Overview
This lecture covers the structure, classification, and identification of epithelial tissue, emphasizing its key characteristics and how to distinguish different types under the microscope.
Introduction to Tissues
- There are four main tissue types: epithelial, connective, muscular, and nervous.
- Epithelial tissue lines open surfaces and serves as barriers and protective layers.
- Accurate tissue identification requires analyzing layer count, cell shape, and arrangement, not just memorizing pictures.
Characteristics of Epithelial Tissue
- High cellularity: tightly packed cells with minimal extracellular matrix.
- Polarity: distinct apical (top, facing open space) and basal (bottom, attached to basement membrane) surfaces.
- Avascular but innervated: lacks direct blood supply but contains nerves.
- Supported by a basement membrane connecting to underlying connective tissue.
Naming and Classification
- Named by layer count and cell shape.
- Layering:
- Simple: one cell layer.
- Stratified: multiple cell layers.
- Pseudostratified: appears layered but all cells touch the basement membrane.
- Cell shape:
- Squamous: flat, tile-like cells.
- Cuboidal: cube-shaped cells with central nucleus.
- Columnar: tall, rectangular cells with nuclei often at the base.
Types of Epithelial Tissue
- Simple squamous: one layer of flat cells; allows rapid diffusion; found in alveoli and blood vessels.
- Simple cuboidal: one layer of square cells; used for secretion and absorption; found in kidney tubules and glands.
- Simple columnar: one layer of tall cells; absorption and secretion; lines GI tract; may have microvilli or goblet cells.
- Pseudostratified columnar: looks stratified but isn’t; nuclei at different levels; often ciliated; lines respiratory tract.
- Stratified squamous: multiple layers, surface cells flat; protects against abrasion.
- Keratinized: surface cells dead and packed with keratin (skin).
- Non-keratinized: moist surfaces such as mouth, esophagus, vagina.
- Transitional: multiple layers, top cells rounded; changes shape when stretched; found in bladder and urinary tract.
Tissue Identification Tips
- Recognize epithelial tissue by finding a lining of an open surface or lumen.
- Use layer count and cell shape to name the tissue.
- Distinguish similar tissues (e.g., pseudostratified vs. simple columnar) by nucleus pattern and presence of cilia or surface features.
Key Terms & Definitions
- Epithelial tissue — tissue that lines open surfaces of the body.
- Apical surface — the top surface facing an open area.
- Basal surface — bottom surface attached to the basement membrane.
- Basement membrane — layer connecting epithelial and connective tissue.
- Simple — single cell layer.
- Stratified — multiple cell layers.
- Pseudostratified — appears layered, but all cells contact basement membrane.
- Squamous — flat-shaped cell.
- Cuboidal — cube-shaped cell.
- Columnar — tall, rectangular cell.
- Keratinized — containing dead cells filled with keratin.
- Transitional — tissue that changes shape based on stretching.
Action Items / Next Steps
- Practice identifying epithelial tissues in images by layering and cell shape.
- Compare similar tissue types side by side to recognize distinguishing features.
- Review homework or assigned readings on connective tissue for the next class.