The Integumentary System: Structure, Function, and Components

An Introduction to the Integumentary System

The Integument

The integumentary system is the largest organ system of the human body, comprising the skin and its accessory structures. It serves as the primary barrier between the internal environment and the external world.

• Largest system of the body: Accounts for approximately 16% of body weight and covers 1.5 to 2 m2 in area.

• Two main parts:

  1. Cutaneous membrane (skin)

  2. Accessory structures (hair, nails, glands)

Components of the Cutaneous Membrane

• Outer epidermis: Superficial epithelial tissue

• Inner dermis: Connective tissue

Accessory Structures

• Originate in the dermis

• Extend through the epidermis to the skin surface (hair, nails, multicellular exocrine glands)

Connections

• Cardiovascular system: Blood vessels in the dermis

• Nervous system: Sensory receptors for pain, touch, and temperature

Hypodermis (Superficial Fascia or Subcutaneous Layer)

• Loose connective tissue below the dermis

• Location of hypodermic injections

Functions of Skin

• Protection of underlying tissues and organs

• Excretion of salts, water, and organic wastes (glands)

• Maintenance of body temperature (insulation and evaporation)

• Production of melanin and keratin

• Synthesis of vitamin D3

• Storage of lipids

• Detection of touch, pressure, pain, and temperature

5-1 Epidermis

Structure and Function

The epidermis is the outermost layer of the skin, composed of avascular stratified squamous epithelium. Nutrients and oxygen diffuse from capillaries in the underlying dermis.

• Keratinocytes: Most abundant cells, produce keratin

Types of Skin

• Thin skin: Covers most of the body, has four layers of keratinocytes

• Thick skin: Covers palms and soles, has five layers of keratinocytes

Layers of the Epidermis (from deep to superficial)

1. Stratum basale: Attached to basement membrane by hemidesmosomes; contains basal cells (germinative cells), forms epidermal ridges (fingerprints), and dermal papillae.

2. Stratum spinosum: "Spiny layer"; produced by division of stratum basale, contains dendritic (Langerhans) cells for immune response.

3. Stratum granulosum: "Grainy layer"; cells stop dividing and start producing keratin and keratohyalin.

4. Stratum lucidum: "Clear layer"; found only in thick skin, covers stratum granulosum.

5. Stratum corneum: "Horn layer"; exposed surface, 15–30 layers of keratinized cells, water resistant, shed and replaced every two weeks.

Keratinization

• Formation of a layer of dead, protective cells filled with keratin

• Occurs on all exposed skin surfaces except eyes

• Skin life cycle: 7–10 days for a cell to move from stratum basale to stratum corneum

Perspiration

• Insensible perspiration: Interstitial fluid lost by evaporation through stratum corneum

• Sensible perspiration: Water excreted by sweat glands

• Dehydration: Results from damage to stratum corneum (e.g., burns, blisters) or immersion in hypertonic solution (e.g., seawater)

• Hydration: Results from immersion in hypotonic solution (e.g., freshwater)

5-2 Skin Color

Determinants of Skin Color

• Influenced by two pigments:

  1. Carotene: Orange-yellow pigment, found in orange vegetables, accumulates in epidermal cells and fatty tissues, can be converted to vitamin A.

  2. Melanin: Yellow-brown or black pigment, produced by melanocytes in stratum basale, stored in melanosomes, transferred to keratinocytes.

• Blood circulation (red blood cells)

Function of Melanocytes

• Melanin protects skin from ultraviolet (UV) radiation

• UV radiation causes DNA mutations and burns that can lead to cancer and wrinkles

• Skin color depends on melanin production, not number of melanocytes

Capillaries and Skin Color

• Oxygenated blood contributes to skin color

• Blood vessels dilate from heat, skin reddens

• Blood flow decreases, skin pales

• Cyanosis: Bluish skin tint caused by severe reduction in blood flow or oxygenation

Illness and Skin Color

• Jaundice: Yellow skin due to buildup of bile produced by liver

• Addison's disease: Disease of the pituitary gland, skin darkening due to excess MSH

