Compounds of cells present in the tissues and organs of multicellular organisms are formed by complex structures called intercellular contacts . Especially often they are found in the epithelium, the boundary integumentary layers.
Scientists believe that the primary separation of the layer of elements connected by intercellular contacts , ensured the formation and subsequent development of organs and tissues.
Thanks to the use of electron microscopy methods, it was possible to accumulate a large amount of information about the ultrastructure of these bonds. However, their biochemical composition, as well as their molecular structure, are not well understood today.
Next, we consider the features, groups and types of intercellular contacts .
General information
The membrane is very actively involved in the formation of intercellular contacts . In multicellular organisms, complex cell formations are formed due to the interaction of elements. Their conservation can be provided in many ways.
In embryonic, germinal tissues, especially at the initial stages of development, cells maintain bonds with each other due to the fact that their surfaces have the ability to stick together. Such adhesion (bonding) may be associated with the surface properties of the elements.
Specifics of occurrence
Researchers believe that the formation of intercellular contacts is provided due to the interaction of glycocalyx with lipoproteins. When connecting, there is always a small gap (its width is about 20 nm). It contains glycocalyx. When processing the tissue with an enzyme that can violate its integrity or damage the membrane, the cells begin to separate from each other, dissociate.
If the dissociating factor is removed, the cells can again come together. This phenomenon is called reaggregation. So you can dissociate the cells of different in color sponges: yellow and orange. During the experiments, it was found that only 2 types of aggregates appear in the cell connection. Some consist solely of orange, while others are only of yellow cells. Mixed suspensions, in turn, self-organize and restore the primary multicellular structure.
Researchers obtained similar results during experiments with suspensions of divided amphibian embryonic cells. In this case, the ectoderm cells are isolated in space selectively from the mesenchyme and endoderm. If we use tissues of the later stages of embryo development to restore bonds, different cell groups that differ in organ and tissue specificity will independently assemble in a test tube, and epithelial aggregates will be formed that resemble the renal tubules.
Physiology: types of intercellular contacts
Scientists distinguish 2 main groups of relationships:
- Simple. They can form compounds that vary in shape.
- Complicated. These include slit-shaped, desmosomal, tight intercellular contacts , as well as adhesive bands and synapses.
Consider their brief characteristics.
Simple connections
Simple intercellular contacts are the sites of interaction of the supmembrane cell complexes of plasmolemma. The distance between them is no more than 15 nm. Intercellular contacts provide adhesion of elements due to mutual "recognition". Glycocalyx is equipped with special receptor complexes. They are strictly individual for each individual organism.
The formation of receptor complexes is specific within a particular population of cells or certain tissues. They are represented by integrins and cadherins, which have an affinity for similar structures of cells in the neighborhood. When interacting with related molecules located on adjacent cytomembranes, they stick together - adhesion.
Intercellular contacts in histology
Among adhesive proteins, there are:
- Integrins.
- Immunoglobulins.
- Selectins.
- Cadherins.
Some proteins with adhesive properties do not belong to any of these families.
Characteristics of Families
Some glycoproteins of the surface cell apparatus belong to the main class 1 histocompatibility complex. Like integrins, they are strictly individual for an individual organism and specific for the tissue formations in which they are located. Some substances are found only in certain tissues. For example, E-cadherins are specific for the epithelium.
Integrins are called integral proteins, which consist of 2 subunits - alpha and beta. Currently, 10 variants of the first and 15 types of the second have been identified. Intracellular sites are associated with thin microfilaments using special protein molecules (tannin or vinculin) or directly with actin.
Selectins are monomeric proteins. They recognize certain carbohydrate complexes and attach to them on the surface of the cells. Currently, the most studied are L, P and E-selectins.
Immunoglobulin-like adhesive proteins are similar in structure to classical antibodies. Some of them are receptors for immunological reactions, others are intended only for the implementation of adhesive functions.
The intercellular contacts of cadherins occur only in the presence of calcium ions. They are involved in the formation of permanent bonds: P and E-cadherins in the epithelial tissues, and N-cadherins in the muscular and nervous.
Appointment
It should be said that intercellular contacts are not only for simple adhesion of elements. They are necessary to ensure the normal functioning of tissue structures and cells in the formation of which are involved. Simple contacts control the maturation and movement of cells, prevent hyperplasia (an excessive increase in the number of structural elements).
Variety of compounds
In the course of research, different types of intercellular contacts in shape were established. They can be, for example, in the form of "tiles". Such bonds are formed in the stratum corneum of the squamous multilayered keratinizing epithelium, in the arterial endothelium. Toothed and finger-shaped types are also known. In the first, the protrusion of one element is immersed in the concave part of the other. Due to this, the mechanical strength of the connection is significantly increased.
Difficult connections
These types of intercellular contacts are specialized for the implementation of a specific function. Such compounds are represented by small paired specialized sections of the plasma membranes of 2 neighboring cells.
The following types of intercellular contacts exist:
- Locking.
- Interlocking.
- Communication.
Desmosomes
They are complex macromolecular formations, through which a strong connection of neighboring elements is ensured. With electron microscopy, this type of contact is very noticeable, because it is distinguished by a high electron density. The local area looks like a disk. Its diameter is about 0.5 microns. The membranes of neighboring elements in it are located at a distance of 30 to 40 nm.
