Epithelium

An epithelium is a layer or sheet of cells that covers a surface or lines a cavity. Functions of epithelia include formation of a protective layer (epidermis), absorption of water and solutes (intestine), secretion (intestine, various glands) and excretion (kidney tubules). Classification of epithelia is generally based upon two criteria: number of cell layers and cell shape. Simple epithelia are one cell layer thick and stratified epithelia are two or more cell layers thick. Pseudostratified epithelium is an intermediate type that appears stratified but really is one cell layer thick. The shape of epithelial cells may be squamous, cuboidal, or columnar; intermediate forms are often encountered. Cuboidal and columnar cells may form glandular epithelia. Stratified epithelia are classified according to the shape of the cells at the free surface. Transitional epithelia (urothelia in the urinary system) line cavities, which may be distended, and the thickness of the epithelium varies with the degree of distention.

Learning objectives:

• Recognize the different types of epithelia.
• Relate characteristics of particular epithelia to their function, keeping in mind their essential features including junctions, apical modifications and polarity.

Lab Activities

1. Simple Squamous Epithelium
2. Simple Cuboidal and Simple Columnar Epithelia
   2.1 Simple Cuboidal Epithelia
   2.2 Simple Columnar Epithelia
3. Pseudostratified Epithelium
4. Stratified Epithelium
5. Transitional Epithelium
6. Electron Micrographs
7. Review
8. Answers