Tissue: The Living Fabric – Structure and Function of Epithelial Tissue

Tissue: The Living Fabric

Introduction to Tissues

Tissues are groups of cells with similar structure that perform a common or related function, 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 together.

• Muscle tissue: Contracts to cause movement.

• Nervous tissue: Enables internal communication by transmitting electrical impulses.

Microscopy of Human Tissue

Preparation and Visualization

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

• Fixation: Preserves tissue using a solvent.

• Sectioning: Slices tissue thin enough for light or electron transmission.

• Staining: Enhances contrast; light microscopy uses colored dyes, electron microscopy uses heavy metals.

Additional info: Artifacts may be introduced during preparation, causing distortions not present in living tissue.

Epithelial Tissue

Definition and Functions

Epithelial tissue (epithelium) consists of sheets of cells that cover body surfaces or line cavities. It has 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

• Polarity: Cells have an apical (top, exposed) and basal (bottom, attached) surface, each with distinct structures and functions.

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

• Supported by connective tissues: All epithelial sheets rest on a basement membrane composed of basal and reticular lamina.

• Avascular but innervated: No blood vessels; nutrients diffuse from underlying connective tissue. Nerve fibers are present.

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

Clinical Note: Cancerous epithelial cells can breach the basement membrane, invading underlying tissues and spreading cancer.

Classification of Epithelia

By Layers and Cell Shape

Each epithelial tissue is named according to two criteria:

• Number of layers: Simple (one layer) or stratified (two or more layers).

• Cell shape: Squamous (flattened), cuboidal (cube-like), or columnar (tall, column-like).

In stratified epithelia, the shape is named according to the apical layer.

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. Found in digestive tract, gallbladder, ducts of some glands, bronchi, and uterine tubes.

• 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: Multiple layers; protects underlying tissues in areas of abrasion. Keratinized type forms the skin; nonkeratinized type lines moist cavities (e.g., esophagus, mouth, vagina).

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

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

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

Glandular Epithelia

Definition and Classification

A gland consists of one or more cells that 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

Endocrine glands are ductless; they secrete hormones directly into the interstitial fluid, which is then picked up by the circulatory system. Hormones act as chemical messengers to regulate target organs.

Exocrine Glands

Exocrine glands secrete their products onto body surfaces or into body cavities, usually via ducts. They are more numerous than endocrine glands and include mucous, sweat, oil, and salivary glands. Exocrine glands can be unicellular or multicellular.

Unicellular Exocrine Glands

The main unicellular exocrine glands are mucous cells and goblet cells, which produce mucin that forms mucus for protection and lubrication.

Multicellular Exocrine Glands

These consist of a duct and a secretory unit, often supported by connective tissue. They are classified by structure (simple or compound; tubular, alveolar, or tubuloalveolar) 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).

Summary Table: Epithelial Tissue Types and Functions

Type	Structure	Function	Location
Simple squamous	Single layer, flat cells	Diffusion, filtration	Air sacs of lungs, lining of heart, blood vessels
Simple cuboidal	Single layer, cube-shaped cells	Secretion, absorption	Kidney tubules, ducts of small glands
Simple columnar	Single layer, tall cells	Absorption, secretion	Digestive tract, gallbladder
Pseudostratified columnar	Single layer, varying heights	Secretion, propulsion of mucus	Upper respiratory tract
Stratified squamous	Multiple layers, flat apical cells	Protection	Skin, mouth, esophagus, vagina
Transitional	Multiple layers, shape varies	Stretches, permits distension	Bladder, ureters, part of urethra