The Integumentary System: Structure and Function

The Integumentary System

Overview

The integumentary system is the organ system that protects the body from various kinds of damage, such as loss of water or abrasion from outside. It is composed of the skin and its associated appendages, including hair, nails, and glands. This system serves as the body's first line of defense and plays a vital role in maintaining homeostasis.

• Main components: Skin, hair, nails, exocrine glands

• Functions: Protection, sensation, thermoregulation, excretion, vitamin D synthesis

Skin

General Characteristics

The skin is the largest organ of the human body. It is composed of several layers of tissue, each containing different types of cells. The skin is divided into three major layers, each with distinct structures and functions:

• Epidermis: Outermost layer, provides a waterproof barrier and creates our skin tone.

• Dermis: Beneath the epidermis, contains tough connective tissue, hair follicles, and sweat glands.

• Hypodermis (Subcutaneous layer): Deeper subcutaneous tissue made of fat and connective tissue.

Layers of the Skin

Epidermis

The epidermis is the outermost layer of the skin, composed of stratified squamous epithelial cells. It is avascular (lacks its own blood supply), so nutrients diffuse from the underlying dermis. The epidermis is organized into five distinct layers (from superficial to deep):

• Stratum corneum: Outermost layer, 15-30 layers of dead, keratinized cells; provides a tough, protective barrier.

• Stratum lucidum: Thin, translucent layer found only in thick skin (e.g., palms, soles); composed of dead, flattened cells.

• Stratum granulosum: Cells contain granules that release lipids and proteins, forming a hydrophobic barrier; cells begin to lose nuclei and organelles.

• Stratum spinosum: Thickest layer of living keratinocytes; cells have spine-like extensions and begin keratin production.

• Stratum basale: Deepest layer, single row of cuboidal stem cells (keratinocytes) that undergo mitosis; contains melanocytes (produce melanin), Merkel cells (touch receptors), and immune cells.

Mnemonic: "Can Lily Get Some Books" (Corneum, Lucidum, Granulosum, Spinosum, Basale)

Basement Membrane

The basement membrane is a thin, non-cellular layer composed mainly of collagen fibers. It separates and anchors the epidermis to the dermis.

Dermis

The dermis lies beneath the epidermis and is composed of connective tissue. It provides strength, flexibility, and houses blood vessels, nerves, hair follicles, and glands. The dermis has two main regions:

• Papillary region: Upper layer, contains dermal papillae and terminal blood capillaries.

• Reticular region: Deeper layer, provides skin with strength and elasticity.

Hypodermis (Subcutaneous Layer)

The hypodermis is the deepest layer of the integumentary system. It anchors the skin to underlying structures and is composed of:

• Fibrous bands: Provide anchoring

• Adipose cells: Store fat

• Macrophages: Provide immune protection

Accessory Structures of the Skin

Hair

Most of the skin is covered in hair, though much of it is fine and lightly pigmented. Hair is absent from certain areas such as the palms, soles, and parts of the external genitalia. The visible part of the hair is the shaft, while the root is anchored in a tubular hair follicle that extends into the dermis or hypodermis. Hair growth occurs in cycles:

• Anagen: Growth phase (6-8 years for scalp hair)

• Catagen: Transition phase; hair follicle shrinks, hair dies

• Telogen: Resting phase (2-3 months); old hair falls out, new growth begins

The arrector pili muscle causes hair to stand up (goosebumps).

Exocrine Glands

• Sudoriferous (Sweat) Glands:

  • Apocrine: Found mainly in armpits and groin; ducts open into hair follicles; active at puberty; may produce pheromones.

  • Eccrine: More common; ducts open directly onto skin surface; produce watery sweat for thermoregulation.

• Modified Sweat Glands: Ceruminous (ear wax) and mammary (milk) glands.

• Sebaceous Glands: Secrete sebum (oil) into hair follicles; lubricates skin and inhibits bacterial growth; absent on palms and soles.

Nails

Nails are plates of stratified, squamous epithelial cells with hard keratin. They protect the distal ends of fingers and toes. Nail growth occurs at the lunula (the white crescent at the base of the nail). The cuticle is a fold of stratum corneum at the proximal end.

Physiology of the Integumentary System

• Thermoregulation: Sweat glands and blood vessels help regulate body temperature.

• Vitamin D Synthesis: Skin produces vitamin D when exposed to sunlight.

• Protection: Acts as a barrier to fluid loss and entry of pathogens; melanin protects against UV radiation.

• Absorption: Can absorb certain medications and chemicals.

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

• Excretion: Eliminates water, salts, and small amounts of waste.

Clinical Considerations

Burns

Degree	Layers Involved	Symptoms	Healing
First	Epidermis only	Redness, slight edema, pain	Heals in days, no scarring
Second	Epidermis + part of dermis	Red/tan/white, blistered, very painful	Heals in weeks/months, may scar
Third	Epidermis + dermis + possibly deeper tissues	Painless (nerves destroyed), may have surrounding painful areas	Requires grafts, risk of fluid loss and infection

Skin Cancer

Type	Origin	Appearance	Prognosis
Basal cell carcinoma	Stratum basale	Small, shiny bump; pearly edge	Most common, rarely metastasizes
Squamous cell carcinoma	Stratum spinosum	Raised, red, scaly; often on ears/hands	Can metastasize
Melanoma	Melanocytes	Irregular, multicolored, >6mm, evolving	Most aggressive, poor prognosis

ABCDE Rule for Melanoma: Asymmetry, Border irregularity, Color variation, Diameter >6mm, Evolving

Risk Factors for Skin Cancer

• Genetic: Family history, fair skin, light eyes/hair, many or unusual moles, certain skin conditions

• Personal: Previous skin cancer, frequent/intense sun exposure, blistering sunburns (especially in childhood), tanning bed use, immunosuppression

Additional Information

UV Radiation and Sunscreen

• UVA: Lower energy, causes tanning, contributes to skin aging and cancer

• UVB: Higher energy, causes sunburn, also contributes to skin cancer

• SPF (Sun Protection Factor): SPF 15 blocks 93% of UVB, SPF 30 blocks 97%. Effectiveness depends on application frequency and water resistance.

Note: Sunburn and skin cancer are caused by different mechanisms; sunscreen may not fully prevent skin cancer even if it prevents sunburn.

Practice Questions

• What is the outermost layer of the epidermis? Stratum corneum

• Which layer of the epidermis is composed completely of dead cells? Stratum corneum

• Nail growth occurs at which structure? Lunula

• The most aggressive type of skin cancer is? Melanoma

• The 'crescent moon' at the bottom of your nail is the? Lunula

• The most easily treated skin cancer is? Basal cell carcinoma

• Keratinocytes and melanocytes are found in the? Stratum basale

• The growth stage of the hair cycle is? Anagen