Chapter 4: Tissue—The Living Fabric

Introduction to Tissues

Tissues are groups of cells with similar structure and function, forming the "fabric" of the human body. Understanding tissue types is essential for monitoring health and diagnosing conditions such as bedsores.

• Tissue: Groups of cells performing a common function.

• Histology: The study of tissues.

• Four basic tissue types: epithelial, connective, muscle, and nervous.

Epithelial tissue forms boundaries, protects, secretes, absorbs, and filters. Connective tissue supports and binds other tissues. Muscle tissue contracts for movement. Nervous tissue enables internal communication.

Microscopy of Human Tissue

To study tissues, samples must be prepared for microscopy:

• Fixed: Preserved with solvent.

• Sectioned: Cut into thin slices.

• Stained: Enhanced contrast for visualization.

• Light microscopy uses colored dyes; electron microscopy uses heavy metal coatings.

Epithelial Tissue

Overview and Functions

Epithelial tissue (epithelium) is a sheet of cells covering surfaces or lining cavities. It is classified as covering/lining or glandular epithelia.

• Covering and lining epithelia: External/internal surfaces (e.g., skin).

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

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

Special Characteristics of Epithelial Tissues

Epithelial tissues have five key characteristics:

• Polarity: Distinct apical (top) and basal (bottom) surfaces.

• Specialized contacts: Tight junctions and desmosomes bind cells together.

• Supported by connective tissues: Basement membrane (basal and reticular lamina) reinforces and defines boundaries.

• Avascular, but innervated: No blood vessels; nourished by diffusion; supplied by nerve fibers.

• Regeneration: High capacity for renewal, especially in areas exposed to friction or hostile environments.

Classification of Epithelia

Epithelial tissues are classified by cell layers and cell shape:

• Number of cell layers:

  • Simple epithelia: Single layer; functions in absorption, secretion, filtration.

  • Stratified epithelia: Multiple layers; functions in protection.

• Cell shape:

  • Squamous: Flattened, scale-like.

  • Cuboidal: Box-like, cube-shaped.

  • Columnar: Tall, column-like.

Types of Simple Epithelia

Simple Squamous Epithelium

Single layer of flattened cells; sparse cytoplasm. Functions in rapid diffusion and filtration.

• Locations: Kidney glomeruli, air sacs of lungs, lining of heart, blood vessels, lymphatic vessels, serosae.

• Special types:

  • Endothelium: Lines lymphatic vessels, blood vessels, heart.

  • Mesothelium: Serous membranes in ventral body cavity.

Simple Cuboidal Epithelium

Single layer of cube-shaped cells with large, central nuclei. Functions in secretion and absorption.

• Locations: Kidney tubules, ducts and secretory portions of small glands, ovary surface.

Simple Columnar Epithelium

Single layer of tall, closely packed cells. May have microvilli or cilia; often contains goblet cells. Functions in absorption and secretion of mucus, enzymes, and other substances.

• Locations: Digestive tract, gallbladder, ducts of some glands, bronchi, uterine tubes.

Pseudostratified Columnar Epithelium

Single layer of cells of varying heights, giving a false impression of stratification. Many cells are ciliated; contains goblet cells. Functions in secretion and movement of mucus.

• Locations: Upper respiratory tract, ducts of large glands, tubules in testes.

Types of Stratified Epithelia

Stratified Squamous Epithelium

Several layers; basal cells are cuboidal/columnar, surface cells are squamous. Protects against abrasion.

• Locations: Nonkeratinized—moist linings of esophagus, mouth, vagina; keratinized—epidermis of skin.

Stratified Cuboidal and Columnar Epithelium

Rare types; stratified cuboidal found in sweat and mammary glands; stratified columnar found in pharynx, male urethra, and some glandular ducts.

Usually occur at transition areas between other epithelia.

Transitional Epithelium

Resembles both stratified squamous and cuboidal; basal cells cuboidal/columnar, surface cells dome-shaped. Stretches to permit distension of urinary organs.

• Locations: Ureters, bladder, part of urethra.

Glandular Epithelia

Overview of Glands

A gland is one or more cells that secrete an aqueous fluid called a secretion. Glands are classified by site of product release (endocrine or exocrine) and number of cells (unicellular or multicellular).

• Endocrine glands: Ductless; secrete hormones into interstitial fluid, then into blood or lymph.

• Exocrine glands: Secrete products onto body surfaces or into cavities; more numerous; can be unicellular (e.g., goblet cells) or multicellular (e.g., salivary glands).

Unicellular Exocrine Glands

Unicellular glands, such as goblet cells, are found in epithelial linings of intestinal and respiratory tracts. They produce mucin, which forms mucus when hydrated.

Multicellular Exocrine Glands

Multicellular exocrine glands consist of a duct and a secretory unit. They are classified by structure (simple or compound ducts) and mode of secretion (tubular, alveolar, tubuloalveolar).

• Simple glands: Unbranched ducts.

• Compound glands: Branched ducts.

• Tubular: Secretory cells form tubes.

• Alveolar: Secretory cells form sacs.

• Tubuloalveolar: Both tube and sac structures.

Modes of Secretion

Multicellular exocrine glands secrete by different mechanisms:

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

• Holocrine: Accumulate products, then rupture (e.g., sebaceous oil glands).

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

Clinical Relevance

Cancerous epithelial cells can penetrate the basement membrane, invading underlying tissues and spreading cancer.

Summary Table: Epithelial Tissue Types