Chapter 5: The Integumentary System – Structure and Function of Skin

Overview of the Integumentary System

Main Regions of the Skin

The integumentary system is composed of the skin and its derivatives. The skin itself is organized into three major regions, each with distinct structure and function:

• Epidermis: The outermost, avascular layer composed primarily of epithelial cells.

• Dermis: The middle, vascularized layer containing connective tissue, blood vessels, and nerves.

• Hypodermis (subcutaneous tissue): The deepest layer, consisting mainly of adipose tissue and connective tissue. It anchors the skin to underlying structures.

Additional info: The hypodermis is not technically part of the skin but is often studied with it due to its close association and functional importance.

Functions of the Integumentary System

The integumentary system serves several vital roles in maintaining homeostasis and protecting the body:

• Protection: Acts as a physical, chemical, and biological barrier against pathogens, UV radiation, and mechanical injury.

• Sensation: Contains sensory receptors for touch, pain, temperature, and pressure.

• Thermoregulation: Regulates body temperature through sweat production and blood flow adjustments.

• Excretion: Eliminates waste products such as urea and salts through sweat.

• Vitamin D Synthesis: Initiates the synthesis of vitamin D when exposed to UV light.

• Reservoir: Stores blood and fat, serving as an energy reserve and contributing to overall fluid balance.

Epidermis

Types of Cells in the Epidermis

The epidermis is a stratified squamous epithelium composed of several specialized cell types:

• Keratinocytes: The most abundant cells, producing keratin, a tough, fibrous protein that provides protection and waterproofing.

• Melanocytes: Cells that synthesize melanin, the pigment responsible for skin color and protection against UV radiation.

• Epidermal Dendritic (Langerhans) Cells: Immune cells that help detect and fight pathogens.

• Tactile (Merkel) Cells: Sensory receptors for touch, associated with nerve endings.

Layers of the Epidermis

The epidermis is organized into distinct layers (strata), each with specific characteristics and functions. From superficial to deep:

• Stratum Corneum: Outermost layer; consists of dead, flattened keratinocytes filled with keratin. Provides a durable, water-resistant barrier.

• Stratum Lucidum (only in thick skin, e.g., palms and soles): A thin, translucent layer of dead keratinocytes.

• Stratum Granulosum: Cells begin to die and accumulate keratohyalin granules, contributing to keratin formation.

• Stratum Spinosum: Several layers of keratinocytes connected by desmosomes; cells appear spiny under a microscope.

• Stratum Basale (germinativum): Deepest layer; a single row of mitotically active stem cells that give rise to all other epidermal layers. Contains melanocytes and Merkel cells.

Keratinization is the process by which keratinocytes move from the stratum basale to the stratum corneum, accumulating keratin and eventually dying.

Dermis

Cell Types in the Dermis

The dermis is a connective tissue layer that supports and nourishes the epidermis. Major cell types include:

• Fibroblasts: Produce collagen and elastin fibers, providing strength and elasticity.

• Macrophages: Immune cells that engulf pathogens and debris.

• Mast Cells/White Blood Cells: Involved in immune responses and inflammation.

Layers of the Dermis

• Papillary Layer: Thin, superficial layer composed of loose connective tissue. Contains dermal papillae (projections that interlock with the epidermis), which form friction ridges (fingerprints).

• Reticular Layer: Thicker, deeper layer made of dense irregular connective tissue. Contains collagen and elastin fibers for strength and flexibility.

Skin Markings

• Cleavage Lines (Langer's lines): Natural orientation of collagen fibers in the dermis, important for surgical incisions.

Skin Color

Pigments Contributing to Skin Color

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

• Carotene: Yellow to orange pigment from diet, accumulates in the stratum corneum and hypodermis.

• Hemoglobin: Red pigment in blood, visible through the skin, especially in fair-skinned individuals.

Skin Accessories (Derivatives of the Epidermis)

• Sweat Glands

• Sebaceous (Oil) Glands

• Hair and Hair Follicles

• Nails

All these structures originate from the epidermis and serve specialized functions.

Sweat Glands

• Types: Eccrine (merocrine) glands (widely distributed, important for thermoregulation) and apocrine glands (found in axillary and genital areas, become active at puberty).

Sebaceous (Oil) Glands

• Distribution: Found throughout the skin except palms and soles.

• Secretions: Produce sebum, an oily substance that lubricates skin and hair.

• Structure: Usually associated with hair follicles.

• Function: Prevents drying and inhibits bacterial growth.

Hair

• Distribution: Covers most of the body except palms, soles, lips, and parts of external genitalia.

• Components: Shaft (above skin), root (within skin), pigments (melanin for color).

• Structure: Medulla (core), cortex (middle layer), cuticle (outer layer).

• Function: Protection, sensation, and insulation.

Hair Follicle

• Structure: Two-layered wall, bulb (base), hair follicle receptor (sensory nerve ending), arrector pili (smooth muscle causing hair to stand), papilla (provides nutrients).

Types of Hair

• Vellus: Fine, pale body hair.

• Terminal: Coarse, long hair of scalp, eyebrows, and after puberty, axillary and pubic regions.

Hair Growth

• Growth Phase: Active mitosis in hair matrix.

• Regressive Stage: Hair growth slows, follicle shrinks.

• Resting Phase: Follicle is inactive before shedding hair.

Homeostatic Imbalance: Alopecia is hair loss due to various causes.

Nails

• Structure: Nail bed, lunule (white crescent), free edge, eponychium (cuticle), proximal and lateral nail folds.

Homeostatic Imbalances of the Skin

Skin Cancer

• Basal Cell Carcinoma: Most common, least malignant; arises from stratum basale.

• Squamous Cell Carcinoma: Arises from keratinocytes of stratum spinosum; can metastasize.

• Melanoma: Most dangerous; arises from melanocytes. The ABCD Rule is used for early detection:

  • Asymmetry

  • Border irregularity

  • Color variation

  • Diameter > 6 mm

Burns

• Types of Burns: Classified by depth and extent.

• Rule of Nines: Used to estimate the percentage of body surface area affected by burns.

• Degree of Burns:

  • First Degree: Only epidermis damaged; redness, swelling, pain.

  • Second Degree: Epidermis and upper dermis damaged; blisters form.

  • Third Degree: Entire thickness of skin destroyed; may appear white, red, or blackened; not painful due to nerve destruction.

• Severity of Burns: Depends on depth, area, and location.

Rule of Nines Table

The Rule of Nines is a method for estimating the total body surface area (TBSA) affected by burns in adults:

Body Region	Percentage of TBSA
Head and Neck	9%
Each Upper Limb	9% (each)
Each Lower Limb	18% (each)
Anterior Trunk	18%
Posterior Trunk	18%
Perineum	1%

Developmental Considerations

• Infancy: Vernix caseosa (waxy coating) and milia (white spots) are common.

• Adolescence: Increased activity of sebaceous glands; acne may develop.

• Old Age: Thinning skin, decreased elasticity, and slower healing.

Clinical Scenarios

• Acne Treatment: Example of a young patient prescribed retinoids for acne management.

• Burn Management: Example of a patient with third-degree burns requiring emergency care.

Additional Resources

• Khan Academy: What is Skin?

• PurposeGames: Integumentary System Skin Structure