Functions and structure of epithelial tissue. Structure of epithelial and connective tissue

Cloth is a collection of cells and intercellular substance. It has common features of structure and performs the same functions. In the body there are four types of tissues: epithelial, nervous, muscular and connective.

The structure of the epithelial tissue of man and animals is due, first of all, to its localization. Epithelial tissue is the boundary layer of cells lining the integument of the body, the mucous membranes of internal organs and cavities. Also, many of the glands in the body are formed by the epithelium.

general characteristics

The structure of epithelial tissue has a number of features that are inherent only in the epithelium. The main feature is that the tissue itself looks like a continuous layer of cells that fit tightly together.

The epithelium lining all surfaces in the body has the appearance of a formation, whereas in the liver, pancreas, thyroid, salivary and other glands it is a cluster of cells. In the first case, it is located over the basement membrane, which 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, in the lymphatic system, alternating cells of epithelial and connective tissue are observed. This type of epithelium is called atypical.

High regenerative capacity is another feature of the epithelium.

The cells of this tissue are polar, which is due to the difference in the basal and apical parts of the cell center.

The structure of epithelial tissue is largely due to its borderline position, which, in turn, makes the epithelium an important link in the metabolic processes. This tissue is involved in the absorption of nutrients from the intestine into the blood and lymph, in the release of urine through the epithelium of the kidneys, etc. Also, one must not forget the protective function, which is to protect the tissues from damaging effects.

The structure of the substance forming the basal membrane shows that it contains a large amount of mucopolysaccharides, and there is also a network of fine fibrils.

How is epithelial tissue laid?

Features of the structure of epithelial tissue of animals and humans are largely dictated by the fact that its development is carried out from all three embryonic sheets. This feature is inherent only in this type of fabric. Ectoderm gives rise to the epithelium of the skin, the mouth, a large part of the esophagus, the cornea of the eye; Endoderm - the epithelium of the gastrointestinal tract; And mesoderm - the epithelium of the genito-urinary organs and serous membranes.

In embryonic development begins to form at the earliest stages. Since there is a sufficient amount of epithelial tissue in the placenta, it is a participant in the metabolism between the mother and the embryo.

Maintaining the integrity of epithelial cells

Interaction of neighboring cells in the formation 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 slit-shaped gap between the halves of the desmos is a substance of carbohydrate origin.

In cases where the intercellular spaces are wide, desmosomes are located at the ends of the cytoplasmic cytoplasm directed towards each other on the contacting cells. If you consider a pair of these bulging under a microscope, you can find that they look like an intercellular bridge.

In the small intestine, the integrity of the formation is maintained by the fusion of the cell walls 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 that ensure integrity. Then the contact of neighboring cells is due to the contact of the even or sinuous surfaces of the cells. The edges of the cells can be tiled overlapping.

Structure of the epithelial cell

The features of the cells of epithelial tissue include the presence on their surface of the plasma membrane.

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 are the so-called cells in the science that form epithelial tissues. Features of the structure, the functions of epithelial cells are in close interrelation. So, in form they are divided into flat, cubic and columnar. The nucleus is dominated by euchromatin, due to which it has a light color. The nucleus is large enough, its shape coincides with the shape of the cell.

The pronounced polarity determines the location of the nucleus in the basal part, above it are the mitochondria, the Golgi complex and the centrioles. In cells performing secretory function, the endoplasmic reticulum and the Golgi complex are especially well developed. Epithelium, which experiences a high mechanical load, has in its cells a system of special threads - tonofibrils, which create a barrier, designed to protect cells from deformation.

Microvilli

Some cells, or rather their cytoplasm, on the surface can form the smallest, directed to the outside, outgrowths - microvilli. The largest of their clusters are found on the apical surface of the epithelium in the small intestine and in 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 which can be viewed under an optical microscope. In addition, microvilli in these places contain a number of enzymes.

Classification

The peculiarities of the structure of epithelial tissues of different localization allow us to classify them 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-layered and multilayered.

Also, the glandular epithelium that secrets the secretory function is secreted.

Single-layered epithelium

The name of the single-layered epithelium speaks for itself: in it all cells are located on the basement membrane in one layer. If the shape of all cells is the same (that is, they are isomorphic), and the nuclei of the cells are on the same level, then they speak of a single-row epithelium. And if in the single-layered epithelium the alternation of cells of different forms is observed, their nuclei are located at different levels, then this is a multi-row or anisomorphic epithelium.

Multilayered epithelium

In multilayer epithelium, only the lower layer contacts the basal membrane, and the other layers are above it. The cells of different layers differ in shape. The structure of epithelial tissue of this type allows us to distinguish several types of multilayer epithelium depending on the shape and condition of the cells of the outer layer: multilayered planar, multilayer keratinizing (there are keratinized scales on the surface), multilayered non-keratinizing.

There is also the so-called transitional epithelium lining the organs of the excretory system. Depending on whether the organ shrinks or stretches, the fabric takes on a different appearance. Thus, 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 (shortened) form, the epithelial tissue sharply thickens, the cells of the basal layer become polymorphic and their nuclei are at different levels. The integumentary cells acquire a pear-shaped form and overlap each other.

Histogenetic classification of epithelia

The structure of the epithelial tissue of animals and humans is often the subject of scientific and medical research. In these cases, most often the histogenetic classification developed by Academician NG Khlopin is used. According to her, five types of epithelium are distinguished. The criterion is that of which primordia tissue developed in embryogenesis.

1. Epidermal type, the beginning of which was given by ectoderm and prechordal plate.

2. Entero-dermal type, the development of which occurred from the intestinal endoderm.

3. Whole-nephrodermal type, developed from coelomic lining and nephrotoma.

4. Angiodermal type, the development of which began from the site of the mesenchyme, which forms the vascular endothelium, which is called angioblast.

5. The ependymaglial type, the beginning of which gave a neural tube.

Features of the structure of epithelial tissues that form glands

The glandular epithelium performs the secretory function. This type of tissue is an accumulation of glandular (secretory) cells, called granulocytes. Their function is to carry out the synthesis, as well as to isolate specific substances - secrets.

It is through secretion that the body is able to perform many important functions. Glands secrete secrets 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 epithelial tissue of this type can be different in view of the fact that granulocytes can take different forms. It depends on the phase of secretion.

Both types of glands (endocrine and exocrine) can consist of one cell (unicellular) or of a multitude of cells (multicellular).