High electron density regions can also be considered on the inner membrane surfaces of both interacting cells. Intermediate filaments are attached to them. In the epithelial tissue, these elements are represented by tonofilaments, which form clusters - tonofibrils. Cytokeratins are present in tonofilaments. An electron-dense zone is also found between the membranes, which corresponds to the adhesion of protein complexes of neighboring cellular elements.
As a rule, desmosomes are found in epithelial tissue, but they can also be detected in other structures. In this case, the intermediate filaments contain substances characteristic of this tissue. For example, vimentins are present in the connective structures, desmins, etc., are present in the muscles.
The interior of the desmosome at the macromolecular level is represented by desmoplakins - supporting proteins. Intermediate filaments join with them . Desmoplakins, in turn, are bonded to desmogleins with the help of placoglobins. This triple compound passes through the lipid layer. Desmogleins bind to proteins located in a neighboring cell.
However, another option is possible. The attachment of desmoplakins is carried out to integral proteins located in the membrane - desmokolins. They, in turn, bind to similar proteins in the adjacent cytomembrane.
Belt desmosome
It is also presented as a mechanical connection. However, its distinguishing feature is the form. The belt desmosome looks like a tape. Like a rim, the adhesion band covers the cytolemma and neighboring cell membranes.
This contact is characterized by a high electron density both in the membrane region and in the location of the intercellular substance.
Vinculin is present in the adhesion belt, a supporting protein that acts as a site for microfilament attachment to the inside of the cytomembrane.
Adhesive tape can be found in the apical region of a single-layer epithelium. She often adjoins close contact. A distinctive feature of this compound is that its structure includes actin microfilaments. They are parallel to the surface of the membrane. Due to their ability to contract in the presence of minimiosins and instability, the whole layer of epithelial cells, as well as the microrelief of the surface of the organ that they line, can change their shape.
Slit contact
It is also called a nexus. As a rule, endotheliocytes are so connected. Slit-type intercellular contacts are disk-shaped. Its length is 0.5-3 microns.
At the junction, adjacent membranes are located at a distance of 2-4 nm from each other. On the surface of both contacting elements there are integral proteins - connectins. They, in turn, integrate into connexons - protein complexes consisting of 6 molecules.
Connexon complexes are adjacent to each other. In the central part of each is a time. Elements with a molecular mass of no more than 2 thousand can freely pass through it. The pores in adjacent cells are tightly attached to each other. Due to this, the molecules of inorganic ions, water, monomers, low molecular weight biologically active substances move only into the neighboring cell, and they do not penetrate into the intercellular substance.
Nexus Functions
Due to the slit-like contacts, excitation is transmitted to neighboring elements. For example, this is how impulses pass between neurons, smooth myocytes, cardiomyocytes, etc. Due to nexuses, the unity of cell bioreactions in tissues is ensured. In the nerve tissue structures, the slit-like contacts are called electrical synapses.
The tasks of the nexus are the formation of intercellular interstitial control over the bioactivity of cells. In addition, such contacts perform several specific functions. For example, without them there would be no unity in the reduction of cardiac cardiomyocytes, synchronous reactions of smooth muscle cells, etc.
Tight contact
It is also called the locking zone. It is presented as a site of fusion of the surface membrane layers of neighboring cells. These zones form a continuous network that is “crosslinked” by the integral protein molecules of the membranes of neighboring cellular elements. These proteins form a structure similar to a mesh network. She surrounded the perimeter of the cell in the form of a belt. In this case, the structure connects adjacent surfaces.
Often tape desmosomes adhere to close contact. This area is impervious to ions and molecules. Therefore, it closes the intercellular clefts and, in fact, the internal environment of the whole organism from external factors.
The value of the locking zones
Close contact prevents diffusion of the compounds. For example, the contents of the gastric cavity are protected from the internal environment of its walls, protein complexes cannot move from the free epithelial surface to the intercellular space, etc. The locking zone also contributes to cell polarization.
Close contacts are the basis of various barriers present in the body. In the presence of locking zones, the transfer of substances into neighboring media is carried out exclusively through the cell.
Synapses
They are specialized compounds located in neurons (nerve structures). Due to them, information is transferred from one cell to another.
The synaptic connection is found in specialized areas and between two nerve cells, and between a neuron and another element included in the effector or receptor. For example, secrete neuroepithelial, neuromuscular synapses.
These contacts are divided into electrical and chemical. The former are similar to slit-like bonds.
Intercellular adhesion
Cells attach at the expense of cytolemma receptors to adhesive proteins. For example, receptors for fibronectin and laminin in epithelial cells provide adhesion to these glycoproteins. Laminin and fibronectin are adhesive substrates with a fibrillar element of the basement membranes (type IV collagen fibers).
Half desmosome
From the side of the cell, its biochemical composition and structure is similar to a dysmosome. Special anchor filaments depart from the cell into the intercellular substance . Due to them, the membrane with the fibrillar framework and anchoring fibrils of type VII collagen fibers are combined.
Point contact
It is also called focal. Point contact is part of the linking group. It is considered the most characteristic for fibroblasts. The cell in this case does not adhere to adjacent cellular elements, but to intercellular structures. Receptor proteins interact with adhesive molecules. These include chondronectin, fibronectin, etc. They bind cell membranes with extracellular fibers.
The formation of point contact is due to actin microfilaments. They are fixed on the inside of the cytolemma using integral proteins.