The Integumentary System: Structure and Function

The Integumentary System

Overview

The integumentary system is composed of the skin and its accessory structures. It serves as the body's first line of defense and plays a vital role in protection, sensation, and homeostasis.

• Skin is the largest organ of the body, consisting of multiple layers and specialized cells.

• Accessory structures include hair, nails, and glands.

Structure of Skin

Main Layers of the Skin

The skin is composed of two primary layers, with an additional subcutaneous layer beneath:

• Epidermis: The superficial, avascular layer made of epithelial tissue.

• Dermis: The deeper, vascular layer composed mainly of fibrous connective tissue.

• Hypodermis (Superficial Fascia): A subcutaneous layer not technically part of the skin, but important for fat storage, insulation, and anchoring the skin to underlying structures (mainly muscles).

Epidermis

Cell Types in the Epidermis

• Keratinocytes: Produce keratin, a protein that provides protective properties; most abundant cell type.

• Melanocytes: Spider-shaped cells that produce melanin, the pigment responsible for skin color and UV protection.

• Dendritic (Langerhans) Cells: Star-shaped macrophages involved in immune defense.

• Tactile (Merkel) Cells: Sensory receptors for touch.

Layers of the Epidermis

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

1. Stratum Basale (Basal Layer): Deepest layer; single row of mitotically active stem cells. Contains melanocytes.

2. Stratum Spinosum (Prickly Layer): Several layers thick; keratinocytes appear spiky; contains melanosomes and dendritic cells.

3. Stratum Granulosum (Granular Layer): 4-6 cell layers; keratinization begins; cells flatten and organelles disintegrate.

4. Stratum Lucidum (Clear Layer): Present only in thick skin; thin, translucent band of dead keratinocytes.

5. Stratum Corneum (Horny Layer): 20-30 layers of dead, keratinized cells; provides protection and prevents water loss.

Dermis

Structure and Layers

The dermis is a strong, flexible connective tissue layer containing nerves, blood vessels, lymphatics, hair follicles, and glands.

• Papillary Layer (about 20%): Superficial areolar connective tissue; contains dermal papillae with capillary loops, nerve endings, and Meissner's corpuscles (touch receptors).

• Reticular Layer (about 80%): Dense irregular connective tissue; provides strength and elasticity; contains collagen and elastic fibers, and the cutaneous plexus (blood vessel network).

Dermal Modifications

• Dermal Ridges: In thick skin, dermal papillae sit atop dermal ridges, forming epidermal ridges (fingerprints).

• Cleavage (Tension) Lines: Patterns of collagen fibers in the reticular layer; important for surgical incisions.

Skin Color

Pigments Affecting Skin Color

• Melanin: Produced by melanocytes; shields DNA from UV radiation. More melanin = darker skin.

• Carotene: Yellow-orange pigment from diet; most visible in palms and soles; can be converted to vitamin A.

• Hemoglobin: Red pigment in blood; gives fair skin a pinkish hue due to low melanin.

Clinical Significance of Skin Color

• Cyanosis: Blue skin due to low oxygenation.

• Pallor: Pale color from anemia, low blood pressure, or emotional stress.

• Erythema: Redness from fever, hypertension, or inflammation.

• Jaundice: Yellow cast from liver disorders.

• Bruises: Black-and-blue marks from clotted blood beneath the skin.

Accessory Structures of the Skin

Hair

• Composed of dead, keratinized cells; absent on palms, soles, lips, nipples, and parts of external genitalia.

• Functions: Sensory detection, protection from trauma, heat loss, and sunlight.

• Structure: Shaft (above skin), root (within skin), follicle (extends into dermis), bulb (base of follicle), matrix (growth zone), arrector pili muscle (causes 'goose bumps'), and papilla (supplies nutrients).

• Color: Determined by melanins; gray/white hair results from decreased melanin and air bubbles in shaft.

Nails

• Modified epidermis with hard keratin; protects distal phalanges.

• Structure: Nail plate, nail bed, matrix (growth area), cuticle (eponychium), and hyponychium (under free edge).

• Abnormalities in color or shape can indicate disease.

Glands

• Sudoriferous (Sweat) Glands:

  • Eccrine (Merocrine) Glands: Most numerous; abundant on palms, soles, and forehead; secrete watery sweat for thermoregulation.

  • Apocrine Glands: Found in axillary and anogenital areas; secrete viscous, milky sweat; function at puberty; responsible for body odor.

  • Specialized Apocrine Glands: Ceruminous (earwax) and mammary (milk) glands.

• Sebaceous (Oil) Glands: Widely distributed except in thick skin; secrete sebum (oily, bactericidal, softens skin/hair); usually associated with hair follicles; become active at puberty.

Functions of the Integumentary System

Protection

• Chemical Barrier: Skin secretions and melanin.

• Physical Barrier: Continuity of skin and keratinized cells.

• Biological Barrier: Dendritic cells, macrophages, and DNA.

Body Temperature Regulation

• Via sweat gland activity and blood vessel dilation/constriction.

Cutaneous Sensations

• Receptors for touch, pressure, pain, and temperature (e.g., Meissner's and Pacinian corpuscles).

Metabolic Functions

• Synthesizes vitamin D for calcium absorption.

• Enzymes in keratinocytes can neutralize carcinogens and aid collagen turnover.

Blood Reservoir

• Holds up to 5% of the body's blood volume; can redirect blood as needed.

Excretion

• Eliminates nitrogenous wastes (ammonia, urea, uric acid) and excess salts via sweat.

Skin Disorders

Skin Cancer

• Basal Cell Carcinoma: Most common, least malignant; arises from stratum basale; slow-growing, rarely metastasizes.

• Squamous Cell Carcinoma: Second most common; arises from keratinocytes of stratum spinosum; can metastasize.

• Melanoma: Cancer of melanocytes; most dangerous due to high metastatic potential and resistance to chemotherapy. Early detection is critical (ABCD rule: Asymmetry, Border, Color, Diameter).

Burns

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

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

• Third-degree: Full-thickness burn; entire skin destroyed; area may appear white, red, or blackened; not painful due to nerve destruction; requires skin grafting.

Rule of Nines: Used to estimate the extent and severity of burns for treatment planning.

Aging and the Integumentary System

• Skin cell cycle slows; skin becomes thinner, drier, and less elastic.

• Loss of subcutaneous fat; increased sensitivity to cold.

• Decreased melanocyte activity; hair whitens.

• Reduced number of hair follicles and impaired hair growth.

• Decline in sensory receptors and vitamin D production.

• Slower wound healing.

Key Terms

• melan-: black

• leuko-: white

• hypo-: under, below

• derm-: skin

• cytic-: small bag

• kerat-: horn

• seb-: grease

• sudor-: sweat

• basi-: base, foundation

• lucid-: light, clear

• papill-: nipple