The Integumentary System: Structure, Function, and Clinical Aspects

The Integumentary System

Overview and Components

The integumentary system consists of the skin and its derivatives, including nails, hair, sudoriferous (sweat) glands, and sebaceous (oil) glands. It serves as the body's primary barrier to the external environment and plays vital roles in protection, sensation, and regulation.

• Skin (cutaneous membrane): The largest organ of the body, covering all external surfaces.

• Derivatives: Nails, hair, sweat glands, and sebaceous glands.

• Functions: Protection, sensation, thermoregulation, metabolic functions, and excretion.

• Dermatology: The scientific study and treatment of the skin.

Key facts:

• Visual indicator of physiology and health.

• Composed of all four tissue types: epithelial, connective, muscle, and nervous tissue.

• Contains nervous tissue for touch, pressure, temperature, and pain sensation.

Layers of the Integument

Major Layers

The skin is organized into two main layers, with an additional subcutaneous layer beneath:

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

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

• Subcutaneous (hypodermis) layer: Not technically part of the skin, but supports it with areolar and adipose connective tissue.

Structure of the Skin (Diagram Description)

The skin consists of several distinct layers and structures, as shown in the cross-sectional diagram:

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

• Dermis: Contains tough connective tissue, hair follicles, and sweat glands. Divided into:

  • Papillary layer: Upper dermal region with areolar connective tissue and tactile (sensory) receptors.

  • Reticular layer: Deeper dermal region with dense irregular connective tissue, blood vessels, and nerves.

• Subcutaneous (hypodermis): Contains adipose tissue, blood vessels, and nerves.

• Associated structures: Hair follicles, sebaceous glands, sweat glands, arrector pili muscles, sensory nerve fibers, and blood vessels (arteries and veins).

Epidermis

Structure and Layers

The epidermis is a stratified squamous epithelium, organized into distinct layers (strata). The number of layers varies by location:

• Thick skin (palms, soles): 5 layers

• Thin skin (most of the body): 4 layers (lacking stratum lucidum)

Layers of the Epidermis (from deep to superficial):

1. Stratum basale: Deepest layer; single row of cuboidal or columnar cells attached to the basement membrane. Contains stem cells that divide to form new keratinocytes.

2. Stratum spinosum: Several layers of keratinocytes joined by desmosomes; provides strength and flexibility.

3. Stratum granulosum: 3-5 layers of flattened cells; site of keratinization where cells begin to die and fill with keratin.

4. Stratum lucidum: Thin, translucent layer found only in thick skin; filled with the protein eleidin.

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

Dermis

Structure and Function

The dermis lies beneath the epidermis and is primarily composed of connective tissue. It provides structural support and houses many accessory structures.

• Papillary layer: Superficial, areolar connective tissue; contains capillaries and sensory receptors.

• Reticular layer: Deeper, dense irregular connective tissue; contains collagen and elastic fibers, blood vessels, nerves, hair follicles, and glands.

• Functions: Sensation, thermoregulation, and support for epidermal structures.

Subcutaneous Layer (Hypodermis)

The subcutaneous layer is not technically part of the skin but is important for fat storage, insulation, and anchoring the skin to underlying tissues.

• Composed of areolar and adipose connective tissue.

• Acts as an energy reservoir and provides thermal insulation.

• Common site for subcutaneous injections.

Accessory Structures of the Skin

Hair, Nails, and Glands

• Hair: Provides protection, sensation, and helps reduce heat loss.

• Nails: Protect the distal tips of fingers and toes; assist in grasping objects.

• Glands:

  • Sudoriferous (sweat) glands: Eccrine glands secrete watery sweat for thermoregulation; apocrine glands secrete a thicker, odoriferous sweat in specific regions (axillae, groin).

  • Sebaceous (oil) glands: Secrete sebum to lubricate skin and hair; have antibacterial properties.

  • Ceruminous glands: Found in the ear canal; produce cerumen (earwax) for protection.

Functions of the Integumentary System

Major Functions

• Protection: Physical barrier against mechanical injury, pathogens, and dehydration.

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

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

• Metabolic regulation: Synthesizes vitamin D (cholecalciferol) upon UV exposure, which is converted to calcidiol in the liver and then to calcitriol in the kidney. Calcitriol increases calcium and phosphate absorption in the intestine.

• Excretion: Eliminates waste products through sweat.

Clinical Aspects

Nail Disorders

• Spoon nails: Concave nails, often a sign of iron deficiency.

• Ingrown nails: Nail edge grows into the skin, causing pain and possible infection.

• Onychomycosis: Fungal infection of the nails.

Psoriasis

• Chronic autoimmune disease where T-lymphocytes attack keratinocytes.

• Leads to rapid overproduction of skin cells, resulting in scaly, whitish patches.

• Symptoms: Itching, pain, skin cracking.

• Treatments: Corticosteroids, UV light therapy, medications to slow cell production.

Burns

• First-degree: Involves only the epidermis; redness and pain.

• Second-degree: Involves epidermis and part of dermis; blisters, pain, possible scarring.

• Third-degree: Involves epidermis, dermis, and subcutaneous layer; severe scarring, risk of dehydration and infection, may require skin grafts.

Rule of Nines: Used to estimate the percentage of body surface area affected by burns. Burns are considered critical if:

• Over 25% of body has second-degree burns

• Over 10% of body has third-degree burns

• Third-degree burns on face, hands, or feet

Skin Cancer

• Most common types: Basal cell carcinoma, squamous cell carcinoma, and melanoma.

• Melanoma: Most dangerous due to aggressive growth and metastasis.

• ABCDE Rule for Melanoma Detection:

  • A: Asymmetry

  • B: Border irregularity

  • C: Color variation

  • D: Diameter greater than 6 mm

  • E: Evolving (changes in size, shape, or color)

Table: Layers and Structures of the Integument

Layer	Main Tissue Type	Key Structures	Functions
Epidermis	Stratified squamous epithelium	Keratinocytes, melanocytes, Langerhans cells	Protection, water resistance, UV protection
Dermis (Papillary)	Areolar connective tissue	Capillaries, tactile receptors	Nourishes epidermis, sensation
Dermis (Reticular)	Dense irregular connective tissue	Hair follicles, glands, blood vessels, nerves	Strength, elasticity, houses accessory structures
Subcutaneous (Hypodermis)	Areolar and adipose tissue	Fat cells, larger blood vessels	Insulation, energy storage, anchors skin

Key Equations

• Vitamin D Synthesis Pathway:

$\text{7-dehydrocholesterol (in skin)} \xrightarrow{UV\ light} \text{Cholecalciferol (Vitamin D}_3\text{)} \xrightarrow{liver} \text{Calcidiol} \xrightarrow{kidney} \text{Calcitriol (active Vitamin D)}$

• Rule of Nines (Burn Assessment):

$\text{Total Body Surface Area Burned} = \sum \text{(Area % for each region affected)}$

Additional info: The above notes expand on the provided content with standard academic context, definitions, and a summary table for clarity.