Chapter 4: Tissues – Epithelial Tissue Structure, Function, and Classification

Chapter 4 - Tissues

Overview

This chapter introduces the major types of tissues in the human body, focusing on epithelial tissue. It covers the structure, function, and classification of epithelial cells, as well as their specialized connections and roles in protection, secretion, and absorption.

Epithelial Tissue

Definition and General Features

• Epithelial tissue consists of layers of cells that cover external surfaces or line internal surfaces of the body.

• Includes epithelia (covering/lining) and glands (secreting).

• Cells are polarized (have an attached base surface and a free apical surface).

• Epithelial tissue is avascular (lacks blood vessels).

• Regenerates rapidly due to high mitotic activity.

Functions of Epithelial Tissue

• Physical protection: Shields underlying tissues from abrasion, dehydration, and destruction by chemical or biological agents.

• Control permeability: Regulates substances entering or leaving the body; some epithelia are highly impermeable, others allow selective passage.

• Sensation: Contains sensory nerve endings for detecting environmental changes (e.g., touch, sight, smell).

• Specialized secretions: Gland cells produce secretions for physical protection or chemical regulation.

Intercellular Connections – Support and Communication

Epithelial cells are tightly bound together by specialized structures that provide support and facilitate communication.

• Cell adhesion molecules (CAMs): Transmembrane proteins that connect cells to each other and to extracellular materials.

• Proteoglycans: Glycosaminoglycans (GAGs) like hyaluronan help bind cells and retain water.

Types of Cell Junctions

• Gap junctions: Allow rapid communication by permitting ions and small molecules to pass between cells.

• Tight junctions: Prevent diffusion of fluids and solutes between cells, maintaining distinct tissue compartments.

• Desmosomes: Provide strong adhesion and resist stretching; spot desmosomes connect to intermediate filaments, hemidesmosomes anchor cells to the basement membrane.

Attachment to Basement Membrane

• Epithelial cells are anchored to a basement membrane composed of two layers:

  • Basal lamina: Closest to the epithelium, restricts movement of proteins and large molecules.

  • Reticular lamina: Deeper portion, provides support and anchors the basement membrane to underlying tissues.

Classification of Epithelia

Epithelia are classified by cell shape and the number of layers between the basement membrane and the exposed surface.

• Cell shapes: Squamous (flat), cuboidal (box-like), columnar (tall).

• Layers: Simple (one layer), stratified (multiple layers).

Types of Epithelial Tissue

Squamous Epithelia

• Cells: Thin, flat, irregular in shape.

• Simple squamous: Single layer, found in protected regions for absorption or diffusion (e.g., alveoli of lungs, lining of heart and blood vessels).

• Functions: Reduces friction, controls vessel permeability, performs absorption and secretion.

• Stratified squamous: Multiple layers, found where mechanical stresses are severe (e.g., skin, mouth, throat, esophagus, rectum, anus, vagina).

  • Keratized: Surface cells packed with keratin, tough and water-resistant (skin).

  • Nonkeratinized: Surface cells remain moist (oral cavity, pharynx, anus, esophagus, vagina).

  • Function: Protection against abrasion, pathogens, and chemical attack.

Cuboidal Epithelia

• Cells: Hexagonal boxes, appear square in section.

• Simple cuboidal: Single layer, limited protection, secretion, absorption.

• Locations: Glands, ducts, kidney tubules, thyroid gland.

• Stratified cuboidal: Rare, found in ducts of sweat glands and larger ducts of mammary glands.

Transitional Epithelia

• Cells can change between squamous and cuboidal shapes.

• Locations: Urinary bladder, renal pelvis, ureters.

• Function: Permits repeated cycles of stretching without damage.

Columnar Epithelia

• Cells: Taller and thinner than cuboidal; height is several times the distance between adjacent nuclei.

• Simple columnar: Single layer, found where absorption or secretion occurs (e.g., lining of stomach, intestine, gallbladder, uterine tubes, collecting ducts of kidneys).

• Functions: Protection, secretion, absorption.

Summary Table: Epithelial Tissue Types

Type	Cell Shape	Layers	Locations	Functions
Simple Squamous	Flat	Single	Alveoli, blood vessels, serous membranes	Absorption, diffusion, reduces friction
Stratified Squamous	Flat	Multiple	Skin, mouth, throat, esophagus, anus, vagina	Protection against abrasion, pathogens
Simple Cuboidal	Box-like	Single	Glands, ducts, kidney tubules, thyroid	Secretion, absorption, limited protection
Stratified Cuboidal	Box-like	Multiple	Sweat gland ducts, mammary gland ducts	Protection, secretion, absorption
Transitional	Variable	Multiple	Urinary bladder, renal pelvis, ureters	Stretching, expansion
Simple Columnar	Tall	Single	Stomach, intestine, gallbladder, uterine tubes	Absorption, secretion, protection

Key Terms

• Epithelium: Layer of cells covering surfaces or lining cavities.

• Basement membrane: Thin, fibrous layer anchoring epithelium to underlying tissue.

• Cell junctions: Specialized connections between cells (gap, tight, desmosomes).

• Keratin: Tough, water-resistant protein found in skin.

• Microvilli: Finger-like projections increasing surface area for absorption.

Example Application

• Simple squamous epithelium in alveoli allows rapid gas exchange in the lungs.

• Stratified squamous epithelium in the skin protects against mechanical injury and pathogens.

• Transitional epithelium in the bladder enables expansion and contraction during urine storage and release.

Additional info: The notes above expand on the brief points in the original material, providing definitions, examples, and a summary table for clarity and completeness.