Chapter 5: The Integumentary System – Structure and Function

Integumentary System Overview

Introduction

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

• Protection: Shields internal tissues from physical damage, pathogens, and dehydration.

• Coordination of Immune Response: Responds to pathogens and cancer in skin.

• Sensory Information: Detects touch, pressure, pain, and temperature.

• Synthesis of Vitamin D3: Initiated by exposure to sunlight.

• Excretion and Thermoregulation: Removes waste and regulates body temperature via sweat.

Structure of the Skin

Layers of the Skin

The skin is composed of three main layers: the epidermis, dermis, and hypodermis (subcutaneous layer). Each layer has distinct structures and functions.

• Epidermis: Outermost layer, primarily made of keratinocytes.

• Dermis: Middle layer, contains connective tissue, blood vessels, and sensory receptors.

• Hypodermis (Subcutaneous Layer): Deepest layer, mainly adipose tissue, provides insulation and energy storage.

Epidermis

Keratinocytes and Keratin

Keratinocytes are the predominant cell type in the epidermis, responsible for producing keratin, a tough, fibrous protein that provides structural strength and waterproofing to the skin, hair, and nails.

• Keratin: Main structural component of hair and nails.

Layers of the Epidermis

The epidermis is organized into distinct layers, from deep to superficial:

• Stratum basale

• Stratum spinosum

• Stratum granulosum

• Stratum lucidum (present only in thick skin)

• Stratum corneum

Stratum Basale

The deepest layer of the epidermis, attached to the basement membrane. It contains stem cells and specialized cells.

• Hemidesmosomes: Attach cells to the basement membrane.

• Epidermal ridges: Increase surface area for attachment; form fingerprints.

• Basal cells: Stem cells that divide to replace keratinocytes.

• Tactile (Merkel) cells: Sensory cells sensitive to touch, found in skin surfaces.

Stratum Spinosum

Composed of several layers of keratinocytes bound together by desmosomes. Contains immune cells.

• Dendritic (Langerhans) cells: Immune cells that defend against pathogens and superficial skin cancer.

Stratum Granulosum

Consists of layers of keratinocytes derived from the stratum spinosum. Cells begin to die and accumulate keratin.

• Keratin: Protein that provides strength.

• Keratohyalin: Protein that promotes dehydration and cross-linking of keratin fibers.

Stratum Lucidum

Present only in thick skin (palms of hands, soles of feet). Composed of flattened, densely packed cells filled with keratin.

• Function: Provides extra protection and durability.

Stratum Corneum

The outermost layer, consisting of many layers of dead, keratinized cells. It is water-resistant but not waterproof.

• Insensible Perspiration: Evaporative water loss by diffusion through the skin or lungs.

• Sensible Perspiration: Water loss due to secretion by sweat glands.

Epidermal Growth Factor (EGF)

Role and Functions

Epidermal Growth Factor is produced by salivary glands and glands of the duodenum. It regulates growth and repair of the epidermis.

• Stimulates division of basal cells in the stratum basale.

• Accelerates keratin production (keratinization).

• Promotes epidermal development and repair after injury.

• Stimulates secretory product synthesis and secretion by epithelial glands.

Dermis

Papillary Layer

The superficial layer of the dermis, composed of areolar tissue. Contains capillaries, lymphatic vessels, and sensory nerve fibers.

• Dermatitis: Inflammation of the skin, often due to infection, irritation, or mechanical stimuli.

Reticular Layer

The deeper layer of the dermis, made of dense irregular connective tissue. Contains collagen and elastic fibers, providing strength and elasticity.

• Collagen fibers: Strong, resist stretching.

• Elastic fibers: Permit stretching and return to original length.

• Skin turgor: The ability of the skin to return to its original shape after deformation.

• Tension lines (cleavage lines): Patterns formed by collagen and elastic fibers; incisions parallel to these lines heal better.

Dermal Blood Supply

Blood vessels in the dermis regulate temperature and supply nutrients to the skin.

• Controls blood flow and glandular secretion rates.

• Monitors sensory receptors in the dermis and deeper layers.

Hypodermis (Subcutaneous Layer)

Structure and Function

The hypodermis is not technically part of the skin but stabilizes its position relative to underlying tissues. It consists mainly of adipose tissue and provides insulation, energy storage, and shock absorption.

