratum basale

• Squamous cell carcinoma: Second most common, can metastasize, arises from stratum spinosum

• Melanoma: Cancer of melanocytes, most dangerous, highly metastatic

Burns

• First-degree: Epidermal damage only, redness, pain

• Second-degree: Epidermal and upper dermal damage, blisters

• Third-degree: Entire skin thickness, gray-white, cherry red, or blackened, nerve endings destroyed

• Critical if >25% second-degree, >10% third-degree, or face/hands/feet involved

• Treatment: Debridement, antibiotics, temporary covering, skin grafts

Developmental Aspects

Infancy to Adulthood

• Vernix caseosa: Protective substance in fetus

• Milia: Small white spots in newborns

• Skin thickens, gland activity increases, acne may develop

• Optimal skin appearance in 20s–30s

Aging Skin

• Epidermal replacement slows, skin thins, becomes dry and itchy

• Subcutaneous fat and elasticity decrease, leading to wrinkles and cold intolerance

• Increased risk of cancer due to decreased melanocytes and dendritic cells

• Hair thinning

• UV protection can delay aging