The Integumentary System: Structure, Function, and Clinical Aspects

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The Integumentary System

Overview and Major Components

The integumentary system is the body's largest organ system, serving as the primary barrier between the internal environment and the external world. It consists of the cutaneous membrane (skin) and accessory structures such as hair, nails, and exocrine glands.

Cutaneous membrane (skin): Composed of the epidermis and dermis.
Accessory structures: Include hair, nails, and exocrine glands (sebaceous and sweat glands).

The Integumentary System: labeled diagram of hair, skin, and nails General structure of the integumentary system, showing layers and accessory structures

Subcutaneous Layer (Hypodermis)

The subcutaneous layer, or hypodermis, is a loose connective tissue layer beneath the dermis. It is not technically part of the integumentary system but is closely associated with it.

Separates the skin from deeper tissues and organs.
Contains adipose tissue for insulation and energy storage.
Common site for subcutaneous injections due to absence of vital organs.

General structure of the skin, showing the subcutaneous layer

Functions of the Integumentary System

General Functions

Protection: Shields underlying tissues from mechanical, chemical, and biological damage; prevents fluid loss.
Temperature maintenance: Regulates heat exchange with the environment.
Synthesis and storage of nutrients: Epidermis synthesizes vitamin D3; dermis stores lipids.
Sensory reception: Contains receptors for touch, pressure, pain, and temperature.
Excretion and secretion: Glands excrete salts, water, and organic wastes; mammary glands secrete milk.

The Epidermis

Structure and Cell Types

The epidermis is a stratified epithelial tissue that is avascular and relies on diffusion from the underlying dermis for nutrients. The majority of cells are keratinocytes, which produce the protein keratin.

Keratinocytes: Main cell type, produce keratin for waterproofing and protection.
Melanocytes: Produce melanin pigment for UV protection.
Tactile (Merkel) cells: Sensory receptors for touch.

Histology of thin skin showing epidermal layers

Thick Skin vs. Thin Skin

Thick skin: Found on palms and soles; has five layers, including a prominent stratum corneum.
Thin skin: Covers most of the body; has four layers and a thinner stratum corneum.

Histology of thick skin showing multiple layers Labeled diagram of thick skin layers

Strata of the Epidermis

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

Stratum basale (germinativum): Deepest layer; contains stem cells, melanocytes, and tactile cells. Site of cell division.
Stratum spinosum: Several layers of keratinocytes; contains dendritic cells for immune defense.
Stratum granulosum: Cells stop dividing and begin producing keratin.
Stratum lucidum: Present only in thick skin; clear, densely packed cells filled with keratin.
Stratum corneum: Outermost layer; 15–30 layers of dead, keratinized cells that are shed in sheets.

Labeled diagram of epidermal layers

Epidermal Ridges and Dermal Papillae

Epidermal ridges (from the stratum basale) interlock with dermal papillae, forming the basis for fingerprints and enhancing grip.

Epidermal ridges and fingerprint formation

Skin Color and Pigmentation

Pigments

Carotene: Orange-yellow pigment from diet, accumulates in epidermal cells.
Melanin: Brown-black or red-yellow pigment produced by melanocytes; stored in melanosomes and transferred to keratinocytes.

Melanocytes and melanin distribution in the epidermis

Role of Sunlight

UV exposure increases melanin production, protecting deeper tissues.
Sunlight enables synthesis of vitamin D3, which is converted to calcitriol for calcium and phosphorus absorption.

Dermal Circulation and Skin Color

Oxygenated blood gives skin a reddish tint; vasodilation causes flushing, vasoconstriction causes paleness.
Cyanosis (bluish color) indicates low oxygen levels.

Skin Cancer

Basal cell carcinoma: Most common, originates in stratum basale.
Squamous cell carcinoma: Arises from superficial layers.
Malignant melanoma: Most dangerous, can metastasize rapidly.

Images of basal cell carcinoma and melanoma

The Dermis

Layers of the Dermis

Papillary layer: Superficial, areolar tissue; contains capillaries, lymphatics, and sensory neurons.
Reticular layer: Deeper, dense irregular connective tissue; contains collagen and elastic fibers, blood vessels, nerves, and accessory organs.

Dermal layers and accessory structures

Blood Supply

Cutaneous plexus: Supplies blood to the hypodermis and deeper dermis.
Subpapillary plexus: Supplies blood to the superficial dermis and epidermis.

Blood supply in the dermis

Accessory Structures

Hair and Hair Follicles

Hair is a nonliving, keratinized structure produced by hair follicles. It serves protective, sensory, and thermoregulatory functions.

Hair follicle: Extends into the dermis and hypodermis; contains hair matrix (site of cell division) and hair papilla (connective tissue with capillaries and nerves).
Hair structure: Root (below skin), shaft (visible part), cuticle (outer layer), cortex (middle), medulla (core).

Longitudinal section of hair follicle Cross-section of hair shaft structure

Hair Growth and Color

Hair grows in cycles; growth phase followed by rest phase.
Color determined by type and amount of melanin; graying occurs with age as pigment production declines.

Exocrine Glands of the Skin

Sebaceous (oil) glands: Secrete sebum into hair follicles or directly onto skin; lubricates and protects.
Apocrine sweat glands: Found in armpits, nipples, and pubic region; produce odorous secretion.
Eccrine (merocrine) sweat glands: Widely distributed; secrete watery sweat for thermoregulation.
Modified sweat glands: Mammary glands (milk), ceruminous glands (earwax).

Sebaceous glands and their relationship to hair follicles Sweat glands in the skin

Nails

Nails are protective structures on the dorsal surfaces of fingers and toes, composed of dense, keratinized cells.

Nail body: Visible part, covers the nail bed.
Nail root: Site of nail production, covered by the cuticle (eponychium).
Lunula: Pale crescent near the root.

Structure of a nail, superficial and cross-sectional views Longitudinal section of a nail

Repair and Regeneration of the Integument

Phases of Skin Repair

Inflammatory phase: Bleeding and inflammation occur; mast cells trigger response.
Migratory phase: Scab forms; epithelial cells migrate; phagocytes remove debris; granulation tissue forms.
Proliferation phase: Epidermal cells migrate over collagen meshwork; fibroblasts produce new tissue.
Scarring phase: Scab is shed; scar tissue forms, elevating the epidermis.

Inflammatory phase of skin repair Migratory phase of skin repair Proliferation phase of skin repair Scarring phase of skin repair

Keloids

Keloids are thickened areas of scar tissue with a shiny, smooth surface, resulting from excessive collagen formation during healing.

A keloid scar

Clinical Aspects: Burns and Aging

Burns

First-degree (superficial) burns: Affect only the epidermis; cause redness and mild pain (e.g., sunburn).
Second-degree (partial-thickness) burns: Damage epidermis and part of dermis; cause blistering and pain.
Third-degree (full-thickness) burns: Destroy epidermis and dermis; may extend into hypodermis; require skin grafting.

Burns compromise fluid balance, thermoregulation, and protection from infection.

Effects of Aging

Epidermis thins; stem cell activity declines.
Immune response weakens due to fewer macrophages.
Vitamin D3 production decreases, affecting bone strength.
Melanin production declines, increasing sun sensitivity.
Glandular activity decreases, causing dry, scaly skin.
Hair thins and changes color; skin becomes less elastic and more prone to injury.
Skin repair slows, increasing infection risk.