Anatomy & Physiology: The Integumentary System – Structure and Function

The Integumentary System

Overview

The integumentary system is the body's largest organ system, primarily composed of the skin and its associated structures. It serves as a protective barrier, regulates temperature, and provides sensory information.

• Epidermis: Composed of epithelial cells; forms the outermost protective shield of the body.

• Dermis: Composed of dense connective tissue; provides structural strength and elasticity.

• Subcutaneous tissue (Hypodermis): Composed of loose connective tissue and adipose tissue; anchors skin to underlying structures.

Layers of the Epidermis

Keratinized Stratified Squamous Epithelium

The epidermis is a keratinized stratified squamous epithelium, providing a tough, water-resistant barrier. It is organized into several distinct layers, each with specialized functions.

Layer	Function
Stratum Basale	Single row of keratinocytes; site of cell division; contains melanocytes and tactile (Merkel) cells
Stratum Spinosum	Several layers of keratinocytes; cells connected by desmosomes; contains dendritic (Langerhans) cells
Stratum Granulosum	Keratinization begins; cells accumulate keratohyalin granules and glycolipids for waterproofing
Stratum Lucidum	Thin, clear layer of dead keratinocytes; present only in thick skin (palms, soles)
Stratum Corneum	Multiple layers of dead, flattened keratinocytes; provides protection and prevents water loss

Cells of the Epidermis

• Keratinocytes: Produce keratin, a tough fibrous protein; most abundant cell type in the epidermis.

• Melanocytes: Synthesize melanin pigment, which protects against UV radiation.

• Dendritic (Langerhans) cells: Immune cells that detect and present antigens.

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

Dermis

Structure and Composition

The dermis is a strong, flexible connective tissue layer beneath the epidermis. It contains blood vessels, nerves, hair follicles, and glands.

• Papillary Layer: Areolar connective tissue; contains dermal papillae, capillary loops, and sensory receptors.

• Reticular Layer: Dense irregular connective tissue; contains collagen and elastic fibers, providing strength and elasticity.

Dermal Features

• Friction ridges: Form fingerprints; enhance grip.

• Cleavage (tension) lines: Patterns of collagen fiber bundles; important for surgical incisions.

• Flexure lines: Dermal folds at joints (e.g., palms, fingers).

• Stretch marks (striae): Tearing of dermal collagen due to rapid stretching.

Skin Color

Pigments and Factors

• Melanin: Brown to black pigment produced by melanocytes; protects against UV damage.

• Carotene: Yellow to orange pigment from diet; accumulates in stratum corneum and fat.

• Hemoglobin: Oxygen-carrying protein in red blood cells; gives skin a pinkish hue.

Clinical Signs of Skin Color Changes

Color Change	Cause	Clinical Significance
Blueness (Cyanosis)	Low oxygenation of hemoglobin	Indicates respiratory or cardiovascular issues
Pallor	Reduced blood flow or anemia	May signal shock, low blood pressure, or illness
Redness (Erythema)	Increased blood flow (inflammation, fever, allergy)	May indicate infection or allergic reaction
Yellowing (Jaundice)	Accumulation of bilirubin	Sign of liver dysfunction
Bruising (Hematoma)	Blood vessel damage	Indicates trauma or clotting disorder
Brown/black neck (Acanthosis nigricans)	Increased skin pigmentation	May be a sign of insulin resistance

Hair Structure and Function

Anatomy of a Hair

• Medulla: Central core; contains soft keratin; absent in fine hairs.

• Cortex: Surrounds medulla; contains flattened cells and pigment granules.

• Cuticle: Outermost layer; most heavily keratinized; provides strength and protection.

Associated Structures

• Arrector pili muscle: Smooth muscle that causes hair to stand (goosebumps).

• Hair follicle: Structure that produces hair; extends from epidermal surface into dermis.

• Hair papilla: Contains capillaries that nourish growing hair.

Hair Growth and Cycle

• Hair grows from the base of the follicle by mitosis of matrix cells.

• Growth phase (anagen), regression phase (catagen), and resting phase (telogen).

• Hair color determined by type and amount of melanin.

Wall of Hair Follicle

• Peripheral connective tissue sheath: Derived from dermis; forms outer wall.

• Glassy membrane: Thickened basement membrane.

• Epithelial root sheath: Derived from epidermis; forms inner wall.

Example:

Goosebumps occur when the arrector pili muscle contracts, pulling the hair follicle upright in response to cold or emotional stimuli.

Additional info: The notes also reference the importance of skin color changes in clinical diagnosis, the role of fingerprints in identification, and the structural differences between thick and thin skin.