Cell adhesion. Cell adhesion Intercellular contacts Plan I Definition
The most important receptors on the surface of animal cells, responsible for cell recognition of each other and their binding, are adhesion receptors. They are necessary for the regulation of morphogenetic processes during embryonic development and maintaining tissue stability in an adult organism.
The ability for specific mutual recognition allows cells of different types to associate into certain spatial structures characteristic of different stages of animal ontogenesis. In this case, cells of the embryo of one type interact with each other and are separated from other cells that differ from them. As the embryo develops, the nature of the adhesive properties of cells changes, which underlies processes such as gastrulation, neurulation and somite formation. In early embryos of animals, for example, amphibians, the adhesive properties of the cell surface are so pronounced that they are able to restore the original spatial arrangement of cells of different types (epidermis, neural plate and mesodere) even after their disaggregation and mixing (Fig. 12).
Fig. 12. Restoration of embryonic structures after disaggregation
Currently, several families of receptors involved in cell adhesion have been identified. Many of them belong to the family of immunoglobulins, which provide Ca ++ -independent intercellular interaction. The receptors included in this family are characterized by the presence of a common structural basis - one or several domains of amino acid residues homologous to immunoglobulins. The peptide chain of each of these domains contains about 100 amino acids and is folded into a structure of two antiparallel β-sheets stabilized by a disulfide bond. Figure 13 shows the structure of some receptors of the immunoglobulin family.
Glycoprotein Glycoprotein T-cell Immunoglobulin
MHC class I MHC class II receptor
Fig. 13. Schematic representation of the structure of some immunoglobulin family receptors
Receptors of this family include, first of all, receptors that mediate the immune response. Thus, the interaction of three types of cells that occurs during the immune reaction - B lymphocytes, T helper cells and macrophages is due to the binding of cell surface receptors of these cells: the T cell receptor and MHC class II glycoproteins (major histocompatibility complex).
Structurally similar and phylogenetically related to immunoglobulins are the receptors involved in the recognition and binding of neurons, the so-called cell adhesion molecules (N-CAM). They are integral monotopic glycoproteins, some of which are responsible for the binding of nerve cells, others for the interaction of nerve cells and glial cells. For most N-CAM molecules, the extracellular part of the polypeptide chain is the same and is organized in the form of five domains, homologous to the domains of immunoglobulins. Differences between nerve cell adhesion molecules concern mainly the structure of transmembrane regions and cytoplasmic domains. There are at least three forms of N-CAM, each encoded by a separate mRNA. One of these forms does not penetrate the lipid bilayer because it does not contain a hydrophobic domain, but connects to the plasma membrane only through a covalent bond with phosphatidylinositol; another form of N-CAM is secreted by cells and incorporated into the extracellular matrix (Fig. 14).
Phosphatidylinositol
Fig. 14. Schematic representation of the three forms of N-CAM
The process of interaction between neurons consists of the binding of receptor molecules of one cell to identical molecules of another neuron (homophilic interaction), and antibodies to the proteins of these receptors suppress the normal selective adhesion of cells of the same type. Protein-protein interactions play the main role in the functioning of receptors, while carbohydrates have a regulatory function. Some forms of CAM perform heterophilic binding, in which adhesion of neighboring cells is ensured by various surface proteins.
It is assumed that the complex pattern of neuronal interaction during brain development is not due to the participation of a large number of highly specific N-CAM molecules, but to the differential expression and post-translational modifications of the structure of a small number of adhesion molecules. In particular, it is known that during the development of an individual organism, different forms of nerve cell adhesion molecules are expressed at different times and in different places. In addition, the regulation of the biological functions of N-CAM can be carried out by phosphorylation of serine and threonine residues in the cytoplasmic domain of proteins, modifications of fatty acids in the lipid bilayer or oligosaccharides on the cell surface. It has been shown, for example, that during the transition from the embryonic brain to the brain of an adult organism, the number of sialic acid residues in N-CAM glycoproteins significantly decreases, causing an increase in cell adhesiveness.
Thus, unique cellular systems are formed through the receptor-mediated recognition abilities of immune and nerve cells. Moreover, if the network of neurons is relatively rigidly fixed in space, then the continuously moving cells of the immune system only temporarily interact with each other. However, N-CAM not only “glues” cells together and regulates intercellular adhesion during development, but also stimulates the growth of neural processes (for example, the growth of retinal axons). Moreover, N-CAM is transiently expressed during critical stages in the development of many non-neural tissues, where these molecules help hold specific cells together.
