The 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.

• 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

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

• About 50% of epidermal cells are too far from blood supply to survive, so the superficial layers are made of dead cells.

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

Functions of the Integumentary System

• Protection: Acts as a barrier against pathogens, chemicals, and physical trauma.

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

• Thermoregulation: Maintains body temperature through sweat production and blood flow regulation.

• Excretion: Removes waste products through sweat.

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

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

1. Receptor: Detects the change in temperature.

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

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

4. In Homeostatic Range: Body temperature returns to normal.

The Epidermis

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

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

• Keratinocytes are linked by desmosomes, providing structural integrity.

• Layers of the Epidermis: From deep to superficial:

  • Stratum basale

  • Stratum spinosum

  • Stratum granulosum

  • Stratum lucidum (only in thick skin)

  • 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 cells of the epidermis:

  • 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 the 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 dense irregular connective tissue, lamellated (Pacinian) corpuscles for pressure, and skin markings such as epidermal ridges (fingerprints) and tension (cleavage) lines due to collagen bundle orientation.

Module 5.4 Skin Pigmentation

Skin color is determined by the amount and type of pigment produced by melanocytes and other factors.

• Melanin: Main pigment; protects against UV radiation.

• Mole (nevus): Localized overgrowth of melanocytes.

• 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).

  • Hair types:

    • 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: Proximal part under the skin.

  • Nail matrix: Site of nail growth.

  • Proximal nail fold, medial/lateral nail folds, hyponychium, lunula: Structures 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) to lubricate skin and hair.

Module 5.6 Pathology of the Skin

Wounds and Burns

• Wound: Any break in the skin's integrity.

• Burns:

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

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

  • Third-degree burns: Destroy epidermis and dermis; may damage deeper tissues.

Skin Cancer

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

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

• Squamous cell carcinoma: Arises from stratum spinosum; can metastasize.

• Malignant melanoma: Most dangerous; arises from melanocytes. Key features (ABCDE):

  • A: Asymmetry

  • B: Border irregularity

  • C: Color variation

  • D: Diameter > 6 mm

  • E: Evolving shape and size

Early detection and treatment (surgical removal, radiation, chemotherapy) are essential to prevent metastasis.