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The epithelium, the name used to refer to epithelial tissue , is the type of tissue that covers all surfaces of the body, both internal cavities and external surfaces. Epithelia can be classified according to several criteria but the most common are the cell shape and the number of layers of cells that make up the tissue. The main classification criteria and the types of epithelia that exist in the human body are:
The epithelial tissue has important functions, vital for survival, and the same epithelium can play several at once. According to the main function we can distinguish:
- Covering or paving epithelium: the lining epithelium is the tissue that lines the external surface of the body (skin, mucous membranes) and the light of internal ducts and cavities of the body (endothelium). The so-called transitional epithelium is a type of coating epithelium that is characterized because its cells respond by changing shape according to the tension that is on them. It can be found in the urinary tract and was thought to be a transitional epithelium between two types of epithelia until it was observed to be an epithelium whose cells change shape allowing the urinary ducts to stretch and contract.
- Glandular epithelium : the glands are covered by a type of epithelial tissue specially designed for the secretory and substance-producing activity of glandular function.
- Sensory epithelium : the epithelial tissue of sensory function, such as the taste epithelium or the olfactory epithelium, has a high number of nerve endings and specialized cells in the reception of specific stimuli.
- Respiratory epithelium : the epithelial tissue that covers all the respiratory tracts is specialized in the exchange of gases between the blood and the air that is breathed.
- Intestinal epithelium : the epithelium lining the intestine and, as a distinctive feature, we can find individual cells with very specific functions that respond to very specific stimuli. It is an epithelium with a high degree of individualization of its cells.
According to the cellular morphology we can distinguish the following types of epithelia:
- Squamous or flat epithelium : epithelia whose cells have a width greater than their height giving rise to a flattened morphology; the cell nucleus also usually appears flattened and the cells acquire a scaled disposition.
- Cubic epithelium : in the cubic epithelia the cell width and height is very similar.
- Cylindrical or columnar epithelium : the cells of these epithelia have a cylindrical morphology, with a height clearly greater than the width, and are arranged as columns.
Number of cell strata
The classification according to the stratification or number of cellular layers of the epithelium is widely used, often in combination with cell morphology, and divides the epithelia into the following types:
Types of Epithelial tissues (monostratified)
The simple type epithelium, also called monostratified, is formed by a single layer or cell layer. In addition, when observing the cells, the nuclei appear in line, with a similar height. In combination with the morphology of the cells, we can have several subtypes:
- Simple flat epithelium : a single layer of flat cells. It is a type of epithelium usually associated with tra function . The nucleus, also flattened, occupies a good part of the cytoplasm. It is typical of coating surfaces where substance exchange takes place; for example, the loop of Henle (in the kidney) or in the alveoli (lungs).
- Simple cubic epithelium : a single layer of cubic cells. The cell nucleus is rounded and located in the center of the cytoplasm. It is associated with epithelia of absorption and secretion; for example endocrine glands and renal tubules .
- Simple columnar epithelium: epithelium with a single layer of columnar cells with an oval-shaped nucleus. It is also associated with epithelia whose function is absorption and secretion, for example it is common in the intestinal epithelium. The goblet cells are columnar cells specialized in the production and secretion of mucus typical of this type of epithelium (they also appear in columnar stratified epithelia).
Stratified epithelium (or polystratified)
The stratified epithelium, more exactly called polystratified, is that formed by at least two cellular strata . Considering the cellular morphology of the superficial stratum (apical side), we have:
- Flat stratified epithelium : the most common lining epithelium. For example, it is the one that forms the skin and the one that covers most mucous membranes. The superficial layer of this type of epithelium is formed by flat cells that, in the case of the skin, are keratinized. On the last layer accumulate dead cells full of keratin that form a water impermeable barrier, impenetrable by microorganisms and very resistant to friction.
- Columnar stratified epithelium : it is a rare type of epithelium and is usually associated with a protective function. Examples can be found in the urethra or conjunctiva of the eye.
- Stratified cubic epithelium : in humans it is only present in the sweat glands.
The pseudostratified epithelial tissue is formed by a single layer of cells alternating cells with a wide base (on the side of the basement membrane) and a narrow apical end (on the side of the epithelial surface) with narrow base and wide end cells. This characteristic makes it confused with a stratified epithelium but in reality there is only one cell stratum. This type of epithelium can be found in some parts of the urethra, the nasal passages or the trachea .
Accessory cellular structures
The specialized function of the epithelial tissue has led to the plasma membrane on the apical side having developed specialized structures, referred to as accessory structures, that allow it to perform its function in a more efficient manner. Among these structures are cilia, flagella and microvilli.
- Ciliated epithelium : the cilia are projections of the apical membrane towards the lumen of the duct or cavity that covers the epithelium. In each cell there is a high number of cilia that move in a coordinated way, usually next to the cilia of neighboring cells, and cause what is known as ciliary wind responsible for the transport of fluids in many parts of the body. For example, movement of mucus in the respiratory system.
- Flagellated epithelium : the flagella are structures similar to cilia but their movement is more independent of each other. The number of flagella is much smaller than that of cilia and, in addition to transport function, there are some with sensory functions.
- Epithelium with microvilli : the microvilli are folds of the apical plasma membrane and are characteristic of absorption epithelia, for example the intestinal epithelium. These folds greatly increase the membrane surface, which in turn increases the absorption capacity.