Cell surface glycoproteins that do not belong to the immunoglobulin family, but have some structural similarity to them, form a family of intercellular adhesion receptors called cadherins. Unlike N-CAM and other immunoglobulin receptors, they ensure interaction between the contacting plasma membranes of neighboring cells only in the presence of extracellular Ca ++ ions. In vertebrate cells, more than ten proteins belonging to the cadherin family are expressed, all of them are transmembrane proteins that pass through the membrane once (Table 8). The amino acid sequences of different cadherins are homologous, and each of the polypeptide chains contains five domains. A similar structure is also found in the transmembrane proteins of desmosomes—desmogleins and desmocollins.
Cell adhesion mediated by cadherins exhibits a homophilic interaction pattern in which dimers protruding from the cell surface are tightly coupled in an antiparallel orientation. As a result of this “adhesion”, a continuous cadherin zipper is formed in the contact zone. To bind cadherins of neighboring cells, extracellular Ca ++ ions are required; when they are removed, tissues are divided into individual cells; in its presence, dissociated cells reaggregate.
Table 8
Types of cadherins and their localization
To date, E-cadherin, which plays an important role in holding together cells of various epithelia, is the best characterized. In mature epithelial tissues, with its participation, actin filaments of the cytoskeleton are bound and held together, and in the early periods of embryogenesis, it ensures compaction of blastomeres.
Cells in tissues, as a rule, come into contact not only with other cells, but also with insoluble extracellular components of the matrix. The most extensive extracellular matrix, where cells are located quite freely, is found in connective tissues. Unlike epithelia, here the cells are attached to the matrix components, while the connections between individual cells are not so significant. In these tissues, the extracellular matrix, surrounding the cells on all sides, forms their frame, helps maintain multicellular structures and determines the mechanical properties of the tissues. In addition to performing these functions, it is involved in processes such as signal transmission, migration and cell growth.
The extracellular matrix is a complex complex of various macromolecules that are locally secreted by cells in contact with the matrix, mainly fibroblasts. They are represented by glycosaminoglycan polysaccharides, usually covalently linked to proteins in the form of proteoglycans and fibrillar proteins of two functional types: structural (for example, collagen) and adhesive. Glycosaminoglycans and proteoglycans form extracellular gels in an aqueous environment, into which collagen fibers are immersed, strengthening and organizing the matrix. Adhesive proteins are large glycoproteins that ensure the attachment of cells to the extracellular matrix.
A special specialized form of extracellular matrix is the basement membrane - a strong, thin structure built from type IV collagen, proteoglycans and glycoproteins. It is located at the border between the epithelium and connective tissue, where it serves for cell attachment; separates individual muscle fibers, fat and Schwann cells, etc. from the surrounding tissue. At the same time, the role of the basement membrane is not limited to just a supporting function; it serves as a selective barrier for cells, affects cellular metabolism, and causes cell differentiation. Its participation in the processes of tissue regeneration after damage is extremely important. When the integrity of muscle, nervous or epithelial tissue is damaged, the preserved basement membrane acts as a substrate for the migration of regenerating cells.
Special receptors belonging to the family of so-called integrins (integrate and transfer signals from the extracellular matrix to the cytoskeleton) participate in the attachment of cells to the matrix. By binding to proteins of the extracellular matrix, integrins determine the shape of the cell and its movement, which is crucial for the processes of morphogenesis and differentiation. Integrin receptors are found in all vertebrate cells, some of them are present in many cells, others have a fairly high specificity.
Integrins are protein complexes containing two types of nonhomologous subunits (α and β), and many integrins are characterized by similarities in the structure of the β subunits. Currently, 16 varieties of α- and 8 varieties of β-subunits have been identified, the combinations of which form 20 types of receptors. All types of integrin receptors are constructed in fundamentally the same way. These are transmembrane proteins that simultaneously interact with extracellular matrix proteins and cytoskeletal proteins. The outer domain, in which both polypeptide chains participate, binds to the adhesive protein molecule. Some integrins are able to bind simultaneously not to one, but to several components of the extracellular matrix. The hydrophobic domain crosses the plasma membrane, and the cytoplasmic C-terminal region is in direct contact with submembrane components (Fig. 15). In addition to receptors that ensure the binding of cells to the extracellular matrix, there are integrins involved in the formation of intercellular contacts - intracellular adhesion molecules.
