Tissue: The Living Fabric

Introduction to Tissues

Tissues are groups of cells similar in structure that perform common or related functions, essential for maintaining homeostasis in the human body. The study of tissues is known as histology. There are four basic tissue types: epithelial, connective, muscle, and nervous tissue.

• Epithelial tissue: Covers body surfaces and lines cavities.

• Connective tissue: Supports, protects, and binds other tissues.

• Muscle tissue: Contracts to produce movement.

• Nervous tissue: Enables internal communication.

Microscopy of Human Tissue

To study tissues under a microscope, they must be properly prepared:

• Fixed: Preserved with a solvent to prevent decay.

• Sectioned: Cut into thin slices for light or electron transmission.

• Stained: Enhanced contrast using dyes (light microscopy) or heavy metals (electron microscopy). Note: Staining may introduce artifacts (distortions).

Epithelial Tissue

Definition and Functions

Epithelial tissue (epithelium) is a sheet of cells that covers body surfaces or lines body cavities. It exists in two main forms:

• Covering and lining epithelia: Found on external and internal surfaces (e.g., skin, lining of digestive tract).

• Glandular epithelia: Forms secretory tissue in glands (e.g., salivary glands).

Main functions include protection, absorption, filtration, excretion, secretion, and sensory reception.

Special Characteristics of Epithelial Tissues

Epithelial tissues have five distinguishing characteristics:

1. Polarity: Cells have an apical (top) surface exposed to the exterior or cavity, and a basal (bottom) surface attached to underlying tissue. The apical surface may have microvilli for absorption.

2. Specialized contacts: Cells are closely packed and joined by tight junctions and desmosomes to form continuous sheets.

3. Supported by connective tissues: The basement membrane (basal lamina + reticular lamina) reinforces the epithelium and resists stretching and tearing.

4. Avascular, but innervated: Contains no blood vessels but is supplied by nerve fibers; nutrients diffuse from underlying connective tissue.

5. Regeneration: High regenerative capacity, especially in areas exposed to friction or hostile environments.

Classification of Epithelia

All epithelial tissues are classified by two criteria:

• Number of cell layers:

  • Simple epithelia: Single cell layer (for absorption, secretion, filtration).

  • Stratified epithelia: Two or more layers (for protection).

• Cell shape:

  • Squamous: Flattened and scale-like.

  • Cuboidal: Box-like, as tall as they are wide.

  • Columnar: Tall and column-shaped.

Types of Epithelia

Simple Epithelia

• Simple squamous epithelium: Single layer of flat cells; allows rapid diffusion and filtration. Found in kidney glomeruli, air sacs of lungs, lining of heart, blood vessels, and lymphatic vessels. Special types include endothelium (lining vessels) and mesothelium (serous membranes).

• Simple cuboidal epithelium: Single layer of cube-shaped cells; functions in secretion and absorption. Found in kidney tubules and ducts of small glands.

• Simple columnar epithelium: Single layer of tall cells; may have microvilli or cilia, and goblet cells. Functions in absorption and secretion of mucus and enzymes. Found in digestive tract, gallbladder, and some ducts.

• Pseudostratified columnar epithelium: Appears multilayered but is a single layer; often ciliated. Functions in secretion and movement of mucus. Found in upper respiratory tract and ducts of large glands.

Stratified Epithelia

• Stratified squamous epithelium: Several layers; protects underlying tissues in areas of abrasion. Keratinized type forms the skin; nonkeratinized type lines moist cavities (mouth, esophagus, vagina).

• Stratified cuboidal epithelium: Rare; found in some sweat and mammary glands (usually two layers).

• Stratified columnar epithelium: Rare; found in pharynx, male urethra, and some glandular ducts.

• Transitional epithelium: Resembles both stratified squamous and cuboidal; stretches to allow urinary organs to expand. Found in bladder, ureters, and part of urethra.

Glandular Epithelia

Definition and Classification

A gland consists of one or more cells that make and secrete an aqueous fluid called a secretion. Glands are classified by:

• Site of product release: Endocrine (internally secreting, e.g., hormones) or exocrine (externally secreting, e.g., sweat).

• Number of cells: Unicellular (e.g., goblet cells) or multicellular (e.g., salivary glands).

Endocrine Glands

• Ductless glands; secrete hormones directly into interstitial fluid, which is then picked up by the circulatory system.

• Hormones act as chemical messengers to target organs throughout the body.

Exocrine Glands

• Secrete products onto body surfaces or into body cavities via ducts.

• Include mucous, sweat, oil, and salivary glands.

• Can be unicellular or multicellular.

Unicellular Exocrine Glands

• Main types: mucous cells and goblet cells.

• Found in epithelial linings of intestinal and respiratory tracts.

• Produce mucin, which forms mucus for protection and lubrication.

Multicellular Exocrine Glands

• Composed of a duct and a secretory unit, often surrounded by connective tissue.

• Classified by structure (simple vs. compound; tubular vs. alveolar) and mode of secretion.

Modes of Secretion

• Merocrine: Secrete products by exocytosis (e.g., sweat, pancreas).

• Holocrine: Accumulate products until the cell ruptures (e.g., sebaceous oil glands).

• Apocrine: Accumulate products, then only the apex ruptures (controversial in humans; possibly mammary glands).

Clinical Connection: Homeostatic Imbalance

Cancerous epithelial cells can penetrate the basement membrane and invade underlying tissues, leading to the spread of cancer (metastasis). The integrity of the basement membrane is crucial for containing epithelial cells.