Chapter 4: Tissue—The Living Fabric (Epithelial and Connective Tissues)

Tissue: The Living Fabric

Introduction to Tissues

Tissues are groups of cells similar in structure that perform common or related functions, maintaining homeostasis in the body. The study of tissues is called 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 cause movement.

• Nervous tissue: Initiates and transmits electrical impulses.

4.1 Tissue Preparation for Microscopy

Steps in Tissue Preparation

To study tissues under a microscope, samples must be:

• Fixed: Preserved with a solvent to prevent decay.

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

• Stained: Treated with dyes to enhance contrast; light microscopy uses colored dyes, while electron microscopy uses heavy metal salts.

Transmission electron microscopy (TEM) shows internal structures, while scanning electron microscopy (SEM) reveals surface details.

4.2 Epithelial Tissue

Definition and Functions

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

• Covering and lining epithelium: Forms the outer layer of skin and lines open cavities and organs.

• Glandular epithelium: Forms glands that secrete substances (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) surface exposed to the exterior or cavity, and a basal (bottom) surface attached to underlying tissues. The apical surface may have microvilli for absorption.

• Specialized contacts: Cells are tightly joined by junctions (tight junctions, desmosomes) to form continuous sheets.

• Supported by connective tissue: The basement membrane (basal and reticular lamina) reinforces the epithelium and resists stretching.

• Avascular but innervated: Contains no blood vessels but is supplied by nerves; nutrients diffuse from underlying tissues.

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

Classification of Epithelial Tissue

Epithelia are classified by the number of cell layers and cell shape:

• Simple epithelium: Single cell layer; allows absorption, secretion, and filtration.

• Stratified epithelium: Two or more layers; provides protection in high-abrasion areas.

Cell shapes:

• Squamous: Flattened and scale-like.

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

• Columnar: Tall and column-shaped.

Types of Epithelial Tissue

Simple Squamous Epithelium

Single layer of flattened 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-like cells; functions in secretion and absorption. Located in kidney tubules, ducts, and secretory portions of small glands.

Simple Columnar Epithelium

Single layer of tall, closely packed cells; may have microvilli or cilia. Functions in absorption and secretion of mucus, enzymes, and other substances. Found in digestive tract, gallbladder, ducts of some glands, bronchi, and uterine tubes.

Pseudostratified Columnar Epithelium

Single layer of cells of varying heights, giving a false impression of stratification. Often ciliated; functions in secretion and movement of mucus. Located in upper respiratory tract, ducts of large glands, and tubules in testes.

Stratified Epithelia (Overview)

Stratified epithelia involve two or more layers of cells, providing protection in high-wear areas (e.g., skin, mouth lining). The most widespread type is stratified squamous epithelium. Other types include stratified cuboidal and columnar (rare), and transitional epithelium (lines urinary organs, stretches as needed).

Glandular Epithelia

Glands are one or more cells that secrete an aqueous fluid (secretion). They are classified by:

• Site of product release: Endocrine (ductless, hormones into blood) or exocrine (onto surfaces or into cavities via ducts).

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

Exocrine glands can be further classified by structure (simple/compound, tubular/alveolar) and mode of secretion (merocrine, holocrine, apocrine).

Summary Table: Epithelial Tissue Types

Type	Structure	Function	Location
Simple Squamous	Single layer, flat cells	Diffusion, filtration	Lungs, blood vessels
Simple Cuboidal	Single layer, cube cells	Secretion, absorption	Kidney tubules, glands
Simple Columnar	Single layer, tall cells	Absorption, secretion	Digestive tract, ducts
Pseudostratified Columnar	Single layer, varied height	Secretion, mucus movement	Respiratory tract
Stratified Squamous	Multiple layers, flat cells	Protection	Skin, mouth, esophagus
Transitional	Multiple layers, shape varies	Stretching	Bladder, ureters

Additional info: For rare types (stratified cuboidal/columnar), see histology atlases for images and further details.