Overview
The lecture covers the types, structure, classification, and functions of epithelial tissue, emphasizing histological identification and their roles as protective layers and in secretion.
Functions and Importance of Epithelial Tissue
- Epithelial tissue covers internal and external surfaces like skin and blood vessel linings.
- Main functions are protection and secretion of substances.
- Forms the parenchyma (functional tissue) in organs like the liver.
- Epithelial tissue is avascular and relies on diffusion for nutrients.
Structure and Organization
- Epithelial cells are polar, having a basal pole (attached to basement membrane) and apical pole (facing lumen or external environment).
- Basement membrane separates epithelium from underlying connective tissue (lamina propria).
- Apical surfaces may have specializations like cilia.
- Lateral surfaces between cells contain junctions: gap junctions, tight junctions, and desmosomes.
Classification of Epithelial Cells
- Named by arrangement (simple or stratified) and cell shape (squamous, cuboidal, columnar).
- Simple epithelium: single cell layer; good for absorption and secretion.
- Stratified epithelium: multiple cell layers; provides protection.
- Pseudostratified epithelium: appears multilayered but all cells touch the basement membrane.
- Keratinized stratified epithelium: top layers are dead, filled with keratin (as in skin).
- Transitional epithelium: allows stretching; found in the bladder.
Glandular Epithelium
- Glandular epithelium specializes in secretion.
- Endocrine glands release hormones into the bloodstream (e.g., thyroid, pineal gland, Leydig cells in testes).
- Exocrine glands secrete substances outside the body or into body cavities (e.g., goblet cells in GI tract, salivary glands, apocrine glands).
Key Terms & Definitions
- Epithelium — tissue covering body surfaces and lining cavities.
- Basement membrane — thin layer separating epithelium from connective tissue.
- Lamina propria — connective tissue beneath the basement membrane.
- Parenchyma — functional tissue in organs.
- Simple epithelium — single cell layer thick.
- Stratified epithelium — multiple cell layers thick.
- Squamous — flat, pancake-like epithelial cells.
- Cuboidal — cube-shaped epithelial cells.
- Columnar — tall, column-shaped epithelial cells.
- Keratinized — cells filled with keratin protein, typically dead.
- Transitional epithelium — stretchy epithelial tissue, found in bladder.
- Glandular epithelium — specialized for secretion.
- Endocrine gland — secretes hormones into bloodstream.
- Exocrine gland — secretes substances outside body or into cavities.
Action Items / Next Steps
- Practice identifying epithelial tissue types on histology slides by locating basement membrane, apical surface, cell shape, and layering.
- Review digestive system histology for more examples of epithelial tissue.