Each type of fabric has many characteristic features. They consist in structural features, a set of functions performed, the origin, the nature of the update mechanism. These tissues can be characterized by several criteria, but morphofunctional affiliation is the most common. Such a classification of tissues makes it possible to most fully and substantially characterize each type. The following types of tissue are distinguished depending on morphological and functional features : epithelial (integumentary), musculoskeletal and trophic muscular tissue .
Features of the structure of epithelial tissue: general morphological and functional features
Epithelium includes a group of tissues that are widely distributed in the body. They can differ in origin, that is, develop from an ectoderm, mesoderm or endoderm, and also perform different functions.
The list of common morphological and functional features characteristic of all epithelial tissues:
1. Consist of cells called epithelial cells. Between them there are thin intermembrane cracks in which there is no intercellular substance. In it, in turn, there is a supmembrane complex (glycocalyx). It is through it that substances enter the cells and through it are derived from the cells.
2. Cells of epithelial tissues are located very densely, which leads to the formation of layers. It is their presence that allows tissue to perform its functions. Methods for connecting cells to each other can be different: using desmosomes, gap or tight contacts.
3. The connective and epithelial tissues, which are located one below the other, are separated by a basement membrane consisting of proteins and carbohydrates. Its thickness is 100 nm - 1 ΞΌm. There are no blood vessels inside the epithelium, and therefore, their nutrition is carried out diffusely, using the basement membrane.
4. Epithelial cells are characterized by morphofunctional polarity. They have a basal and apical pole. The nucleus of epithelial cells is located closer to the basal, and almost the entire cytoplasm is located at the apical. There may be accumulations of cilia and microvilli.
5. Epithelial tissues are characterized by a pronounced ability to regenerate. They are characterized by the presence of stem, cambial and differentiated cells.
Different classification approaches
From the point of view of evolution, epithelial cells formed earlier than cells of other tissues. Their primary function was to delimit the body from the external environment. At the present stage of evolution, epithelial tissues perform several functions in the body. According to this symptom, these types of this tissue are distinguished: integumentary, absorbing, excretory, secretory and others. The classification of epithelial tissues according to morphological characteristics takes into account the shape of the epithelial cells and the number of their layers in the reservoir. So, single-layer and multilayer epithelial tissues are distinguished.
Characterization of single-layer single-row epithelium
The structural features of the epithelial tissue, which is usually called a single layer, are that the layer consists of a single layer of cells. When the same height is characteristic of all cells in the formation, then we are talking about a single-layer single-row epithelium. The height of the epithelial cells determines the following classification, according to which they say that the body has a flat, cubic and cylindrical (prismatic) single-layer single-row epithelium.
A single-layered squamous epithelium is localized in the respiratory parts of the lungs (alveoli), small ducts of the glands, testes, middle ear cavity, serous membranes (mesothelium). Formed from the mesoderm.
Locations of a single-layer cubic epithelium are ducts of the glands and tubules of the kidneys. The height and width of the cells are approximately the same, the nuclei are rounded and are located in the center of the cells. The origin may be different.
This type of single-layer single-row epithelial tissue, such as a cylindrical (prismatic) epithelium, is located in the gastrointestinal tract, ducts of glands, and collecting ducts of the kidneys. Cell height significantly exceeds width. It has a different origin.
Characterization of a single-layered multilayer ciliated epithelium
If a single-layer epithelial tissue forms a layer of cells of different heights, then we are talking about multirow ciliated epithelium. Such tissue lines the surface of the airways and some parts of the reproductive system (vas deferens and egg ducts). The structural features of this type of epithelial tissue are that its cells are of three types: short insertion, long ciliary and goblet. All of them are located in one layer, but the insertion cells do not reach the upper edge of the formation. When they grow, they differentiate and turn into ciliary or goblet. A feature of ciliated cells is the presence of a large number of cilia on the apical pole, goblet cells are able to produce mucus.
Classification and structure of stratified epithelium
Epithelial cells can form several layers. They are located on each other, therefore, direct contact with the basement membrane is present only in the deepest located, basal layer of epithelial cells. It contains stem and cambial cells. When they differentiate, they move outward. The criterion for further classification is the shape of the cells. Thus, multilayer squamous keratinized, stratified squamous non-keratinized and transitional epithelia are distinguished.
Characterization of the stratified squamous keratinized epithelium
Formed from the ectoderm. This tissue consists of the epidermis, which is the surface layer of the skin, and the final portion of the rectum. The structural features of the epithelial tissue of this type are the presence of five layers of cells: basal, spiky, granular, shiny and horny.
The basal layer is one row of tall cylindrical cells. They are tightly connected with the basement membrane and have the ability to reproduce. The thickness of the prickly layer is from 4 to 8 rows of prickly cells. In the granular layer - 2-3 rows of cells. Epithelial cells have a flattened shape, the nuclei are dense. The shiny layer is 2-3 rows of dying cells. The stratum corneum closest to the surface consists of a large number of rows (up to 100) of dead cells of a flat shape. These are horny scales in which keratin is present.
The function of this tissue is to protect deep tissue from external damage.
Structural features of the stratified squamous non-keratinizing epithelium
Formed from the ectoderm. Localization sites are the cornea of ββthe eye, the oral cavity, the esophagus and part of the stomach of some animal species. It has three layers: basal, prickly and flat. The basal layer is in contact with the basement membrane, consists of prismatic cells having large oval nuclei, somewhat shifted to the apical pole. The cells of this layer, dividing, begin to move up. Thus, they cease to come into contact with the basement membrane and pass into a prickly layer. These are several layers of cells having an irregular polygonal shape and an oval nucleus. The prickly layer passes into the surface - flat layer, the thickness of which is 2-3 cells.
Transitional epithelium
Classification of epithelial tissues involves the presence of the so-called transitional epithelium formed from the mesoderm. Locations - renal pelvis, ureters and bladder. Three layers of cells (basal, intermediate and integumentary) vary greatly in structure. The basal layer is characterized by the presence of small cambial cells of various shapes lying on the basement membrane. In the intermediate layer, the cells are light and large, and the number of rows can be different. It directly depends on how full the organ is. In the integumentary layer, the cells are even larger, they are characterized by multinucleation, or polyploidy, capable of secreting mucus, which protects the surface of the layer from harmful contact with urine.
Glandular epithelium
The characteristic of epithelial tissues was incomplete without a description of the structure and functions of the so-called glandular epithelium. This type of tissue is widespread in the body, its cells are able to produce and secrete special substances - secrets. The size, shape, structure of glandular cells is very diverse, as well as the composition and specialization of secrets.
The process in which secrets are formed is quite complicated, proceeds in several stages and is called the secretory cycle.
The structural features of the epithelial tissue, consisting of glandular cells, are due primarily to its purpose. From this type of tissue, the formation of organs occurs, the main function of which will be the secretion. These organs are called glands.