Chapter 4: Tissue—The Living Fabric (Epithelial Tissue)

Tissue: The Living Fabric

Introduction to Tissues

Tissues are groups of cells with similar structure and function, working together to maintain homeostasis in the human body. The study of tissues is known as histology. There are four basic tissue types: epithelial (covers), connective (supports), muscle (moves), and nervous (controls).

• Epithelial Tissue: Covers surfaces and lines cavities.

• Connective Tissue: Supports and binds other tissues.

• Muscle Tissue: Responsible for movement.

• Nervous Tissue: Controls and communicates information.

Microscopy of Human Tissue

Preparation for Microscopy

To study tissues under a microscope, samples must be fixed (preserved), sectioned (sliced thinly), and stained (colored for contrast). Light microscopy uses colored dyes, while electron microscopy uses heavy metal coatings. Staining may introduce artifacts—distortions not present in living tissue.

Epithelial Tissue

Definition and Functions

Epithelial tissue (epithelium) is a sheet of cells covering body surfaces or lining cavities. It exists in two main forms: covering and lining epithelia (e.g., skin) and glandular epithelia (e.g., salivary glands). Its main functions include protection, absorption, filtration, excretion, secretion, and sensory reception.

Special Characteristics of Epithelial Tissue

• Polarity: Cells have distinct apical (top) and basal (bottom) surfaces. The apical surface may have microvilli, while the basal surface attaches to the basal lamina.

• Specialized Contacts: Cells are tightly joined by junctions (tight junctions, desmosomes).

• Supported by Connective Tissue: The basement membrane (basal and reticular lamina) reinforces the epithelium.

• Avascular but Innervated: No blood vessels, but supplied by nerves; nutrients diffuse from underlying connective tissue.

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

Classification of Epithelia

Criteria for Classification

Epithelial tissues are classified by the number of cell layers and cell shape:

• Number of Layers: Simple (one layer) or stratified (multiple layers).

• Cell Shape: Squamous (flat), cuboidal (cube-shaped), columnar (tall).

Types of Epithelial Tissue

• Simple Squamous Epithelium: Single layer of flat cells; allows rapid diffusion. Found in kidneys, lungs, blood vessels (endothelium), and serous membranes (mesothelium).

• Simple Cuboidal Epithelium: Single layer of cube-shaped cells; involved in secretion and absorption. Found in kidney tubules and gland ducts.

• 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, bronchi, and uterine tubes.

• Pseudostratified Columnar Epithelium: Appears multilayered but is single-layered; often ciliated. Functions in secretion and movement of mucus. Found in upper respiratory tract and ducts of large glands.

• Stratified Squamous Epithelium: Multiple layers; protects against abrasion. Keratinized (skin) and nonkeratinized (moist linings like mouth, esophagus).

• Stratified Cuboidal and Columnar Epithelium: Rare; found in sweat glands, mammary glands, pharynx, male urethra.

• Transitional Epithelium: Changes shape when stretched; lines urinary organs (bladder, ureters, urethra).

Glandular Epithelia

Definition and Classification

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, which are transported by blood or lymph to target organs.

• Exocrine Glands: Secrete products onto body surfaces or into cavities via ducts; include sweat, oil, and salivary glands.

Unicellular Exocrine Glands

The main unicellular exocrine glands are mucous cells and goblet cells, found in the linings of the intestinal and respiratory tracts. They produce mucin, which forms mucus for protection and lubrication.

Multicellular Exocrine Glands

Multicellular exocrine glands consist of a duct and a secretory unit, supported by connective tissue. They are classified by structure (simple or compound; tubular, alveolar, or tubuloalveolar) and mode of secretion (merocrine, holocrine, apocrine).

Modes of Secretion

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

• Holocrine: Accumulate products and rupture, releasing cell fragments (e.g., sebaceous glands).

• Apocrine: Accumulate products and only the apex ruptures; existence in humans is debated (possibly mammary glands).

Summary Table: Epithelial Tissue Types

Type	Structure	Function	Location
Simple Squamous	Single layer, flat cells	Diffusion, filtration	Kidney, lungs, blood vessels
Simple Cuboidal	Single layer, cube-shaped cells	Secretion, absorption	Kidney tubules, glands
Simple Columnar	Single layer, tall cells	Absorption, secretion	Digestive tract, gallbladder
Pseudostratified Columnar	Single layer, varying heights	Secretion, movement of mucus	Respiratory tract, ducts
Stratified Squamous	Multiple layers, flat cells	Protection	Skin, mouth, esophagus
Transitional	Multiple layers, shape changes	Stretching	Bladder, ureters, urethra

Key Equations and Concepts

• Diffusion Rate in Simple Squamous Epithelium: $D = \frac{A \cdot (C_1 - C_2)}{d}$ Where D = diffusion rate, A = area, C1 and C2 = concentration difference, d = thickness.

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