Epithelium tissue system Histology : is the study of the microscopic anatomy (microanatomy) of cells and tissues of plants and animals. It is commonly performed by examining cells and tissues under a light microscope or electron microscope. the specimen having been sectioned (cut into a thin cross section with a microtome), stained, and mounted on a microscope slide. Histological studies may be conducted using tissue culture, where live human or animal cells are isolated and maintained in an artificial environment for various research projects. The ability to visualize or differentially identify microscopic structures is frequently enhanced through the use of histological stains. Histology is an essential tool of biology and medicine Histopathology, the microscopic study of diseased tissue, is an important tool in anatomical pathology, since accurate diagnosis of cancer and other diseases usually requires histopathological examination of samples. Trained physicians, frequently licensed pathologists, are the personnel who perform histopathological examination and provide diagnostic information based on their observations. The trained personnel who prepare histological specimens for examination are histotechnicians, histotechnologists, histology technicians (HT), histology technologists (HTL), medical scientists, medical laboratory technicians, or biomedical scientists and Biomedical Science Support Workers. Their field of study is called histotechnology Importance of Histology : Microscopy reveals minor details of objects under study and in Histology, minor details of tissues of human body are revealed. These details help the medical professionals to understand the mechanisms of different processes taking place in specific organs. Knowledge of histology shows how different cells are organized to form tissues and how each cell and tissue show modification according to their functional demands. Knowledge of Histology is gaining
importance in diagnosis of certain diseases like cancers. Such diagnoses are very important because these diseases are fatal and if not diagnosed correctly, they certainly end in death. Epithelium tissue system The outer surface of the body and the luminal surfaces of cavities within the body are lined by one or more layers of cells that completely cover them. Such layers of cells are called epithelia (singular=epithelium). Epithelia also line the ducts and secretory elements of glands (which develop as outgrowths from epithelium lined surfaces). Classification of Epithelia : An epithelium may consist of only one layer of cells when it is called a unilayered or simple epithelium. Alternatively, it may be multi-layered or stratified. Simple epithelia may be further classified according to the shape of the cells constituting them. (1) In some epithelia the cells are flatten, their height being very little as compared to their width. Such an epithelium is called a squamous epithelium. (2) When the height and width of the cells of the epithelium are more or less equal (i.e., they look like squares in section) it is described as a cuboidal epithelium. (3) When the height of the cells of the epithelium is distinctly greater than their width, it is described as a columnar epithelium.
Multilayered epithelia are of two main types. In the most common type the deeper layers are columnar, but in proceeding towards the surface of the epithelium the cells become increasingly flattened (or squamous). Such an epithelium is described as stratified squamous. It may be noted that all cells in this kind of epithelium are not squamous. In the other type of multilayered epithelium all layers are made up of cuboidal, polygonal or rounded cells. The cells towards the surface of the epithelium are not flattened. This type of epithelium is called transitional epithelium (being transitional between unilayered epithelia and stratified squamous epithelium). As transitional epithelium is confined to the urinary tract it is also called urothelium. A third, rather rare type of multilayered epithelium is made up of two or more layers of cuboidal or columnar cells (stratified cuboidal, or stratified columnar epithelium). Lastly, in some situations a columnar epithelium which is really single layered may give the appearance of a stratified epithelium: such an epithelium is referred to as pseudostratified columnar epithelium. The various types of epithelia named above are considered further below. All epithelia rest on a very thin basement membrane Squamous Epithelium : The cytoplasm of cells in this kind of epithelium forms only a thin layer. The nuclei produce bulgings of the cell surface.in surface view the cells have polygonal outlines that interlock with those of adjoining cells. With the EM the junctions between cells are marked by occluding junctions: the junctions are thus tightly sealed and any substance passing through the epithelium has to pass through the cells, and not between them. Squamous epithelium lines the alveoli
of the lungs. It lines the free surface of the serous pericardium, of the pleura, and of the peritoneum: here it is called mesothelium. It lines the inside of the heart, where it is called endocardium; and of blood vessels and lymphatics, where it is called endothelium. Squamous epithelium is also found lining some parts of the renal tubules, and in some parts of the internal ear. Columnar Epithelium : We have seen that in vertical section the cells of this epithelium are rectangular. On surface view (or in transverse section) the cells are polygonal. In keeping with the elongated shape of the cells, the nuclei are also frequently elongated. Columnar epithelium can be further classified according to the nature of the free surfaces of the cells as follows. (a) In some situations the cell surface has no particular specialization: this is simple columnar epithelium. (b) In some situations the cell surface bears cilia. This is ciliated columnar epithelium. (c) In other situations the surface is covered with microvilli. Although the microvilli are visible only with the EM, with the light microscope the region of the microvilli is seen as a striated border (when the microvilli are arranged regularly) or as a brush border (when the microvilli are irregularly placed). Some columnar cells have a secretory function. The apical parts of their cytoplasm contain secretory vacuoles. Simple columnar epithelium (without cilia or microvilli) is present over the mucous membrane of the stomach and the large intestine.
Columnar epithelium with a striated border is seen most typically in the small intestine, and with a brush border in the gall bladder. Ciliated columnar epithelium lines most of the respiratory tract, the uterus, and the uterine tubes. It is also seen in the efferent ductules of the testis, parts of the middle ear and auditory tube; and in the ependyma lining the central canal of the spinal cord and the ventricles of the brain. In the respiratory tract the cilia move mucous accumulating in the bronchi (and containing trapped dust particles) towards the larynx and pharynx. When excessive this mucous is brought out as sputum during coughing. In the uterine tubes the movements of the cilia help in the passage of ova towards the uterus. Secretory columnar cells are scattered in the mucosa of the stomach and intestines. In the intestines many of them secrete mucous which accumulates in the apical part of the cell making it very light staining. These cells acquire a characteristic shape and are called goblet cells. Some columnar cells secrete enzymes.
Cuboidal Epithelium : Cuboidal epithelium is similar to columnar epithelium, but for the fact that the height of the cells is about the same as their width. The nuclei are usually rounded. A typical cuboidal epithelium may be seen in the follicles of the thyroid gland, in the ducts of many glands, and on the surface of the ovary (where it is called germinal epithelium). Other sites are the choroid plexuses, the inner surface of the lens, and the pigment cell layer of the retina. An epithelium that is basically cuboidal (or columnar) lines the secretory elements of many glands. In this situation, however, the parts of the cells nearest the lumen are more compressed (against neighbouring cells) than at their bases, giving them a triangular shape. A cuboidal epithelium with a prominent brush border is seen in the proximal convoluted tubules of the kidneys.