- 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.
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.
Beneath the layer of epithelial cells is an underlying noncellular structure known as the basal lamina which is secreted by the epithelial cells. The basal lamina is often associated with an additional layer secreted by other cells. Together the basal lamina and the underlying layer make up the basement membrane which can usually be seen with light microscopy. Higher magnification (e.g., electron microscopy) is usually required to resolve the basal lamina.