Connections of cells present in 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 a layer of elements interconnected 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, have not been sufficiently studied today.
Next, 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 cellular formations are formed due to the interaction of elements. Their preservationcan be provided in a variety of ways.
In embryonic, germinal tissues, especially at the initial stages of development, cells maintain connections with each other due to the fact that their surfaces have the ability to stick together. Such adhesion (connection) may be related to the surface properties of the elements.
Specific appearance
Researchers believe that the formation of intercellular contacts is provided by the interaction of glycocalyx with lipoproteins. When connecting, a small gap always remains (its width is about 20 nm). It contains the glycocalyx. When a tissue is treated with an enzyme that can disrupt its integrity or damage the membrane, the cells begin to separate from each other and dissociate.
If the dissociating factor is removed, the cells can come together again. This phenomenon is called reaggregation. So you can separate the cells of sponges of different colors: yellow and orange. During the experiments, it was found that only 2 types of aggregates appear in the cell junction. Some are exclusively orange, while others are only yellow cells. Mixed suspensions, in turn, self-organize and restore the primary multicellular structure.
Similar results were obtained by researchers in experiments with suspensions of separated amphibian embryonic cells. In this case, the cells of the ectoderm separate in space selectively from the mesenchyme and endoderm. If we use fabrics of laterstages of development of the embryos, different cell groups differing in organ and tissue specificity will independently assemble in the test tube, epithelial aggregates will form, resembling the renal tubules.
Physiology: types of intercellular contacts
Scientists distinguish 2 main groups of connections:
- Simple. They can form compounds that differ in shape.
- Complicated. These include slit-like, desmosomal, tight intercellular junctions, as well as adhesive bands and synapses.
Let's look at their brief characteristics.
Simple ties
Simple intercellular junctions are sites of interaction between supramembrane cellular complexes of the plasmolemma. The distance between them is no more than 15 nm. Intercellular contacts provide adhesion of elements due to mutual "recognition". The 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 neighboring cells. When interacting with related molecules located on adjacent cytomembranes, they stick together - adhesion.
Intercellular contacts in histology
Among the adhesive proteins are:
- Integrins.
- Immunoglobulins.
- Selectins.
- Cadherins.
Some adhesive proteins do not belong to any of these families.
Characteristics of families
Some glycoproteins of the cell surface apparatus belong to the main histocompatibility complex of the 1st class. 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 to 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 regions bind to thin microfilaments using special protein molecules (tannin or vinculin) or directly to actin.
Selectins are monomeric proteins. They recognize certain carbohydrate complexes and attach to them on the cell surface. Currently, the most studied are L, P and E-selectins.
Immunoglobulin-like adhesive proteins are structurally similar to classical antibodies. Some of them are receptors for immunological reactions, others are intended only for implementation of adhesive functions.
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 epithelial tissues, and N-cadherins– in muscular and nervous.
Destination
It should be said that intercellular contacts are intended 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 they are involved. Simple contacts control the maturation and movement of cells, prevent hyperplasia (excessive increase in the number of structural elements).
Variety of compounds
In the course of research, different types of intercellular contacts in shape have been established. They can be, for example, in the form of "tiles". Such connections are formed in the stratum corneum of the stratified keratinized epithelium, in the arterial endothelium. There are also serrated and finger-shaped types. In the first, the protrusion of one element sinks into the concave part of the other. This significantly increases the mechanical strength of the joint.
Complex connections
These types of intercellular contacts are specialized for the implementation of a particular function. Such compounds are represented by small paired specialized sections of the plasma membranes of 2 neighboring cells.
There are the following types of intercellular contacts:
- Locking.
- Hooks.
- 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 well seen, since it is distinguished by a high electron density. The local area looks like a disk. Its diameter is about 0.5 µm. The membranes of neighboring elements in it are located at a distance of 30 to 40 nm.
You can also consider areas of high electron density 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. The tonofilaments contain cytokeratins. 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 be detected in other structures as well. In this case, the intermediate filaments contain substances characteristic of this tissue. For example, there are vimentins in connective structures, desmins in muscles, etc.
