Skin and Body Membranes: Structure, Function, and Clinical Aspects

Chapter 4: Skin and Body Membranes

Overview of Body Membranes

Body membranes are sheets of tissue that cover surfaces, line body cavities, and form protective sheets around organs. They are classified based on their tissue composition and location.

• Epithelial Membranes: Composed of epithelial tissue and an underlying layer of connective tissue. These include cutaneous, mucous, and serous membranes.

• Connective Tissue Membranes: Made entirely of connective tissue; typically found lining joint cavities (e.g., synovial membranes).

Types of Epithelial Membranes

• Cutaneous Membrane: Commonly known as the skin. It is dry, exposed to air, and protects the body.

• Mucous Membranes: Line body cavities open to the exterior (e.g., digestive, respiratory, urinary, and reproductive tracts). They are moist and adapted for absorption and secretion.

• Serous Membranes: Line body cavities closed to the exterior. They occur in pairs: parietal (lines cavity walls) and visceral (covers organs). Serous fluid between layers reduces friction.

Connective Tissue Membranes

• Synovial Membranes: Line joint cavities; secrete synovial fluid for lubrication and movement.

Skin (Cutaneous Membrane)

Functions of the Skin

The skin is the largest organ of the body and serves multiple vital functions:

• Protection: Acts as a mechanical barrier, keeping internal tissues safe from pathogens, chemicals, and physical injury.

• Prevents Water Loss: Maintains hydration by preventing excessive water loss.

• Regulates Body Temperature: Through sweat production and blood vessel dilation/constriction.

• Excretion: Removes waste products via sweat.

• Vitamin D Synthesis: Skin uses ultraviolet light to convert precursors to active Vitamin D, essential for calcium absorption.

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

• Immunity: Provides a barrier and contains immune cells to fight pathogens.

Structure of the Skin

The skin consists of three main layers, each with distinct functions and cell types.

• Epidermis: Outermost layer; stratified squamous epithelium; avascular (no blood vessels). Contains five sublayers (from deepest to superficial):

  1. Stratum basale: dividing cells, melanocytes

  2. Stratum spinosum: dividing cells, spiny cells

  3. Stratum granulosum: dying cells, granules

  4. Stratum lucidum: clear layer found on palms and soles (not present everywhere)

  5. Stratum corneum: dead cells, thickest layer, contains tough keratin protein

• Dermis: Middle layer; contains collagen and elastic fibers, blood vessels, phagocytes, sensory receptors, and glands.

• Hypodermis (Subcutaneous Layer): Deepest layer; composed mainly of adipose tissue (fat), anchors skin to underlying tissues. Not technically part of the skin.

Skin Color

Skin color is determined by the presence and concentration of pigments:

• Melanin: Brown pigment produced by melanocytes; protects against UV radiation.

• Carotene: Orange pigment from vegetables; accumulates in the stratum corneum.

• Hemoglobin: Red pigment in blood; gives skin a pinkish hue when oxygenated.

• Clinical Color Changes:

  • Cyanosis: Bluish skin due to low oxygen.

  • Erythema: Redness from fever, allergy, or inflammation.

  • Pallor: Pale skin from low blood flow or anemia.

  • Jaundice: Yellow skin from liver disease.

  • Hematoma: Purple bruising from blood leakage.

Skin Appendages

Skin appendages are specialized structures derived from the epidermis:

• Glands: Sudoriferous (sweat) and sebaceous (oil) glands.

• Hair: Composed of root, shaft, follicle, and arrector pili muscle (causes goosebumps).

• Nails: Hard plates of keratin; protect fingertips and enhance sensation.

Infections and Allergies

Common Skin Infections

• Athlete's Foot: Fungal infection; causes itchy, red skin, often between toes.

• Boils and Carbuncles: Bacterial infection of hair follicles and sebaceous glands.

• Impetigo: Bacterial infection; pink, water-filled lesions with yellow crust.

• Cold Sores: Viral infection (Herpes simplex); causes blisters.

• Psoriasis: Autoimmune disease; red lesions with silvery scales, excessive keratinocyte production.

Burns

Classification and Assessment

Burns are classified by depth and severity. The "rule of nines" is used to estimate the percentage of body surface area affected:

• First-degree: Epidermis damaged; redness, swelling, pain. Heals in a few days.

• Second-degree: Epidermis and upper dermis damaged; blisters, pain, possible scarring.

• Third-degree: Full thickness burn; skin appears gray/black, nerves destroyed, no pain, no regeneration.

Life-Threatening Changes

• Severe burns can cause dehydration, shock, and increased risk of infection due to loss of protective skin barrier.

Main Types of Skin Cancer

Skin cancer is classified based on the cell type and appearance. The ABCD rule helps identify malignant melanoma.

Type	Description	Common Location
Basal Cell Carcinoma	Shiny, dome-shaped, pearly edge	Face
Squamous Cell Carcinoma	Scaly, red papule, raised border	Scalp, ears, hands
Malignant Melanoma	Spreading pigmented mole; described by ABCD rule	Anywhere on skin

• ABCD Rule for Melanoma:

  • A = Asymmetry of shape

  • B = Border irregularity

  • C = Color variation

  • D = Diameter greater than 6mm

Additional info:

• Vitamin D synthesis in the skin is essential for bone health and calcium metabolism.

• Keratin is a tough, fibrous protein that provides waterproofing and protection.

• Serous membranes are named according to their location (e.g., pericardium around the heart, pleura around the lungs, peritoneum in the abdomen).