• Contains large arteries and veins in the superficial region.

• Fat distribution varies by age and sex.

Pigmentation

Major Skin Pigments

Skin color is determined by the presence of pigments such as carotene and melanin.

• Carotene: Yellow-orange pigment, most apparent in the stratum corneum.

• Melanin: Produced by melanocytes; exists in two forms: pheomelanin (yellow) and eumelanin (brown/black).

• Melanin protects against UV radiation and is transferred to keratinocytes.

Clinical Changes in Skin Color

• Pallor: Pale skin due to reduced blood supply.

• Cyanosis: Bluish coloration from low oxygen levels in hemoglobin.

• Jaundice: Yellowing due to bilirubin accumulation from liver dysfunction.

• Addison's Disease: Increased ACTH leads to skin darkening.

• Vitiligo: Loss of melanocytes, resulting in patchy skin color.

Vitamin D3 Production

Role of Sunlight

Exposure to sunlight stimulates the synthesis of vitamin D3 in the skin, which is essential for calcium absorption and bone health.

• UV radiation converts a precursor molecule in the skin to vitamin D3.

Accessory Structures

Hair

Hair is produced by hair follicles and serves protective and sensory functions.

• Lanugo: Fine, unpigmented hair present during embryonic development.

• Vellus hairs: "Peach fuzz" found on most of the body.

• Terminal hairs: Heavy, pigmented hair (scalp, eyebrows, etc.).

• Hair texture (straight or curly) is determined by follicle shape.

• Hair color is determined by melanin production in the hair matrix.

Hair Structure

• Hair plexus: Sensory nerves surrounding the base of each hair follicle.

• Arrector pili muscle: Smooth muscle that causes "goosebumps" when contracted.

Exocrine Glands

The skin contains several types of exocrine glands:

• Sebaceous glands: Secrete sebum, an oily substance that lubricates and protects hair and skin.

• Sudoriferous (sweat) glands: Include eccrine (merocrine) and apocrine glands.

• Eccrine glands: Widely distributed, regulate temperature via sweat.

• Apocrine glands: Found in armpits and pubic region, secrete at puberty.

• Ceruminous glands: Produce earwax (cerumen).

Nails

Nails protect and support the tips of fingers and toes. They are formed from tightly packed keratinized cells.

• Nail matrix: Site of nail growth.

Aging and the Integumentary System

Effects of Aging

Aging leads to several changes in the skin and its accessory structures:

• Basal cell activity declines, slowing skin repair.

• Connections between epidermis and dermis weaken, increasing risk of injury.

• Dendritic cell numbers decrease, reducing immune response.

• Vitamin D3 production decreases, affecting calcium absorption.

• Glandular activity declines, impairing temperature regulation.

• Hair follicles become less active, producing thinner hair.

• Elastic fiber network decreases, leading to wrinkles.

• Sex hormone levels change, affecting hair and fat distribution.

Summary Table: Layers of the Epidermis

Layer	Main Features	Cell Types
Stratum basale	Deepest layer, attached to basement membrane	Basal cells, Merkel cells, melanocytes
Stratum spinosum	Several layers of keratinocytes, desmosomes	Keratinocytes, dendritic cells
Stratum granulosum	Keratin and keratohyalin accumulation, cells begin to die	Keratinocytes
Stratum lucidum	Present only in thick skin, densely packed cells	Keratinocytes
Stratum corneum	Many layers of dead, keratinized cells	Keratinocytes

Summary Table: Skin Glands

Gland Type	Location	Secretion	Function
Sebaceous	Associated with hair follicles	Sebum (oil)	Lubricates and protects skin/hair
Eccrine (Merocrine)	Widely distributed	Watery sweat	Thermoregulation
Apocrine	Armpits, pubic region	Thick, odorous sweat	Begins at puberty, pheromone function
Ceruminous	Ear canal	Cerumen (earwax)	Protects ear canal

Key Equations

• Vitamin D3 Synthesis:

$\text{7-dehydrocholesterol} + \text{UV radiation} \rightarrow \text{cholecalciferol (Vitamin D}_3\text{)}$

Additional info: Some explanations and tables have been expanded for clarity and completeness based on standard Anatomy & Physiology textbooks.