Chapter 6: The Integumentary System – Structure and Function

Integumentary System

Overview and Definition

The integumentary system covers the body and consists of the skin and its accessory structures, including nails, hair, sweat glands, and sebaceous glands. It serves as the body's primary barrier to the external environment and is a key indicator of physiological health.

• Integument: Another term for skin.

• Cutaneous membrane: The technical name for the skin.

• Dermatology: The scientific study of the skin.

Composition and Functions of the Integument

Layers of the Integument

The integument consists of multiple layers, each with distinct structures and functions:

• Epidermis: The superficial layer, composed of keratinized stratified squamous epithelium.

• Dermis: The deeper layer, made of areolar and dense irregular connective tissue.

• Subcutaneous layer (hypodermis): Located deep to the dermis, consists of alveolar and adipose connective tissue. Additional info: This layer is not technically part of the integumentary system but is closely associated with it.

Epidermis

Structure and Cell Types

The epidermis is the outermost layer of the skin, composed of keratinized stratified squamous epithelium. It is organized into several strata (layers) from deep to superficial:

• Stratum basale: Deepest layer; single row of cuboidal or columnar cells. Contains three main cell types:

  • Keratinocytes: Found in all layers; large stem cells that divide to regenerate new cells and synthesize keratin, a protein that strengthens the skin.

  • Melanocytes: Scattered among keratinocytes; produce and store the pigment melanin in response to ultraviolet (UV) light. Melanin shields nuclear DNA from UV radiation.

  • Tactile cells (Merkel cells): Sensitive to touch; release chemicals when compressed to stimulate sensory nerve endings.

• Stratum spinosum: Several layers of polygonal keratinocytes; new cells from the stratum basale are pushed into this layer. Contains non-dividing keratinocytes attached by desmosomes and epidermal dendritic cells (Langerhans cells) that initiate immune responses.

• Stratum granulosum: 3–5 layers of keratinocytes; keratinization begins here. Keratinocytes fill with keratin, lose their nucleus and organelles, and die.

• Stratum lucidum: Translucent layer, 2–3 cell layers thick; found only in thick skin (palms and soles). Cells filled with translucent protein eleidin.

• Stratum corneum: Most superficial layer; 20–30 layers of dead, interlocking, anucleate keratinized cells. Provides a dry, thickened surface protective against abrasion and infection.

Thick vs. Thin Skin

• Thick skin: Found on palms and soles; contains all five epidermal layers, sweat glands, but no hair follicles or sebaceous glands.

• Thin skin: Covers most of the body; lacks stratum lucidum, contains sweat glands, hair follicles, and sebaceous glands.

Skin Color

Skin color is determined by three main pigments:

• Hemoglobin: Oxygen-binding protein in red blood cells; gives a bright red color when oxygenated.

• Melanin: Produced by melanocytes; transferred to keratinocytes. Eumelanin (brown/black shades), pheomelanin (lighter shades: tan, yellow, red). Amount varies by heredity and UV exposure.

• Carotene: Yellow-orange pigment acquired from vegetables.

Skin Markings

• Nevus (mole): Localized overgrowth of melanocytes.

• Freckles: Yellowish or brown spots; areas of increased melanocyte activity.

• Hemangiomas: Skin discoloration due to benign blood vessel tumors (e.g., capillary and cavernous hemangiomas).

• Friction ridges: Large folds and valleys of dermis and epidermis; responsible for fingerprints.

Dermis

Structure and Layers

The dermis lies deep to the epidermis and is composed of connective tissue proper. It contains blood vessels, sweat and sebaceous glands, hair follicles, nail roots, sensory nerve endings, arrector pili muscles, and motile dendritic cells.

• Papillary layer: Superficial region; areolar connective tissue. Contains dermal papillae that interlock with epidermal ridges, increasing contact area between layers.

• Reticular layer: Deeper, major portion of dermis; dense irregular connective tissue.

Subcutaneous Layer (Hypodermis)

Structure and Function

The subcutaneous layer is not part of the integument but consists of areolar and adipose connective tissue. It provides protection, energy storage, and insulation. It is a common site for drug injection due to its extensive vascular network, which promotes rapid absorption. Thickness and distribution are influenced by hormones.

Functions of the Integument

Protection

• Protects the body from injury, harmful substances, microbes, extreme temperatures, and UV radiation.

Prevention of Water Loss/Gain

• Epidermis is water-resistant, not waterproof; water lost by sweat and transpiration.

Vitamin D Synthesis

• Formation of vitamin D3, a precursor to calcitriol, which increases absorption of calcium and phosphate from the diet and regulates blood calcium levels.

Secretion and Absorption

• Sweating removes waste products.

• Skin is selectively permeable; suitable for transdermal drug administration.

Temperature Regulation

• Dermal blood vessels can vasoconstrict to conserve heat or vasodilate to release heat.

Immune Function

• Dendritic cells in epidermis and dermis initiate immune responses; skin microbiome protects against pathogens.

Sensory Reception

• Tactile sensory receptors detect stimuli such as touch and pressure.

Nails

Structure and Function

Nails are scale-like modifications of the stratum corneum located at the distal edges of fingers and toes. They protect the tips of digits and assist in grasping objects.

