The Integumentary System: Structure, Function, and Clinical Aspects

The Integumentary System

Overview

The integumentary system is the body's outer covering, providing protection and contributing to homeostasis. It consists of the skin, hair, nails, sweat glands, and sebaceous (oil) glands. The skin is the largest organ and serves as a barrier against environmental hazards.

• Major components: Skin, hair, nails, sweat glands, sebaceous glands

• Main functions: Protection, sensation, temperature regulation, metabolic functions, excretion

Structure of the Skin

Skin Layers

The skin is composed of two main layers: the epidermis and the dermis. Beneath these lies the hypodermis (subcutaneous tissue), which is not technically part of the skin but supports its functions.

• Epidermis: Superficial, avascular layer made of keratinized stratified squamous epithelium

• Dermis: Deeper, vascular layer composed of connective tissue

• Hypodermis: Subcutaneous layer of adipose tissue; absorbs shock, insulates, and anchors skin to muscles

Cells of the Epidermis

Major Cell Types

The epidermis contains four main cell types, each with specialized functions:

• Keratinocytes: Produce keratin, a protein that provides strength and waterproofing; most abundant cell type

• Melanocytes: Produce melanin pigment, which protects against UV radiation

• Dendritic (Langerhans) cells: Immune cells that patrol the epidermis and activate immune responses

• Tactile (Merkel) cells: Sensory receptors for touch

Layers of the Epidermis

Strata of the Epidermis

The epidermis is organized into distinct layers (strata). Thick skin (palms, soles) has five layers; thin skin has four.

• Stratum basale (germinativum): Deepest, single row of mitotic stem cells; contains melanocytes

• Stratum spinosum: Several layers of keratinocytes connected by desmosomes; contains dendritic cells and melanosomes

• Stratum granulosum: 4–6 layers; keratinization begins, cells flatten, organelles disintegrate, lamellar granules release glycolipids

• Stratum lucidum: Only in thick skin; thin, clear layer of dead keratinocytes

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

Cell Turnover and Shedding

Epidermal cells undergo apoptosis as they move toward the surface, eventually sloughing off as dandruff and dander. Humans shed approximately 50,000 skin cells per minute.

The Dermis

Structure and Layers

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, and glands.

• Papillary layer: Superficial areolar connective tissue with dermal papillae (capillary loops, nerve endings, touch receptors)

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

Dermal Modifications and Skin Markings

• Friction ridges: Form fingerprints; enhance grip and tactile sensitivity

• Cleavage (tension) lines: Orientation of collagen fibers; important for surgical incisions

• Flexure lines: Dermal folds at joints; allow skin to accommodate movement

• Striae (stretch marks): Dermal tears from overstretching

Skin Color

Pigments

Skin color is determined by three main pigments:

• Melanin: Produced by melanocytes; protects DNA from UV damage; responsible for brown to black hues

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

• Hemoglobin: Oxygen-carrying pigment in red blood cells; gives pinkish hue to fair skin

Clinical Correlations: Skin Color Changes

• Cyanosis: Blue skin due to low oxygenation

• Pallor: Pale color from anemia, low blood pressure, or shock

• Erythema: Redness from fever, inflammation, or allergy

• Jaundice: Yellow cast from liver disorders

• Bruises: Clotted blood beneath skin (hematomas)

• Hyperpigmentation: May indicate endocrine disorders

Hair

Structure and Function

Hair consists of dead, keratinized cells produced by hair follicles. It functions in protection, sensation, and temperature regulation.

• Shaft: Visible part above the skin; keratinization complete

• Root: Embedded in the skin; keratinization ongoing

• Medulla: Central core with air spaces

• Cortex: Layers of flattened cells surrounding the medulla

• Cuticle: Outermost layer of overlapping cells

Hair Color

Hair color is determined by the type and amount of melanin produced by melanocytes in the hair follicle. Red hair contains pheomelanin. Gray or white hair results from decreased melanin production and air bubbles in the shaft.