• Vitiligo: Loss of melanocytes, loss of color

5-3 Vitamin D3

Vitamin D3 Synthesis

• Epidermal cells produce cholecalciferol (vitamin D3) in the presence of UV radiation

• Liver and kidneys convert vitamin D3 into calcitriol, which aids absorption of calcium and phosphorus

• Insufficient vitamin D3 can cause rickets

5-4 Epidermal Growth Factor (EGF)

Functions of EGF

• Promotes division of germinative cells

• Accelerates keratin production

• Stimulates epidermal repair

• Stimulates glandular secretion

5-5 The Dermis

Structure and Function

The dermis is located between the epidermis and the subcutaneous layer. It anchors accessory structures and consists of two layers:

• Outer papillary layer: Consists of areolar tissue, contains smaller capillaries, lymphatics, and sensory neurons, has dermal papillae projecting between epidermal ridges.

• Deep reticular layer: Consists of dense irregular connective tissue, contains larger blood vessels, lymphatics, and nerve fibers, contains collagen and elastic fibers.

Dermatitis

• Inflammation of the papillary layer

• Caused by infection, radiation, mechanical irritation, or chemicals (e.g., poison ivy)

• Characterized by itch or pain

Dermal Strength and Elasticity

• Collagen fibers: Very strong, resist stretching but bend easily, provide flexibility

• Elastic fibers: Permit stretching and recoil to original length, limit flexibility of collagen fibers to prevent damage

• Skin turgor: Properties of flexibility and resilience

Skin Damage

• Sagging and wrinkles (reduced skin elasticity) caused by dehydration, age, hormonal changes, UV exposure

• Thickened tissue from excessive stretching due to pregnancy or weight gain

Cleavage Lines

• Collagen and elastic fibers arranged in parallel bundles

• Resist force in a specific direction

• Cleavage (tension) lines establish important patterns for surgery

Dermal Blood Supply

• Cutaneous plexus: Network of arteries along the reticular layer

• Papillary plexus: Capillary network from small arteries in papillary layer

• Venous plexus: Capillary return deep to the papillary plexus

• Damage to blood vessels results in "black-and-blue" bruising

Innervation of the Skin

• Nerve fibers in skin control blood flow, gland secretions, and sensory receptors

• Light touch: tactile corpuscles in dermal papillae

• Deep pressure and vibration: lamellated corpuscles in reticular layer

5-6 The Hypodermis

Structure and Function

• Lies below the integument

• Stabilizes the skin

• Allows separate movement

• Made of elastic areolar and adipose tissues

• Few capillaries and no vital organs

• Site of subcutaneous injections using hypodermic needles

Deposits of Subcutaneous Fat

• Distribution patterns determined by hormones

• Reduced by cosmetic liposuction (lipoplasty)

5-7 Hair

Hair, Hair Follicles, Sebaceous Glands, Sweat Glands, and Nails

• Integumentary accessory structures

• Derived from embryonic epidermis

• Located in dermis

• Project through the skin surface

Distribution of Hair

• Human body is covered with hair except: palms, soles, lips, portions of external genitalia

Functions of Hair

• Protects and insulates

• Guards openings against particles and insects

• Sensitive to very light touch

The Hair Follicle

• Located deep in dermis

• Produces nonliving hairs

• Wrapped in dense connective tissue sheath

• Base is surrounded by sensory nerves (root hair plexus)

Regions of the Hair

• Hair root: Lower part of the hair, attached to the integument

• Hair shaft: Upper part of the hair, not attached to the integument

Hair Production

• Begins at the base of a hair follicle, deep in the dermis

• Hair papilla: Contains capillaries and nerves

• Hair bulb: Produces hair matrix (layer of dividing basal cells)

• Pushes hair up and out of skin

Hair Shaft Structure

• Medulla: Central core

• Cortex: Middle layer

• Cuticle: Surface layer

Keratin in Hair

• As hair is produced, it is keratinized

• Medulla contains flexible soft keratin

• Cortex and cuticle contain stiff hard keratin

Additional info:

• Hair growth and loss are influenced by genetics, hormones, and health status.

• Accessory structures such as sebaceous glands lubricate hair and skin, and arrector pili muscles cause "goose bumps".