Overview
This lecture explains the structure, characteristics, naming, and functions of epithelial tissue, with comparisons to connective tissue and an introduction to specific epithelial subtypes.
Epithelial vs. Connective Tissue
- Epithelial tissue consists of tightly packed cells with minimal extracellular material.
- Connective tissue has more space, fibers, and less densely packed cells compared to epithelial tissue.
General Characteristics of Epithelial Tissue
- Always has a free (apical) surface not attached to other tissues.
- Sits on a basement membrane made up of two layers, attaching via hemidesmosomes.
- Avascular (no blood vessels); relies on diffusion from underlying connective tissue for nutrients.
- Highly innervated (rich nerve supply).
- High capacity for proliferation (rapid mitosis).
Naming Epithelial Tissues
- Named by number of layers:
- Simple = one layer
- Stratified = multiple layers
- Pseudostratified = appears layered, but all cells touch basement membrane (only one layer).
- Also named by cell shape:
- Squamous = flat and thin
- Cuboidal = cube-shaped with a round nucleus
- Columnar = tall, column-like with oval nucleus near the base
Unique Features of Columnar Epithelium
- May have cilia (move substances) or microvilli (increase absorption) on apical surface.
- Goblet cells are present, producing mucus.
- Tissues can be described as ciliated or non-ciliated based on surface features.
Functions and Locations of Epithelial Tissue
- Simple epithelia: functions include diffusion, osmosis, filtration, and absorption.
- Stratified epithelia: primarily protective due to multiple layers.
- Found lining body surfaces, lumens (internal spaces), glands, and membranes.
Types of Simple Epithelial Tissues
- Simple squamous: best for diffusion, osmosis, filtration; called mesothelium (lines organ outsides) or endothelium (lines organ insides).
- Simple cuboidal: absorption and filtration; found in kidney tubules and on the surface of ovaries.
- Simple columnar: absorption and secretion; may be ciliated (moves mucus/particles) or non-ciliated (with microvilli for absorption); contains goblet cells.
Key Terms & Definitions
- Basement membrane — thin layer anchoring epithelial tissue to connective tissue.
- Avascular — lacking blood vessels.
- Innervated — supplied with nerves.
- Proliferation — capacity for rapid cell division (mitosis).
- Simple epithelium — single cell layer.
- Stratified epithelium — multiple cell layers.
- Pseudostratified epithelium — one cell layer that appears multi-layered.
- Mesothelium — simple squamous lining organ outsides.
- Endothelium — simple squamous lining organ/vascular insides.
- Cilia — hair-like structures for movement.
- Microvilli — tiny projections increasing surface area for absorption.
- Goblet cells — mucus-secreting epithelial cells.
Action Items / Next Steps
- Review tissue sample images and practice naming based on layers and cell shapes.
- Prepare for further discussion on stratified and pseudostratified epithelial tissues.