Fig. 15. Structure of the integrin receptor
When ligands bind, integrin receptors are activated and accumulate in separate specialized areas of the plasma membrane with the formation of a densely packed protein complex called the focal contact (adhesion plate). In it, integrins, using their cytoplasmic domains, are connected to cytoskeletal proteins: vinculin, talin, etc., which, in turn, are associated with bundles of actin filaments (Fig. 16). This adhesion of structural proteins stabilizes cell contacts with the extracellular matrix, ensures cell motility, and also regulates the shape and changes in cell properties.
In vertebrates, one of the most important adhesion proteins to which integrin receptors bind is fibronectin. It is found on the surface of cells, such as fibroblasts, or circulates freely in the blood plasma. Depending on the properties and localization of fibronectin, three forms are distinguished. The first, a soluble dimeric form called plasma fibronectin, circulates in the blood and tissue fluids, promoting blood clotting, wound healing and phagocytosis; the second forms oligomers that temporarily attach to the cell surface (surface fibronectin); the third is a sparingly soluble fibrillar form located in the extracellular matrix (matrix fibronectin).
Extracellular matrix
Fig. 16. Model of interaction of the extracellular matrix with cytoskeletal proteins with the participation of integrin receptors
The function of fibronectin is to promote adhesion between cells and the extracellular matrix. In this way, with the participation of integrin receptors, contact is achieved between the intracellular and surrounding environment. In addition, cell migration occurs through the deposition of fibronectin in the extracellular matrix: the attachment of cells to the matrix acts as a mechanism to guide cells to their destination.
Fibronectin is a dimer consisting of two structurally similar, but not identical, polypeptide chains connected near the carboxyl end by disulfide bonds. Each monomer has binding sites for the cell surface, heparin, fibrin and collagen (Fig. 17). The binding of the outer domain of the integrin receptor to the corresponding region of fibronectin requires the presence of Ca 2+ ions. The interaction of the cytoplasmic domain with the fibrillar cytoskeletal protein actin is carried out using the proteins talin, tansin and vinculin.
Fig. 17. Schematic structure of the fibronectin molecule
Interaction through integrin receptors of the extracellular matrix and cytoskeletal elements ensures two-way signal transmission. As shown above, the extracellular matrix influences the organization of the cytoskeleton in target cells. In turn, actin filaments can change the orientation of secreted fibronectin molecules, and their destruction under the influence of cytochalasin leads to disorganization of fibronectin molecules and their separation from the cell surface.
Reception with the participation of integrin receptors is analyzed in detail using the example of fibroblast culture. It turned out that during the process of attachment of fibroblasts to the substrate, which occurs in the presence of fibronectin in the medium or on its surface, the receptors move, forming clusters (focal contacts). The interaction of integrin receptors with fibronectin in the area of focal contact induces, in turn, the formation of a structured cytoskeleton in the cell cytoplasm. Moreover, microfilaments play a decisive role in its formation, but other components of the musculoskeletal system of the cell also participate - microtubules and intermediate filaments.
Receptors for fibronectin, contained in large quantities in embryonic tissues, are of great importance in the processes of cell differentiation. It is believed that it is fibronectin that during embryonic development directs migration in the embryos of both vertebrates and invertebrates. In the absence of fibronectin, many cells lose the ability to synthesize specific proteins, and neurons lose the ability to grow in a directed manner. It is known that in transformed cells the level of fibronectin decreases, which is accompanied by a decrease in the degree of their binding to the extracellular environment. As a result, cells become more mobile, increasing the likelihood of metastasis.
Another glycoprotein that ensures the adhesion of cells to the extracellular matrix with the participation of integrin receptors is called laminin. Laminin, secreted mainly by epithelial cells, consists of three very long polypeptide chains arranged in a cross shape and connected by disulfide bridges. It contains several functional domains that bind cell surface integrins, type IV collagen, and other components of the extracellular matrix. The interaction of laminin and type IV collagen, found in large quantities in the basement membrane, serves to attach cells to it. Therefore, laminin is present primarily on the side of the basement membrane that faces the plasma membrane of epithelial cells, while fibronectin ensures the binding of matrix macromolecules and connective tissue cells on the opposite side of the basement membrane.
Receptors of two special families of integrins are involved in platelet aggregation during blood coagulation and in the interaction of leukocytes with vascular endothelial cells. Platelets express integrins that bind fibrinogen, von Willebrand factor, and fibronectin during blood clotting. This interaction promotes platelet adhesion and clot formation. A variety of integrins, found exclusively in white blood cells, allow the cells to attach at the site of infection to the endothelium lining blood vessels and pass through this barrier.
