Answer
The cells of epithelium ,a tissue that covers surfaces or lines cavities are of three main types viz. squamous, cuboidal and columnar; squamous cells are are thin and flat, and usually have one round centrally located nucleus; cuboidal cell are about as tall as they are wide, have roundish nuclei, and may grow microvilli on their apical surfaces; the third type , the columnar cells are typically taller than they are wide , and may also produce microvilli . These cell perform or facilitate various physiological functions and these functions are related to the different shapes and organelles. First, the flat shape of the squamous cells allows quick diffusion of dissolved substances across them. In addition, squamous cells function in filtration of fluids and in serous fluids production.. Squamous cells are found in the lining of the cardiovascular and lymphatic systems (endothelium) and in the pleura, peritoneum and pericardium ( mesothelium). The second main type of epithelial cells is cuboidal cells. These cell are shaped more or less like cubes i.e., the dimensions of their heights and widths are about the same. These cuboid cells have round nuclei, and may develop apical microvilli to facilitate absorption. This type of cell is found in the lining of kidney tubules, in the ducts of thyroid glands, in pancreatic ducts, and in the covering membrane of ovaries. Cuboidal cells are secretory and absorptive: the increased surface area provided by microvilli improves the efficiency of absorption. Next are the columnar cells. In this case, the cells taller than they are wide-- in relative dimensions somewhat like a Greek or Romanesque architectural column.. Columnar epithelial cells have apical microvilli to facilitate absorption, and goblet and other glandular cells for secretion of mucous fluids. This kind of epithelia cells are found in many locations in the body including the lining of the GI tract, of ducts of the gall bladder, of bronchioles, of the respiratory tract, of the ventricles of the brain, and of fallopian tubes. Finally there is transitional epithelial cells. This type of cell is found in the urinary bladder. Transitional cells may vary in shape from squamous to cuboidal and back to squamous, depending on how much the epithelium of the bladder is extended by the volume of urine it contains at any time, and whether it has contracted in the process of micturation..
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Different combinations or arrangement of different types of epithelial cells are to be found in different locations in the body. These arrangement give rise to different compound types of tissues which serve specialized functions. These arrangements may be categorized broadly as simple, epithelium, pseudostratified epithelium and stratified epithelium ; A simple epithelium is a tissue with one type of epithelial cell , that is, squamous, cuboidal or columnar. Simple squamous epithelium consists of flat cells . This type of epithelium is found lining blood vessels and lymphatics where it goes by the name of endothelium. Simple squamous epithelium is also found in the peritoneum , the pleura, and the pericardium, where it is called mesothelium. . This type of tissue best serves the purposes of filtration, for example, in Bowman's capsule of the kidneys, and diffusion, as in capillaries of the air sacs of the lungs. The next arrangement is the pseudostratified arrangement. In this type of epithelial arrangement the cells all reach the basement membrane, but the cell nuclei are at different heights and the tops of the cells do not all extend to the apical surface. Therefore, this type of tissue is technically a simple epithelium. Stratified epithelium, the most complex category, has several sub-types classified in the following way:
Stratified squamous epithelium. This type of epithelium usually presents with two or more upper squamous layers. It may have other lower layers which may be cuboidal or columnar. The basal cells in this tissue divide continuously by mitosis, and the products pass upward towards the apical surface. In some cases, as they move away from the blood supply of the connective tissue, the cells become dehydrated; as their metabolic functions become impaired cytoplasmic proteins become hard and denatured ; eventually these cells die. At the surface, the cell to cell connections loosen and the keratinzed cells slough off, and are replaced by cells that come from the basal layer. The two subdivisions of stratified squamous epithelium are keratinized squamous epithelium and non-keratinized stratified squamous epithelium The keratinized type behaves as has been described above. Its function is to provide protection against heat, chemicals and microbes.. The non-keratinized stratified squamous epithelium is different in the following ways; it develops less keratin in its superficial cells and the surface of the tissue is constantly moistened by mucous fluid in locations where it exists. Keratinized stratified squamous epithelium is found in the epidermis of the skin; non-keratinized stratified squamous epithelium lines the oral cavity, the esophagus, the epiglottis, part of the pharynx, the vagina, and covers the tongue Non -keratinized squamous stratified epithelium also protects against microbes and it also protects against abrasion, dehydration and radiation damage.
There also several types of cuboidal arrangements: Stratified cuboidal epithelium has two or more layers of cuboidal cells. This type of tissue is generally found in ducts of sweat glands, esophageal
glands, and in the male urethra. The functions of this type of arrangement are protection, secretion and absorption
In the stratified columnar arrangement the basal layers of cells are different in shape from the cells near the surface : cells of the apical layers are columnar in shape while the cells of the basal layers are shorter and irregularly shaped. Stratified columnar epithelium may be found in the lining of the urethra, in the lining of esophageal glands , in parts of the anal mucosa, and in part o the conjunctiva of the eye. Stratified columnar epithelium serves both protective and secretory functions
