Which kinds of epithelial tissues cannot be classified easily as typical epithelia?
Pseudostratified columnar epithelium, transitional epithelium, and glandular epithelium cannot be classified as easily as typical epithelium. Pseudostratified columnar epithelium, found in the trachea, bronchi and large bronchioles, and parts of the male reproductive tract, is characterized by the fact that all of its cells are in contact with the basement membrane, but not all of the cells reach the surface. It is called pseudostratified because it gives the false (“pseudo”) impression that it is a multilayered stratification, since the nuclei of the cells appear to be at several different levels.
Transitional epithelium lines the urinary tract, including the ureters, urinary bladder, urethra, and calyxes of the kidneys. The cells vary in shape depending on the amount of fluid the organ contains. For example, when the urinary bladder contains a large quantity of urine, the cells are stretched out and assume a flat, squamous appearance. When the bladder is empty, the cells have a cuboidal or slightly columnar shape.
Glandular epithelium cells are specialized for the synthesis, storage, and secretion of chemical substances, such as saliva or digestive juices. These glands are called exocrine glands.