The participation of integrin receptors in regeneration processes has been shown. Thus, after transection of a peripheral nerve, axons can regenerate with the help of growth cone membrane receptors formed at the cut ends. A key role in this is played by the binding of integrin receptors to laminin or the laminin-proteoglycan complex.
It should be noted that often the division of macromolecules into components of the extracellular matrix and plasma membrane of cells is quite arbitrary. Thus, some proteoglycans are integral proteins of the plasma membrane: their core protein can penetrate the bilayer or be covalently associated with it. By interacting with most components of the extracellular matrix, proteoglycans contribute to the attachment of cells to the matrix. On the other hand, matrix components are also attached to the cell surface using specific receptor proteoglycans.
Thus, the cells of a multicellular organism contain a certain set of surface receptors that allow them to specifically bind to other cells or to the extracellular matrix. For such interactions, each individual cell uses many different adhesive systems, characterized by great similarity of molecular mechanisms and high homology of the proteins involved. Due to this, cells of any type, to varying degrees, have an affinity for each other, which, in turn, makes it possible to simultaneously connect many receptors with many ligands of a neighboring cell or extracellular matrix. At the same time, animal cells are able to recognize relatively small differences in the surface properties of plasma membranes and establish only the most adhesive of many possible contacts with other cells and the matrix. At different stages of animal development and in different tissues, various adhesion receptor proteins are differentially expressed, determining the behavior of cells in embryogenesis. These same molecules appear on cells that are involved in tissue repair after damage.
The activity of cell surface receptors is associated with the phenomenon of cell adhesion.
Adhesion- the process of interaction between specific glycoproteins of contacting plasma membranes of cells recognizing each other or cells and the extracellular matrix. If glycoiroteins form bonds, adhesion occurs, and then the formation of strong intercellular contacts or contacts between the cell and the intercellular matrix.
All cell adhesion molecules are divided into 5 classes.
1. Cadherins. These are transmembrane glycoproteins that use calcium ions for adhesion. They are responsible for the organization of the cytoskeleton and the interaction of cells with other cells.
2. Integrins. As already noted, integrins are membrane receptors for protein molecules of the extracellular matrix - fibronectin, laminin, etc. They connect the extracellular matrix with the cytoskeleton using intracellular proteins talin, vinculin, a-actinin. Both cell-cell and intercellular adhesion molecules function.
3. Selectins. Provide adhesion of leukocytes to the endothelium vessels and thereby - leukocyte-endothelial interactions, migration of leukocytes through the walls of blood vessels into the tissue.
4. Immunoglobulin family. These molecules play an important role in the immune response, as well as embryogenesis, wound healing, etc.
5. Homing molecules. They ensure the interaction of lymphocytes with the endothelium, their migration and colonization of specific zones of immunocompetent organs.
Thus, adhesion is an important link in cell reception and plays an important role in intercellular interactions and interactions of cells with the extracellular matrix. Adhesion processes are absolutely necessary in such general biological processes as embryogenesis, immune response, growth, regeneration, etc. They are also involved in the regulation of intracellular and tissue homeostasis.
CYTOPLASM
HYALOPLASMA. Hyaloplasm is also called cell sap, cytosol, or cell matrix. This is the main part of the cytoplasm, making up about 55% of the cell volume. It carries out the main cellular metabolic processes. Hyalonlasma is a complex colloidal system and consists of a homogeneous fine-grained substance with low electron density. It consists of water, proteins, nucleic acids, polysaccharides, lipids, and inorganic substances. Hyaloplasm can change its state of aggregation: transition from a liquid state (sol) into a denser one - gel. At the same time, the shape of the cell, its mobility and metabolism may change. Functions of hyalonlasma:
1. Metabolic - metabolism of fats, proteins, carbohydrates.
2. Formation of a liquid microenvironment (cell matrix).
3. Participation in cell movement, metabolism and energy. ORGANELLES. Organelles are the second most important essential
cell component. An important feature of organelles is that they have a constant, strictly defined structure and function. By functional sign all organelles are divided into 2 groups:
1. Organelles of general importance. Contained in all cells, as they are necessary for their life. Such organelles are: mitochondria, endoplasmic reticulum (ER) of two types, Golgi complex (CG), centrioles, ribosomes, lysosomes, peroxisomes, microtubules And microfilaments.
2. Organelles of special significance. Found only in those cells that perform special functions. Such organelles are myofibrils in muscle fibers and cells, neurofibrils in neurons, flagella and cilia.
