Features of the structure of epithelial tissue. Characteristics of epithelial tissues

Each type of tissue has many characteristic features. They consist in the features of the structure, the set of functions performed, the origin, the nature of the mechanism of renewal. Characterize these tissues by several criteria, but the most common is the morphofunctional accessory. Such a classification of tissues makes it possible to characterize each type in the most complete and essential way. Depending on the morphofunctional features, the following types of tissue are distinguished : epithelial (integumentary), musculoskeletal and nervous-muscular-trophic.

Features of the structure of epithelial tissue: general morphofunctional features

Epithelium refers to a group of tissues that are widespread in the body. They can differ in origin, that is, develop from ectoderm, mesoderm or endoderm, and also perform different functions.

A list of common morphofunctional traits characteristic of all epithelial tissues:

1. Consist of cells called epithelial cells. Between them there are thin intermembrane fissures, in which there is no intercellular substance. In it, in turn, there is an overmembrane complex (glycocalyx). It is through it that substances enter the cells and through it they are removed from the cells.

2. Cells of epithelial tissues are located very tightly, which causes the formation of seams. It is their presence that allows the tissues to perform their functions. The ways of connecting cells to each other can be different: with the help of desmosomes, slit or dense contacts.

3. The connective and epithelial tissues, which are located one under the other, separate the basal membrane, consisting of proteins and carbohydrates. Its thickness is 100 nm - 1 μm. Inside the epithelium, there are no blood vessels, and therefore, their food is diffusely, using the basal membrane.

4. For the epithelial cells, morpho-functional polarity is characteristic. They have a basal and apical pole. The nucleus of epithelial cells is located closer to the basal, and almost all the cytoplasm is located in the apical region. 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 approaches to classification

From the point of view of evolution, epithelial cells were 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 feature, these types of this tissue are distinguished: integumentary, suction, excretory, secretory and others. Classification of epithelial tissues according to morphological features takes into account the shape of epithelial cells and the number of their layers in the formation. Thus, single-layered and multilayered epithelial tissues are isolated.

Characteristics of single-layered single-row epithelium

Features of the structure of epithelial tissue, which is usually called a single layer, is that the layer consists of a single layer of cells. When the same height is typical for all cells in the layer, then we are talking about a single-layer single-row epithelium. The height of epitheliocytes causes a subsequent classification, according to which there is evidence of the presence in the body of a flat, cubic and cylindrical (prismatic) single-layered single-row epithelium.

Single - layered flat epithelium is localized in respiratory sections of the lungs (alveoli), small ducts of glands, testes, middle ear cavity, serous membranes (mesothelium). It is formed from the mesoderm.

Places of localization of single-layered cubic epithelium are ducts of glands and canaliculus of kidneys. The height and width of the cells are approximately the same, the nuclei are rounded and located in the center of the cells. The origin can be different.

This type of single-layered single-row epithelial tissue, like a cylindrical (prismatic) epithelium, is located in the gastrointestinal tract, ducts of glands, collecting tubules of the kidneys. The height of the cells is much larger than the width. It has a different origin.

Characteristics of single-layer multilayer ciliated epithelium

If single-layered epithelial tissue forms a layer of cells of different heights, then they are talking about the multilayered ciliated epithelium. Such tissue lining the surface of the airways and some parts of the reproductive system (vas deferens and oviducts). The peculiarities of the structure of epithelial tissue of this type consist in the fact that its cells are of three types: short intercalary, long ciliate and goblet. All of them are located in one layer, but the intercalary cells do not reach the upper edge of the formation. When they grow, they differentiate and turn into ciliated or goblet. The peculiarity of ciliate cells is the presence of a large number of cilia on the apical pole, the goblet cells are able to produce mucus.

Classification and structure of multilayer epithelia

Epithelial cells can form several layers. They are located on top of each other, therefore, direct contact with the basal membrane is present only in the deepest, basal layer of epithelial cells. It contains stem and cambial cells. When they differentiate, they move to the outside. The criterion for further classification is the shape of the cells. Thus, a multilayered flat keratinizing, multilayered flat nonkeratinized and transitional epithelium is distinguished.

Characteristic of multilayered flat keratinizing epithelium

It is formed from the ectoderm. This tissue consists of the epidermis, which is the surface layer of the skin, and the terminal portion of the rectum. Features of the structure of epithelial tissue of this type are in the presence of five layers of cells: basal, prickly, granular, shiny and horny.

The basal layer is one row of tall cylindrical cells. They are tightly connected to the basal membrane and have the ability to reproduce. The thickness of the spiny layer is from 4 to 8 rows of spiny cells. In the granular layer there are 2-3 rows of cells. Epitheliocytes have a flattened shape, the nuclei are dense. A shiny layer is 2-3 rows of dead cells. The closest to the surface of the stratum corneum, consists of a large number of rows (up to 100) of dead cells of flat form. These are horny scales, in which there is a horny substance keratin.

The function of this tissue is to protect deeply lying tissues from external damage.

Peculiarities of the structure of multilayer flat nonkeratinized epithelium

It is formed from the ectoderm. Locally, the cornea of the eye, mouth, esophagus and part of the stomach of some animal species. It has three layers: basal, prickly and flat. The basal layer touches the basal membrane, consists of prismatic cells that have large oval nuclei somewhat shifted to the apical pole. The cells of this layer, dividing, begin to move upward. Thus, they cease to touch the basal membrane and pass into the prickly layer. These are several layers of cells that have an irregular polygonal shape and an oval core. The spiky layer turns into a surface layer - a flat layer, the thickness of which is 2-3 cells.

Transitional epithelium

Classification of epithelial tissues provides for the presence of a so-called transitional epithelium, formed from the mesoderm. Localization sites - 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 different 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 body is. In the cover layer, the cells are even larger, they are characterized by multinucleation, or polyploidy, capable of secreting mucus, which protects the surface of the formation 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 release special substances - secrets. The size, shape, structure of glandular cells is very diverse, as is the composition and specialization of secrets.

The process, in the course of which secrets are formed quite complex, proceeds in several stages and is called the secretory cycle.

Features of the structure of epithelial tissue, consisting of glandular cells, are primarily due to its purpose. From this type of tissue there is the formation of organs, the main function of which is the development of a secret. These organs are called glands.