Integumentary System: Structure, Function, and Pathology

Module 5.1 Overview of the Integumentary System

Skin Structure

The skin, also known as the cutaneous membrane, is the largest organ of the body and serves as a protective barrier. It consists of two main layers and several accessory structures.

• Two Main Components:

  • Epidermis: The outermost layer, composed primarily of epithelial cells.

  • Dermis: The deeper, connective tissue layer beneath the epidermis.

• Accessory Structures:

  • Sweat glands

  • Sebaceous glands

  • Hair

  • Nails

  • Sensory receptors and arrector pili muscles (smooth muscle associated with hair follicles)

• Avascularity: The epidermis lacks blood vessels; nutrients diffuse from the underlying dermis.

• Cellular Composition: About 50% of epidermal cells are too far from blood supply to survive; the superficial layers are composed of dead cells.

• Hypodermis: Also called superficial fascia or subcutaneous tissue, this layer lies deep to the dermis and contains adipose tissue.

Functions of the Integumentary System

• Protection: Acts as a barrier against mechanical injury, pathogens, and water loss.

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

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

• Excretion: Eliminates waste products through sweat.

• Vitamin D Synthesis: Initiates the synthesis of vitamin D when exposed to UV light.

Thermoregulation Sequence

When body temperature rises above normal, the following sequence occurs:

1. Receptor: Detects temperature change.

2. Control Center: Processes information (usually the hypothalamus).

3. Effector/Response: Sweat glands activate, blood vessels dilate.

4. Return to Homeostatic Range: Body temperature decreases.

The Epidermis

The epidermis is the most superficial layer of skin, composed of several cell types, primarily keratinocytes.

• Keratinocytes: Make up about 95% of the epidermis and produce keratin, a protein that strengthens the skin.

• Layers of the Epidermis: (from deep to superficial)

  1. Stratum basale

  2. Stratum spinosum

  3. Stratum granulosum

  4. Stratum lucidum (only in thick skin)

  5. Stratum corneum

• Keratinocyte Life Cycle: Cells migrate from the stratum basale to the stratum corneum, a process taking 40–50 days. The stratum corneum is continuously shed and replaced.

• Other Epidermal Cells:

  • Dendritic (Langerhans) cells: Immune defense

  • Merkel cells: Sensory receptors for touch

  • Melanocytes: Produce melanin, the pigment responsible for skin color

Thick and Thin Skin

• Thick Skin: Found on palms and soles; has all five epidermal layers and is adapted to mechanical stress.

• Thin Skin: Covers most of the body; lacks stratum lucidum and has fewer cell layers.

• Callus: Localized thickening of the stratum corneum due to repeated friction.

Module 5.3 The Dermis

The dermis is a connective tissue layer beneath the epidermis, divided into two regions:

• Papillary Layer: Contains dermal papillae and tactile (Meissner) corpuscles for touch sensation.

• Reticular Layer: Contains lamellated (Pacinian) corpuscles for pressure sensation and is responsible for skin markings such as tension (cleavage) lines due to collagen bundle arrangement.

• Skin Markings:

  • Epidermal ridges: Enhance grip and form fingerprints.

  • Tension lines: Indicate the direction of collagen fibers; important in surgical incisions.

Module 5.4 Skin Pigmentation

Skin color is determined by the number and activity of melanocytes and the type and amount of pigment produced.

• Melanin: Main pigment; protects against UV radiation.

• Other Pigments:

  • Carotene: Yellow-orange pigment from diet.

  • Hemoglobin: Red pigment in blood; visible in fair skin.

• Skin Color as a Diagnostic Tool: Changes in skin color can indicate disease:

  • Erythema: Redness

  • Pallor: Paleness

  • Cyanosis: Bluish color due to low oxygen

Module 5.5 Accessory Structures of the Integument: Hair, Nails, and Glands

Accessory Structures (Appendages)

• Hair (Pili):

  • Composed of shaft and root; made of keratinized epithelial cells.

  • Associated with arrector pili muscles, which cause hair to stand up (goosebumps).

  • Types of Hair:

    • Lanugo: Fine, unpigmented fetal hair

    • Vellus hair: Fine, pale body hair

    • Terminal hair: Coarse, pigmented hair (scalp, eyebrows, after puberty)

• Nails:

  • Nail plate: Visible part of the nail

  • Nail root: Embedded in skin

  • Nail matrix: Site of nail growth

  • Proximal nail fold, eponychium, lateral nail folds, hyponychium, lunula: Regions supporting and protecting the nail

  • Function: Protects distal tips of fingers and toes

• Glands:

  • Sweat (sudoriferous) glands:

    • Eccrine sweat glands: Widely distributed, regulate temperature

    • Apocrine sweat glands: Found in axillae, groin; active after puberty

    • Ceruminous glands: Produce earwax

    • Mammary glands: Produce milk

  • Sebaceous glands: Secrete sebum (oily substance)

Module 5.6 Pathology of the Skin

Wounds and Burns

• Wounds: Disruption of skin integrity

• Burns:

  • First-degree burns: Affect only the epidermis

  • Second-degree burns: Affect epidermis and part of dermis

  • Third-degree burns: Destroy epidermis, dermis, and deeper tissues

Skin Cancer

Skin cancer is one of the most common cancers, often caused by DNA mutations from UV exposure. Early detection is critical for successful treatment.

• Types of Skin Cancer:

  • Basal cell carcinoma: Most common, least dangerous; arises from stratum basale

  • Squamous cell carcinoma: Arises from stratum spinosum; may metastasize

  • Malignant melanoma: Most dangerous; arises from melanocytes; high risk of metastasis

• ABCDEs of Melanoma Detection:

  • A: Asymmetry

  • B: Border irregularity

  • C: Color variation

  • D: Diameter > 6 mm

  • E: Evolving shape and size

• Treatment: Surgical removal, radiation therapy, chemotherapy

Summary Table: Layers of the Epidermis

Layer	Location	Main Features
Stratum basale	Deepest	Single row of stem cells; site of mitosis
Stratum spinosum	Above basale	Several layers; contains keratinocytes and dendritic cells
Stratum granulosum	Middle	3-5 layers; keratinization begins
Stratum lucidum	Only in thick skin	Clear, thin layer
Stratum corneum	Superficial	Many layers of dead, keratinized cells

Key Equations and Concepts

• Rule of Nines (for burn assessment): Used to estimate the percentage of body surface area affected by burns.

• Vitamin D Synthesis: UV light + 7-dehydrocholesterol (in skin) → Vitamin D3 (cholecalciferol)

Example: A patient with a third-degree burn on the anterior surface of one leg would have approximately 9% of their body surface area affected (using the Rule of Nines).