Stratified columnar epithelium is a type of epithelial tissue composed of two or more layers of columnar epithelial cells. Epithelial tissue is one of the body’s primary tissues and lines the surfaces and insides of structures in many parts of the body. A rare type of this tissue, stratified columnar epithelium, is only found in a few places in the body, including parts of the anus, conjunctiva in the eye, uterus, and pharynx.
Epithelial tissue is classified in two ways: based on structure, and based on form and function. The principle structural types are simple and stratified. Simple epithelium has only one layer of cells, while stratified epithelium has two or more layers. The form and function classifications of epithelial tissue are cuboidal, squamous, and columnar. Squamous cells have a plate-like shape and typically function in diffusion and protection. Cuboidal cells aid in absorption and secretion in various places in the body and, as one might expect, have a cube-like shape.
Columnar tissue have elongated, cylindrical, rectangular, or conically shaped cells that resemble columns, as their name suggests. Their purpose is usually to secrete mucous to lubricate or protect cavities in the body, or act as a sensory tissue in the tongue. Columnar tissue is typically tightly packed, resting on a basement membrane, or a layer of fibers that connects the epithelia to the underlying structure. The surfaces of some columnar cells may have hair-like structures called cilia that help to move particles in a desired direction. Others may have fingerlike structures called microvilli that increase surface area of the cell to help with absorption.
In the bladder, stratified columnar epithelium can function as a stretchy material to allow distention. Stratified columnar tissue may secrete mucus in the uterus of some animals, such as ruminants, to help with implantation of an egg. Other animals, like the horse or human, have simple columnar epithelium in the uterus instead. This tissue may also be found in the salivary glands, namely the submandibular glands, the mammary glands, the urethra, the vas deferens, and the conjunctiva in the eye. The vas deferens are tubes that carry sperm to the ejaculatory ducts from the testicles and the conjunctiva of the eye is a mucous membrane that protects the whites of the eye, or sclera, and the inner eyelids.
Stratified columnar epithelium is often confused with pseudostratified columnar epithelial tissue, as they can look very similar under a microscope. The cells in pseudostratified columnar epithelial tissue mimic stacked layers of columnar cells, but if one looks closely, the bottom of each cell connects with the basement membrane, so there is actually only one layer of cells. Adding to their layered appearance, pseudostratified columnar cells may have nuclei at the bottom or middle of the cells, unlike stratified columnar epithelium, whose nuclei usually rest near the bottom of the cell. Both tissue types are uncommon in the human body.