Epithelium is one of the four main tissue types in the body, along with muscle, connective tissue, and nervous tissue. It functions mainly to line surfaces in the body, and lies on top of connective tissue, to which it is anchored by a basement membrane consisting mainly of collagen fibers. Glands are also composed of epithelial tissue.
Epithelial tissue is supplied with nerves, but not with blood vessels. It receives nutrition from the underlying connective tissue through diffusion. Epithelial tissue has many functions, including protection, secretion, selective absorption, excretion, diffusion, and sensation.
Different types of epithelial tissue are distinguished based on the shape of the cells and the number of cell layers. Epithelial tissue consisting of a single layer of cells is classified as simple epithelium, while multiple cell layers constitute stratified epithelium. Epithelial cells may be squamous or flat, cuboidal, or columnar. Squamous cells are wider than they are tall, while columnar cells are taller than they are wide, and cuboidal cells are about equal in height, width, and depth.
Simple squamous tissue facilitates diffusion because of its thinness. It lines the blood and lymph vessels, as well as the internal body cavities. Simple cuboidal epithelial tissue is often found in tissues specialized for secretion or absorption, such as glands. Simple columnar epithelium lines parts of the female reproductive system and much of the digestive system, including the stomach and intestines.
Squamous and cuboidal epithelium may be simple or stratified, while the columnar type may be simple, stratified, or pseudostratified. Pseudostratified columnar epithelium is made of cuboidal cells of varying heights, which gives it the microscopic appearance of being stratified, though there is actually only a single layer of cells. It is found in parts of the respiratory and urinary systems, as well as in the male reproductive system. Pseudostratified epithelial tissue may also feature cilia, fine hair-like projections on the surface of the tissue that serve to increase surface area and to move substances across the surface of the tissue in a particular direction.
There are also two specialized types of stratified epithelium: keratinized and transitional. Keratinized epithelial tissue is found only on the external surfaces of the body, including the inside of the mouth. It is characterized by an outermost layer of dead cells rich in the protein keratin, making the tissue surface tough and waterproof. Transitional tissue consists of layers of cells that can stretch, appearing cuboidal or columnar when at rest and squamous when stretched. This type of tissue, sometimes called the urothelium, is found only in the urinary system structures of the renal pelvis, bladder, ureter, and urethra.