Pseudostratified epithelium is a type of epithelial tissue that at first glance appears to be stratified, but is actually only a single layer of cells. Epithelial tissue is one of the primary types of tissues in the body and is divided based on form and function into cuboidal, columnar, and squamous epithelia, and by number of layers into stratified, pseudostratified, and simple epithelium. Stratified epithelia have two or more layers of cells, while simple epithelia have only one layer. Because of the irregular shape of the cells and the placement of the nuclei at different levels in the epithelia, pseudostratified epithelium looks stratified, but each cell is actually rooted at the bottom to a basement membrane.
Epithelial tissue lines the inner and outer surfaces of organs in the body and function in processes of secretion, absorption, filtration, diffusion, transport, protection, and sensation. Pseudostratified epithelium specializes in secretion and absorption. The epithelial cells are anchored to the underlying organ by a fibrous sheet called the basal lamina, or basement membrane. The tissue touching the basement membrane is called the basal surface and the side facing out is called the apical surface.
In stratified epithelium, only the bottom cells have a basal surface and only the top cells have an apical surface. Simple epithelium, however, is only one cell thick, so each cell has both a basal and an apical surface. The nuclei of these cells are placed roughly in the middle of the cell. In pseudostratified epithelium, all cells have a basal surface, because the tissue is also only one cell thick, but not all cells reach the surface. The tops of some cells are higher than other cells and umbrella out over other cells, while the bottom of these tall cells may be very skinny and barely visible at the basement membrane, lending the epithelia a stacked appearance. The nuclei also have irregular placement in the cells, with some near the top and some near the bottom, adding to the layered look of the epithelium.
Pseudostratified epithelium usually takes the columnar form and rarely appears cuboidal or squamous, though there are examples in the body. Pseudostratified columnar epithelium is located in the trachea, upper respiratory tract, and in the male reproductive system, particularly in the vas deferens and epididymis. Pseudostratified epithelial tissue typically, but not always, has goblet cells, which secrete mucin to produce mucus. Most pseudostratified epithelia also have cilia, or fingerlike projections, on the apical surface. This is a good way to tell pseudostratified epithelium from stratified epithelium under a microscope, as stratified epithelium is not ciliated.