Anatomy & Physiology Study Notes: Epithelial Tissue and Glands

Overview of Tissues

Introduction to Histology

Histology is the study of tissues, which are groups of structurally and functionally related cells that perform common functions. Tissues also include the surrounding extracellular matrix (ECM).

• Tissue: A group of similar cells and their ECM performing a specific function.

• Four primary tissue types in the human body:

  • Epithelial

  • Connective

  • Muscle

  • Nervous

Microscopy in Histology

Preparation of Tissue Samples

To study tissues under a light microscope, specific preparation steps are required:

• Fixed: Tissue is preserved with a solvent to prevent decay.

• Sectioned: Tissue is cut into thin slices to allow light transmission.

• Stained: Colored dyes are applied to enhance contrast and visualize structures.

Epithelial Tissue

Characteristics of Epithelial Tissue

Epithelial tissue covers body surfaces, lines cavities, and forms glands. It is specialized for protection, absorption, filtration, excretion, secretion, and sensory reception.

• Composed mostly of tightly packed cells with little ECM.

• Cells are joined by specialized junctions (tight, desmosomes, gap junctions).

• Exhibits polarity: has an apical (free) surface and a basal (attached) surface.

• Supported by a basement membrane (basal lamina and reticular lamina).

• Avascular (no blood vessels); nutrients diffuse from underlying connective tissue.

• Regenerates rapidly if nutrition is adequate.

Cell Junctions in Epithelial Tissue

• Tight junctions: Prevent passage of molecules between adjacent cells.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap junctions: Allow passage of ions and small molecules for cell communication.

Polarity and Surfaces

• Apical surface: Exposed to the body exterior or cavity; may have microvilli (increase surface area) or cilia (move substances).

• Basal surface: Attached to underlying connective tissue via the basement membrane.

Basement Membrane

• Consists of basal lamina (from epithelial cells) and reticular lamina (from connective tissue).

• Supports epithelium and acts as a selective filter.

Classification of Epithelial Tissues

Naming Epithelial Tissues

Epithelial tissues are classified by the number of cell layers and the shape of the cells at the apical surface.

• Simple epithelium: One layer of cells.

• Stratified epithelium: Two or more layers of cells.

• Cell shapes:

  • Squamous: Flat and scale-like.

  • Cuboidal: Cube-shaped, as tall as wide.

  • Columnar: Taller than wide, column-shaped.

Table: Classification of Epithelial Tissues

Type	Layers	Cell Shape	Main Functions
Simple Squamous	1	Flat	Diffusion, filtration
Simple Cuboidal	1	Cube	Secretion, absorption
Simple Columnar	1	Column	Absorption, secretion
Stratified Squamous	2+	Flat (apical layer)	Protection
Stratified Cuboidal	2+	Cube (apical layer)	Protection, secretion
Stratified Columnar	2+	Column (apical layer)	Protection, secretion
Pseudostratified Columnar	1 (appears stratified)	Column	Secretion, propulsion (cilia)
Transitional	Multiple	Varies (dome-shaped when relaxed)	Stretching

Types of Epithelial Tissue

Simple Squamous Epithelium

• Single layer of flat cells with disc-shaped central nuclei.

• Functions: Diffusion, filtration, secretion of serous fluid.

• Locations: Air sacs of lungs (alveoli), lining of heart and blood vessels (endothelium), lining of ventral body cavities (mesothelium), kidney glomeruli.

Simple Cuboidal Epithelium

• Single layer of cube-shaped cells with large, spherical central nuclei.

• Functions: Secretion and absorption.

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

Simple Columnar Epithelium

• Single layer of tall cells with round to oval nuclei.

• May have microvilli (increase surface area) or cilia (move substances).

• Functions: Absorption, secretion of mucus and enzymes, ciliated type propels mucus.

• Locations: Nonciliated type lines digestive tract, gallbladder, excretory ducts; ciliated type lines small bronchi, uterine tubes, and some regions of uterus.

Pseudostratified Columnar Epithelium

• Single layer of cells of differing heights; nuclei at different levels.

• May contain mucus-secreting goblet cells and cilia.

• Functions: Secretion (especially mucus), propulsion of mucus by cilia.

• Locations: Ciliated type lines trachea and most of upper respiratory tract; nonciliated type in male sperm-carrying ducts and large gland ducts.

Stratified Squamous Epithelium

• Several layers; basal cells cuboidal or columnar, surface cells flattened (squamous).

• Functions: Protects underlying tissues in areas subject to abrasion.

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

Stratified Cuboidal and Stratified Columnar Epithelium

• Stratified cuboidal: Two or more layers of cuboidal cells; rare, found in ducts of sweat glands.

• Stratified columnar: Several cell layers; basal cells usually cuboidal, superficial cells columnar; rare, found in male urethra and some gland ducts.

Transitional Epithelium

• Resembles both stratified squamous and stratified cuboidal; surface cells dome-shaped or squamous depending on degree of stretch.

• Functions: Stretches readily, permits distension of urinary organ.

• Locations: Lines ureters, urinary bladder, part of urethra.

Glandular Epithelium

Endocrine Glands

• Secrete hormones directly into interstitial fluid and blood (ductless).

• Hormones act as chemical messengers to target organs.

• Most endocrine glands are multicellular; some are single cells (e.g., enteroendocrine cells in digestive tract).

Exocrine Glands

• Secrete products onto body surfaces or into body cavities via ducts.

• Examples: Sweat, oil, salivary, and mucous glands.

• Unicellular exocrine glands: Goblet cells (produce mucus).

• Multicellular exocrine glands: Composed of a duct and a secretory unit; classified by duct structure (simple or compound) and secretory unit shape (tubular, alveolar, or tubuloalveolar).

Table: Comparison of Endocrine and Exocrine Glands

Feature	Endocrine Glands	Exocrine Glands
Secretion	Hormones	Enzymes, mucus, sweat, etc.
Route	Directly into blood	Via ducts to surface
Examples	Thyroid, pituitary	Salivary, sweat, goblet cells

Goblet Cells (Unicellular Exocrine Glands)

• Most common unicellular exocrine gland.

• Found in epithelial linings of intestinal and respiratory tracts.

• Secrete mucus to protect and lubricate surfaces.

Summary

• Epithelial tissue is essential for protection, absorption, secretion, and filtration in the body.

• It is classified by cell shape and number of layers, with specialized forms for different functions.

• Glandular epithelium forms both endocrine and exocrine glands, which are vital for homeostasis and body function.