Epithelial Cell Classification
Simple epithelium is one cell thick. Every cell is in direct contact with the underlying basement membrane. It is generally found where absorption and filtration occur. The thinness of the epithelial barrier facilitates these processes. Simple epithelial tissues are generally classified by the shape of their cells. The four major classes of simple epithelium are: (1) simple squamous; (2) simple cuboidal; (3) simple columnar; and (4) pseudostratified.
(1) Simple squamous is found lining areas where passive diffusion of gases occurs. Example: walls of capillaries, linings of the pericardium, pleural, and peritoneal cavities, as well as the linings of the alveoli of the lungs. (2) Simple cuboidal cells may have secretory, absorptive, or excretory functions. Examples include small collecting ducts of the kidneys, pancreas, and salivary glands. (3) Simple columnar is found in areas with extremely high secretory function (as in wall of the stomach), or absorptive (as in small intestine) areas. They possess cellular extensions (e.g. microvilli in the small intestine, or cilia found almost exclusively in the female reproductive tract). (4) Pseudostratified epithelia, is also called respiratory epithelium due to its almost exclusive confinement to the larger respiratory airways i.e. the nasal cavity, trachea, bronchi etc.
Squamous cells have the appearance of thin, flat plates. They fit closely together in tissues, providing a smooth, low-friction surface over which fluids can move easily. The shape of the nucleus usually corresponds to the cell form and helps to identify the type of epithelium. Squamous cells tend to have horizontally flattened, elliptical (oval or shaped like an egg) nuclei because of the thin flattened form of the cell. Classically, squamous epithelia are found lining surfaces utilizing simple passive diffusion such as the alveolar epithelium in the lungs.
Cuboidal cells are roughly cuboidal in shape, appearing square in cross section. Each cell has a spherical nucleus in the center. Cuboidal epithelium is commonly found in secretive or absorptive tissue: for example the (secretive) exocrine gland of the pancreas and the (absorptive) lining of the kidney tubules as well as in the ducts of the glands. They also constitute the germinal epithelium that covers the female ovary.
Columnar epithelial cells are elongated and column-shaped. Their nuclei are elongated and are usually located near the base of the cells. Columnar epithelium forms the lining of the stomach and intestines. Some columnar cells are specialized for sensory reception such as in the nose, ears, and the taste buds of the tongue. Goblet cells (unicellular glands) are found between the columnar epithelial cells of the duodenum. They secrete mucus, which acts as a lubricant.
Pseudostratified epithelial consist of simple columnar epithelial cells whose nuclei appear at different heights , giving the misleading (hence "pseudo") impression that the epithelium is stratified when the cells are viewed in cross section. Pseudostratified epithelium can also possess fine hair-like extensions of their apical (luminal) membrane called cilia. In this case, the epithelium is described as "ciliated" pseudostratified epithelium. Cilia are capable of energy dependent pulsatile beating in a certain direction through interaction of cytoskeletal microtubules and connecting structural proteins and enzymes. The wafting effect produced causes mucus secreted locally by the goblet cells (to lubricate and to trap pathogens and particles) to flow in that direction (typically out of the body). Ciliated epithelium is found in the airways (nose, bronchi), but is also found in the uterus and Fallopian tubes of females, where the cilia propel the ovum to the uterus.
Stratified epithelium differs from simple epithelium in that it is multilayered. It is therefore found where body linings have to withstand mechanical or chemical insult such that layers can be abraded and lost without exposing subepithelial layers. Cells flatten as the layers become more apical, although in their most basal layers the cells can be squamous, cuboidal, or columnar. Stratified epithelial tissue also differs from simple epithelial tissue in that stratified epithelial tissues do not contain junctional complexes, and have their cells bound together only by desmosomes. Stratified epithelia (of columnar, cuboidal, or squamous type) can have the following specializations:
Keratinized - in this particular case, the most apical layers (exterior) of cells are dead and lose their nucleus and cytoplasm, and contain a tough, resistant protein called keratin. This specialization makes the epithelium waterproof, so is abundant in the mammalian skin. The lining of the esophagus is an example of a non-keratinized or "moist" stratified epithelium.
Transitional epithelia are found in tissues that stretch and it can appear to be stratified cuboidal when the tissue is not stretched or stratified squamous when the organ is distended and the tissue stretches. It is sometimes called the urothelium since it is almost exclusively found in the bladder, ureters, and urethra.