Epithelial Tissue Overview

Epithelial Tissue Overview

• Epithelial tissues are diverse, including surface epithelia and solid organs
• They lack blood vessels (avascular) and lymph nodes
• Cells are tightly packed with minimal intercellular substance
• Lines the body surface, body cavities, and hollow organs
• Forms glands and their ducts

Characteristics of Epithelial Cells

• Cells are in close contact
• Exhibit functional and morphological polarity (apical and basal poles, lateral domain)
• Reside on a basal lamina
• High regeneration capacity
• Cytoskeleton contains cytokeratin intermediate filaments

Classification of Epithelial Tissues

• Classified by arrangement of cells (e.g., surface, trabecular reticular)
• Number of cell layers (e.g., simple, pseudostratified, stratified)
• Cell shape (e.g., squamous, cuboidal, columnar)
• Dominant functions (e.g., sensory, transport, respiratory, secretory)

Simple Squamous Epithelium

• Facilitates passive substance transport (osmosis, diffusion)
• In specialized epithelia, active transport occurs using membrane transporters
• Located in areas requiring rapid substance exchange

Simple Cuboidal Epithelium

• Single layer of cube-shaped cells
• Found in areas of secretion and absorption (e.g. kidneys)

Simple Columnar Epithelium

• Single layer of column-shaped cells
• Found in areas involved in absorption (digestive tract) or secretion

Simple Columnar Ciliated Epithelium

• Cells possess cilia (hair-like projections)
• Facilitates movement of substances (e.g., respiratory tract)

Pseudostratified Columnar Ciliated Epithelium

• Appears stratified but all cells connect to basement membrane
• Involved in protection and secretion

Stratified Squamous Epithelium

• Multiple layers of cells
• Provides protection from abrasion (e.g, skin)
• Can be keratinized (with a protective protein keratin)

Stratified Squamous Non-keratinized Epithelium

• Multiple layers of cells
• Covers moist surfaces (e.g., oral cavity)
• Provides protection

Transitional Epithelium

• Specialised tissue for organs that stretch
• Consists of three layers (basal, intermediate and surface)
• Surface cells (umbrella cells) can change shape with stretching

Trabecular Epithelium

• Complex, branched and interconnected structure resembling cords or beams
• Found in various organs (like adrenal cortex)

Follicular Endocrine Glandular Epithelium

• This type lines the thyroid gland follicles.
• Epithelial cells exhibit a cuboidal shape
• Colloid secretion occurs within these compartments

Reticular Epithelium

• Support structure in developing tooth.
• A specific type of epithelial tissue.

Tissue Location of Transitional Epithelium

• Found in organs subject to large volume changes (minor renal calyces, renal pelvis, ureters, urinary bladder, urethra)

Epithelia and Glands

• Various gland types exist, including apocrine, eccrine and holocrine

Sweat and Sebaceous Glands

• Found in skin
• Involved in secretion and excretion
• Secrete sweat and sebum

Serous Glands (e.g., Parotid)

• Secrete a watery fluid;
• Found in various organs
• Composed of complex systems of ducts and acini (sac-like structures).