The various types of epithelial tissues are simple, compound and glandular epithelium.(i) Simple epitheliumIt is found generally on secretory and absorptive surfaces. It covers surfaces exposed to mechanical or chemical abrasions because it is not effective in protecting the underlying tissues. It is formed of a single layer of cells resting on the basement membrane. There are various types of simple epithelia. These are, squamous, cuboidal, columnar, ciliated and pseudostratified.(a) Squamous epithelium: Squamous epithelium consists of cells, which are thin, flat and polygonal having a prominent round or oval nucleus. The cells have irregular boundaries that fit closely into those of neighbouring cells. Its main function is protection of the underlying tissues. It forms the inner lining of lung, alveoli, blood vessels and peritoneum of the body cavity.(b) Cuboidal epithelium: Cubiodial epithelium is made up of cells, which are polygonal in outline but appear cubical or cuboidal in vertical section. In addition to protection, these cells participate in secretion, excretion and absorption. The cells of cuboidal epithelium in absorptive surfaces often bear microvilli at their free ends. This type of epithelium is found in proximal tubules of kidneys, lines of small salivary glands, pancreatic ducts, thyroid follicles and ovaries.(c) Columnar epithelium: Columnar epithelium is characterized by the presence of tall pillar-like cells, which resemble polygonal columns. The oval nucleus is generally present at the base of the cell. The function of columnar epithelium is secretion or absorption and it is found in the inner surface of the intestine, stomach and gall bladder. It also occurs in gastric and intestinal glands. The intestinal mucosa is lined by columnar epithelium, whose free ends are thrown into tiny finger-like projections.(d) Ciliated epithelium: Ciliated epithelium is formed of columnar or cubical epithelial cells that bear thin protoplasmic processes called cilia on their free surfaces. Such an epithelium is known as ciliated epithelium. The function of the cilia is to move particles, free cells or mucus in a specific direction over the epithelial surface. Ciliated epithelium lines the inner surfaces of some hollow organs such as fallopian tubes, nasal passages, bronchioles and small bronchi.(e) Pseudo-stratified epithelium: Pseudo-stratified epithelium consists of ciliated or non-ciliated epithelium in which the cells are columnar and arranged in a single layer, but appears multi-layered because some cells are shorter than others. As a result, their nuclei become located at different levels giving a stratified appearance. This type of epithelium is therefore, called pseudo-stratified.(ii) Compound epitheliumIt consists of more than one layer of cells and gives a stratified appearance. Hence, these are also known as stratified epithelium. The cells may be of different shapes in different layers. The deepest layer of cells rests on a basement membrane. The morphology of the superficial layers varies in the different kinds of stratified epithelia. Compound epithelium may be stratified cuboidal, stratified squamous, stratified keratinized or transitional. The main function of this type of epithelium is protection to underlying tissues against mechanical, chemical, thermal or osmotic stresses.In stratified cuboidal epithelium, the superficial cells are cuboidal. It lines the inner surfaces of larger salivary and pancreatic ducts. Stratified squamous non-keratinized epithelium has several layers of interlinked cuboidal or columnar cells. Its cells lying closer to the underlying connective tissue, are cuboidal or columnar, but the superficial cells are flattened, thin-walled and squamous. They retain their nuclei. It covers moist surfaces such as buccal cavity, pharynx, oesophagus, cornea of eyes, etc. When the surface cells are dead and contain insoluble protein deposits, the tissue is called stratified squamous keratinized epithelium. This type of epithelium protects the epidermis of skin, hair, horn, nail, etc. Another type of compound epithelium is the transitional epithelium, which lines the inner surface of the urinary bladder, ureter, etc. Transitional epithelium is much thinner and more stretchable than the stratified epithelium. It has a single layer of cuboidal cells at the base, two to three middle layers of a large polygonal or pear-shaped cells and a superficial layer of large, broad, rectangular or oval cells. (iii) Glandular epitheliaThese are specialized epithelial cells that form glands. Glands produce a fluid secretion that is different from blood or any extra cellular fluid in its composition. Such secretions occur along with the synthesis of intracellular macromolecules such as proteins, lipids, complexes of carbohydrates and proteins or all the three types of macromolecules.The cells of glandular epithelia are generally columnar or cuboidal. The glandular epithelium can be classified into two types: (a) Unicellular – consisting of isolated glandular cells and (b) multicellular - consisting of a cluster of cells. A gland, with a single unbranched duct is called a simple gland. The secretory part of the gland consists of epithelial cells arranged in the form of tubes or sacs or a combination of both. The duct is also made up of epithelial cells. Tubular gland, found in the human intestine, is an example of simple gland. A gland with a branched system of ducts is called a compound gland. In these glands, the secretory tubule or acinus may be coiled or branched and opens into the single duct of the gland.
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