Tissue is a combination of cells and intercellular substance. It has common structural features and performs the same functions. There are four types of tissues in the body: epithelial, nervous, muscular and connective.
The structure of the epithelial tissue of humans and animals is primarily due to its localization. Epithelial tissue is the boundary layer of cells lining the integument of the body, mucous membranes of internal organs and cavities. Also, many glands in the body are formed precisely by the epithelium.
General characteristics
The structure of the epithelial tissue has a number of features that are unique to the epithelium. The main feature is that the tissue itself has the appearance of a continuous layer of cells that fit snugly together.
The epithelium lining all surfaces in the body looks like a layer, while in the liver, pancreas, thyroid, salivary and other glands it is a cluster of cells. In the first case, it is locatedover the basement membrane that separates the epithelium from the connective tissue. But there are exceptions when the structure of epithelial and connective tissue is considered in the context of their interaction. In particular, alternation of epithelial and connective tissue cells is observed in the lymphatic system. This type of epithelium is called atypical.
High regenerative capacity is another feature of the epithelium.
The cells of this tissue are polar, due to the difference in the basal and apical parts of the cell center.
The structure of epithelial tissue is largely due to its boundary position, which, in turn, makes the epithelium an important link in metabolic processes. This tissue is involved in the absorption of nutrients from the intestines into the blood and lymph, in the excretion of urine through the epithelium of the kidneys, etc. Also, one should not forget about the protective function, which consists in protecting tissues from damaging effects.
The structure of the substance that forms the basement membrane shows that it contains a large amount of mucopolysaccharides, and there is also a network of thin fibrils.
How is epithelial tissue laid?
The structural features of the epithelial tissue of animals and humans are largely dictated by the fact that its development is carried out from all three germ layers. This feature is unique to this type of fabric. The ectoderm gives rise to the epithelium of the skin, oral cavity, a significant part of the esophagus, and the cornea of the eye; endoderm - the epithelium of the gastrointestinal tract; and mesoderm- the epithelium of the genitourinary organs and serous membranes.
In embryonic development begins to form at the earliest stages. Since the placenta contains a sufficient amount of epithelial tissue, it is a participant in the metabolism between the mother and the fetus.
Maintaining the integrity of epithelial cells
Interaction of neighboring cells in the layer is possible due to the presence of desmosomes. These are special multiple structures of submicroscopic size, which consist of two halves. Each of them, thickening in certain places, occupies adjacent surfaces of neighboring cells. In the gap between the halves of the desmosomes is a substance of carbohydrate origin.
In cases where intercellular spaces are wide, desmosomes are located at the ends of cytoplasmic bulges directed towards each other on contacting cells. If you look at a pair of these bulges under a microscope, you can find that they look like an intercellular bridge.
In the small intestine, the integrity of the layer is maintained due to the fusion of the cell membranes of neighboring cells at the points of contact. Such places are often called end plates.
There are other cases where there are no special structures to ensure integrity. Then the contact of neighboring cells is carried out due to the contact of even or sinuous surfaces of the cells. The edges of the cells can be tiled over each other.
Structure of an epithelial tissue cell
The peculiarities of epithelial tissue cells include the presence of plasmaticshell.
In cells involved in the release of metabolic products, folding is observed in the plasma membrane of the basal part of the cell body.
Epitheliocytes - this is the name in science for the cells that form epithelial tissues. Structural features, functions of epithelial cells are closely related. So, according to their shape, they are divided into flat, cubic and columnar. Euchromatin predominates in the nucleus, due to which it has a light color. The nucleus is quite large, its shape coincides with the shape of the cell.
Pronounced polarity determines the location of the nucleus in the basal part, above it are mitochondria, the Golgi complex and centrioles. In cells that perform a secretory function, the endoplasmic reticulum and the Golgi complex are especially well developed. The epithelium, experiencing a large mechanical load, in its cells has a system of special threads - tonofibrils, which create a kind of barrier designed to protect cells from deformation.
Microvilli
Some cells, or rather their cytoplasm, on the surface can form the smallest, directed outward, outgrowths - microvilli. Their largest accumulations are found on the apical surface of the epithelium in the small intestine and the main sections of the convoluted tubules of the kidneys. Due to the parallel arrangement of microvilli in the cuticles of the intestinal epithelium and the brush border of the kidneys, strips are formed that can be seen under an optical microscope. In addition, microvilli in these places contain a number of enzymes.
Classification
Features of the structure of epithelial tissues of different localizationallow them to be classified according to several criteria.
Depending on the shape of the cells, the epithelium can be cylindrical, cubic and flat, and depending on the location of the cells - single-layer and multi-layer.
Also secrete glandular epithelium, which performs a secretory function in the body.
Single-layered epithelium
The name of the single-layer epithelium speaks for itself: in it all the cells are located on the basement membrane in one layer. If, in this case, the shape of all cells is the same (i.e., they are isomorphic), and the nuclei of the cells are at the same level, then they speak of a single-row epithelium. And if in a single-layer epithelium there is an alternation of cells of various shapes, their nuclei are located at different levels, then this is a multi-row or anisomorphic epithelium.
Squamous epithelium
In stratified epithelium, only the lower layer is in contact with the basement membrane, while the other layers are above it. Cells of different layers differ in shape. The structure of this type of epithelial tissue makes it possible to distinguish several types of stratified epithelium depending on the shape and condition of the cells of the outer layer: stratified squamous, stratified keratinized (there are keratinized scales on the surface), stratified non-keratinized.
There is also the so-called transitional epithelium,lining the organs of the excretory system. Depending on whether the organ is contracting or stretching, the tissue takes on a different appearance. So, when the bladder is stretched, the epithelium is in a thinned state and forms two layers of cells - basal and integumentary. And when the bladder is in a compressed (contracted) form, the epithelial tissue thickens sharply, the cells of the basal layer become polymorphic and their nuclei are at different levels. Integumentary cells become pear-shaped and layer on top of each other.
Histogenetic classification of epithelia
The structure of the epithelial tissue of animals and humans often becomes the subject of scientific and medical research. In these cases, more often than others, the histogenetic classification developed by Academician N. G. Khlopin is used. According to her, there are five types of epithelium. The criterion is from which rudiments the tissue developed in embryogenesis.
1. Epidermal type, originating from the ectoderm and prechordal plate.
2. Enterodermal type, the development of which originated from the intestinal endoderm.
3. Coelonephroderm type developed from coelomic lining and nephrotome.
4. Angiodermal type, the development of which began from a section of the mesenchyme that forms the vascular endothelium, which is called an angioblast.
5. Ependymoglial type, the origin of which was given by the neural tube.
Features of the structure of epithelial tissues that form glands
Glandular epithelium performs a secretory function. This type of tissue is a collectionglandular (secretory) cells called granulocytes. Their function is to carry out the synthesis, as well as the release of specific substances - secrets.
It is thanks to secretion that the body is able to perform many important functions. The glands secrete secretions on the surface of the skin and mucous membranes, inside the cavities of a number of internal organs, as well as into the blood and lymph. In the first case, we are talking about exocrine, and in the second - about endocrine secretion.
Exocrine secretion allows the production of milk (in the female body), gastric and intestinal juice, saliva, bile, sweat and sebum. The secrets of the endocrine glands are hormones that perform humoral regulation in the body.
The structure of this type of epithelial tissue may be different due to the fact that granulocytes can take on different shapes. It depends on the phase of secretion.
Both types of glands (endocrine and exocrine) can consist of a single cell (unicellular) or multiple cells (multicellular).