The inner part of the desmosome at the macromolecular level is represented by desmoplakins - supporting proteins. Intermediate filaments are connected to them. Desmoplakins, in turn, are linked to desmogleins by placoglobins. This triple compound passes through the lipid layer. Desmogleins bind to proteins in the neighboring cell.
However, another option is also possible. Attachment of desmoplakins is carried out to integral proteins located in the membrane - desmocolins. These, in turn, bind to similar proteins in the adjacent cytomembrane.
Girdle desmosome
It is also presented as a mechanical connection. However, its distinctive feature is the form. The belt desmosome looks like a ribbon. Like a rim, the grip band wraps around the cytolemma and adjacent cell membranes.
This contact is characterized by high electron density both in the region of membranes and in the area where the intercellular substance is located.
Vinculin is present in the clutch belt, a support protein that acts as an attachment site for microfilaments to the inside of the cytomembrane.
Adhesive tape can be found in the apical section of the single layer epithelium. It is often adjacent to tight contact. A distinctive feature of this compound is that its structure includes actin microfilaments. They are parallel to the membrane surface. Due to their ability to contract in the presence of minimyosins and instability, a whole layer of epithelial cells, as well as the microrelief of the surface of the organ that they line, can change their shape.
Gap contact
It is also called the nexus. As a rule, endotheliocytes are connected in this way. Intercellular junctions of the slot-like type are disk-shaped. Its length is 0.5-3 microns.
At the connection site, adjacent membranes are at a distance of 2-4 nm from each other. Integral proteins, connectins, are present on the surface of both contacting elements. They, in turn, are integrated into connexons - protein complexes consisting of 6 molecules.
Connexon complexes are adjacent to each other. In the central part of each there is a pore. Elements whose molecular weight does not exceed 2 thousand can freely pass through it. The pores in neighboring cells are tightly attached to each other. Due to this, the molecules of inorganic ions, water, monomers, low-molecular biologically active substances move only to the neighboring cell, and they do not penetrate into the intercellular substance.
Nexus features
Due to slot-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 neural tissue structures, gap junctions are called electrical synapses.
The tasks of nexuses are to form intercellular interstitial control over cell bioactivity. In addition, such contacts perform several specific functions. For example, without them there would be no unity of contraction 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, which is "crosslinked" by the integral protein molecules of the membranes of neighboring cellular elements. These proteins form a mesh-like structure. It surrounds the perimeter of the cell in the form of a belt. In this case, the structure connects adjacent surfaces.
Often to tight contactadjoining banded desmosomes. This area is impermeable to ions and molecules. Consequently, it locks the intercellular gaps and, in fact, the internal environment of the whole organism from external factors.
Meaning of blocking zones
Tight contact prevents diffusion of 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 blocking zone also contributes to cell polarization.
Tight junctions are the basis of the various barriers present in the body. In the presence of blocking zones, the transfer of substances to neighboring environments is carried out exclusively through the cell.
Synapses
They are specialized compounds located in neurons (nerve structures). Due to them, information is transmitted from one cell to another.
A 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, neuro-epithelial, neuromuscular synapses are isolated.
These contacts are divided into electrical and chemical. The former are similar to gap bonds.
Intercellular substance adhesion
Cells are attached by cytolemmal receptors to adhesive proteins. For example, receptors for fibronectin and laminin in epithelial cells provide adhesion to theseglycoproteins. Laminin and fibronectin are adhesive substrates with the fibrillar element of basement membranes (type IV collagen fibers).
Hemidesmosome
From the side of the cell, its biochemical composition and structure is similar to a dismosome. Special anchor filaments extend from the cell into the intercellular substance. Due to them, the membrane is combined with a fibrillar framework and anchoring fibrils of type VII collagen fibers.
Point contact
It is also called focal. Point contact is included in the group of coupling connections. It is considered most characteristic of fibroblasts. In this case, the cell does not adhere to neighboring cellular elements, but to intercellular structures. Receptor proteins interact with adhesive molecules. These include chondronectin, fibronectin, etc. They bind cell membranes to extracellular fibers.
Formation of a point contact is carried out by actin microfilaments. They are fixed on the inside of the cytolemma with the help of integral proteins.