• Nail plate: Composed of the free edge, nail body, and nail root.

• Nail bed: Layer of living epidermis covered by the nail body.

• Nail matrix: Actively growing part at the proximal end of the nail body.

• Lunula: Whitish, semilunar area at the proximal end of the nail body.

• Nail folds: Skin folds overlapping the nail.

• Eponychium (cuticle): Narrow band of epidermis from the margin of the nail wall onto the nail bed.

• Hyponychium: Thickened stratum corneum under which the free nail edge projects.

Hair

Types and Distribution

• Lanugo: Fine, unpigmented, downy hair appearing in the last trimester.

• Vellus: Fine hair, primary human hair, found on upper and lower limbs.

• Terminal hair: Coarser, pigmented, longer; found on scalp, eyebrows, eyelashes, and men's beards. Replaces vellus hair in axillary and pubic regions during puberty.

Structure of Hair and Follicles

• Hair bulb: Swelling at the base where hair originates in the dermis; surrounds the hair papilla.

• Hair matrix: Structure at the base of the hair bulb; epithelial cells divide here to produce new cells.

• Medulla: Remnant of matrix; flexible, soft keratin.

• Cortex: Flattened cells closer to the outer hair surface; relatively hard.

• Cuticle: Single cell layer around cortex.

• Hair follicle: Oblique tube surrounding the hair root; extends into dermis and sometimes subcutaneous layer.

• Arrector pili: Thin ribbons of smooth muscle; contraction elevates hair, causing "goosebumps" (cutis anserina).

Functions of Hair

• Protection (e.g., on head from sunburn and injury; in respiratory system and ears to trap particles).

• Heat retention.

• Sensory reception (hair plexuses detect light touch).

• Visual identification.

Exocrine Glands of the Skin

Sweat Glands

• Merocrine (eccrine) sweat glands: Most numerous; simple, coiled tubular glands that discharge secretions onto the skin surface by exocytosis. Sweat is composed of 99% water and 1% other substances (electrolytes, metabolites, waste products). Major role in thermoregulation.

• Apocrine sweat glands: Coiled, tubular glands that discharge products into hair follicles; located in axillae, around nipples, pubic and anal regions. Produce viscous, cloudy excretion containing proteins and lipids; produce odor when acted on by bacteria. Begin functioning at puberty.

Sebaceous Glands

• Holocrine glands that produce oily substance called sebum; lubricates skin and hair, bactericidal. Discharges into hair follicles; excretion stimulated by hormones, especially during puberty.

Other Integumentary Glands

• Ceruminous glands: Modified apocrine sweat glands in the external ear canal; produce waterproof earwax (cerumen) that traps foreign material and lubricates the acoustic meatus and eardrum.

• Mammary glands: Modified apocrine sweat glands of the breast; function during pregnancy to produce breast milk.

Repair and Regeneration of the Integument

Wound Healing

Skin is repaired by regeneration or fibrosis:

• Regeneration: Replacement of damaged or dead cells with the same cell type; restores organ function.

• Fibrosis: Scar tissue formation; collagen produced by fibroblasts. Functional activities may not be restored.

Steps of Wound Healing

1. Cut blood vessels bleed into the wound.

2. Blood clot forms and leukocytes clean the wound; clot acts as a temporary barrier.

3. Blood vessels regrow and granulation tissue forms; vascular connective tissue initially forms in the wound.

4. Epithelium regenerates and connective tissue fibrosis occurs.

Inflammation sets the stage for healing by releasing inflammatory chemicals, increasing vessel permeability, and allowing white blood cells and clotting proteins to reach the injured area. Clotting seals off the area, and a scab forms. Organization restores blood supply, fibroblasts produce growth factors and collagen, and macrophages remove debris. Regeneration and fibrosis complete the repair, resulting in regenerated epithelium with underlying scar tissue.

Clinical View: Burns

Types and Severity

• First-degree burns: Involve only the epidermis; cause redness and pain. Treatment: immerse in cool water.

• Second-degree burns: Involve epidermis and part of dermis; skin blistered and painful, may cause slight scarring.

• Third-degree burns: Involve epidermis, dermis, and subcutaneous layer; require hospitalization, treatment for dehydration and infection, and may need skin grafts. Severity measured by the "rule of nines" (estimates surface area affected).

Treatment for Burns

• Manage fluid loss

• Manage pain

• Remove dead tissue

• Control infection

Cancer

Types of Skin Cancer

• Benign: Does not spread (encapsulated).

• Malignant: Metastasizes to other parts of the body.

• Basal cell carcinoma: Most common type (~80% of skin cancers); arises from stratum basale.

• Squamous cell carcinoma: Arises from stratum spinosum; can metastasize to lymph nodes if not removed; often sun-induced.

• Malignant melanoma: Most deadly; arises from melanocytes; metastasizes rapidly. Detection uses the ABCD rule:

Characteristic	Description
A - Asymmetry	Two sides of the pigmented area do not match
B - Border	Exhibits indentations
C - Color	Varied: black, brown, tan, sometimes red or blue
D - Diameter	Larger than 6 mm (size of a pencil eraser)