Hair Follicle Structure

The hair follicle extends from the epidermis into the dermis and consists of several layers:

• Peripheral connective tissue sheath: Derived from dermis

• Glassy membrane: Thickened basal lamina

• Epithelial root sheath: Derived from epidermis

• Hair matrix: Actively dividing cells producing hair

• Arrector pili: Smooth muscle causing "goose bumps"

• Hair papilla: Dermal tissue supplying nutrients

Types and Growth of Hair

• Vellus hair: Fine, pale body hair

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

• Growth: Averages 2.25 mm/week; affected by nutrition and hormones

Hair Thinning and Baldness

• Alopecia: Age-related hair thinning

• Male pattern baldness: Genetically determined, influenced by DHT

• Telogen effluvium: Sudden hair loss due to stress or illness

Nails

Structure and Function

Nails are scale-like modifications of the epidermis containing hard keratin. They protect the distal tips of fingers and toes and aid in manipulation.

• Free edge: Extends beyond the finger or toe

• Nail plate: Visible attached portion

• Nail root: Embedded in the skin

• Nail matrix: Site of nail growth

• Lunule: White crescent at the base of the nail

• Eponychium (cuticle): Skin fold over the nail

• Hyponychium: Area under the free edge

Clinical Correlations: Nail Appearance

• Yellow nails: May indicate respiratory or thyroid disorders

• Thickened yellow nails: Fungal infection

• Spoon nails (koilonychia): Iron deficiency

• Beau’s lines: Severe illness

Cutaneous Glands

Types of Glands

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

• Apocrine sweat glands: Axillary/anogenital areas; secrete viscous sweat; function at puberty

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

• Sebaceous (oil) glands: Secrete sebum into hair follicles; lubricate and protect skin/hair

Gland Type	Function	Secretion	Location
Eccrine Sweat Gland	Temperature control, antibacterial	Hypotonic filtrate of blood plasma (mostly water, some salts)	Everywhere, especially palms, soles, forehead
Apocrine Sweat Gland	May act as sexual scent gland	Filtrate of blood plasma with added proteins and fatty substances	Axillary and anogenital regions
Sebaceous Gland	Lubricate skin/hair, antibacterial	Oily sebum	Everywhere except palms and soles

Clinical Correlations: Gland Disorders

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

• Seborrhea (cradle cap): Overactive sebaceous glands in infants

Functions of the Skin

Protection

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

• Physical barrier: Keratinized cells, glycolipids

• Biological barrier: Dendritic cells, macrophages, DNA (UV absorption)

Body Temperature Regulation

• Insensible perspiration: Unnoticeable sweat at rest (~500 ml/day)

• Sensible perspiration: Noticeable sweat during heat or exercise (up to 12 L/day)

• Vasoconstriction: Reduces heat loss in cold environments

Cutaneous Sensations

• Exteroreceptors: Detect temperature, touch, pain

• Free nerve endings: Sense pain

Metabolic Functions

• Synthesizes vitamin D for calcium absorption

• Detoxifies some chemicals and activates hormones

• Makes collagenase for collagen turnover

Blood Reservoir and Excretion

• Holds up to 5% of blood volume

• Excretes nitrogenous wastes, salt, and water

Skin 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: Most dangerous; cancer of melanocytes; highly metastatic

Burns

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

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

• Third-degree: Full-thickness; gray-white, cherry red, or blackened; no pain (nerve endings destroyed); requires grafting

Rule of Nines

The Rule of Nines is used to estimate the extent of burns and guide fluid resuscitation. The body is divided into sections, each accounting for 9% (or multiples) of total body surface area.

• Burns are critical if: >25% of body has second-degree burns, >10% has third-degree burns, or face/hands/feet have third-degree burns

• Treatment includes debridement, antibiotics, temporary coverings, and skin grafts