By structural feature all organelles are divided into: 1) membrane-type organelles And 2) non-membrane type organelles. In addition, non-membrane organelles can be built according to fibrillar And granular principle.
In membrane-type organelles, the main component is intracellular membranes. Such organelles include mitochondria, EPS, CG, lysosomes, and peroxisomes. Non-membrane organelles of the fibrillar type include microtubules, microfilaments, cilia, flagella, and centrioles. Non-membrane granular organelles include ribosomes and polysomes.
MEMBRANE ORGANELLES
ENDOPLASMIC RETICULUM (ER) is a membrane organelle described in 1945 by K. Porter. Its description was made possible thanks to an electron microscope. The ER is a system of small channels, vacuoles, and sacs that form a continuous complex network in the cell, the elements of which can often form isolated vacuoles that appear in ultrathin sections. The ER is built from membranes that are thinner than the cytolemma and contain more protein due to the numerous enzyme systems located in it. There are 2 types of EPS: granular(rough) and agranular, or smooth. Both types of EPS can mutually transform into each other and are functionally interconnected by the so-called transitional, or transient, zone.
Granular EPS (Fig. 3.3) contains ribosomes on its surface (polysomes) and is an organelle for protein biosynthesis. Polysomes or ribosomes bind to the EPS using the so-called docking protein. At the same time, the ER membrane contains special integral proteins ribophorins, also binding ribosomes and forming hydrophobic trapembrane channels for the transport of synthesized polypentide value into the lumen of the granular ER.
Granular EPS is visible only in an electron microscope. In a light microscope, a sign of developed granular EPS is basophilia of the cytoplasm. Granular ER is present in every cell, but the degree of its development varies. It is most developed in cells that synthesize protein for export, i.e. in secretory cells. The granular EPS reaches its maximum development in neurocytes, in which its cisterns acquire an ordered arrangement. In this case, at the light microscopic level, it is revealed in the form of regularly located areas of cytoplasmic basophilia, called basophilic substance of Nissl.
Function granular EPS - protein synthesis for export. In addition, initial post-translational changes in the polypeptide chain occur in it: hydroxylation, sulfation and phosphorylation, glycosylation. The last reaction is especially important because leads to the formation glycoproteins- the most common product of cellular secretion.
The agranular (smooth) ER is a three-dimensional network of tubules that do not contain ribosomes. Granular ER can continuously transform into smooth ER, but can exist as an independent organelle. The place where the granular EPS transitions into the agranular one is called transitional (intermediate, transient) part. From it, vesicles with synthesized protein are separated And transport them to the Golgi complex.
Functions smooth EPS:
1. Division of the cell cytoplasm into sections - compartments, each of which has its own group of biochemical reactions.
2. Biosynthesis of fats and carbohydrates.
3. Formation of peroxisomes;
4. Biosynthesis of steroid hormones;
5. Detoxification of exo- and endogenous poisons, hormones, biogenic amines, drugs due to the activity of special enzymes.
6. Deposition of calcium ions (in muscle fibers and myocytes);
7. Source of membranes for restoration of the karyolemma in the telophase of mitosis.
PLATE GOLGI COMPLEX. This is a membrane organelle described in 1898 by the Italian neurohistologist C. Golgi. He named this organelle intracellular mesh apparatus due to the fact that in a light microscope it has a mesh appearance (Fig. 3.4, A). Light microscopy does not provide a complete picture of the structure of this organelle. In a light microscope, the Golgi complex looks like a complex network in which cells can be connected to each other or lie independently of each other (dictyosomes) in the form of separate dark areas, sticks, grains, concave disks. There is no fundamental difference between the reticular and diffuse forms of the Golgi complex; a change in the forms of this orgamella can be observed. Even in the era of light microscopy, it was noted that the morphology of the Golgi complex depends on the stage of the secretory cycle. This allowed D.N. Nasonov to suggest that the Golgi complex ensures the accumulation of synthesized substances in the cell. According to electron microscopy, the Golgi complex consists of membrane structures: flat membrane sacs with ampullary extensions at the ends, as well as large and small vacuoles (Fig. 3.4, b, c). The collection of these formations is called a dictyosome. The dictyosome contains 5-10 sac-like cisternae. The number of dictyosomes in a cell can reach several dozen. In this case, each dictyosome is connected to the neighboring one using vacuoles. Each dictyosome contains proximal, immature, emerging, or CIS-zone, facing the nucleus, and distal, TRANS zone. The latter, in contrast to the convex cis-surface, is concave, mature, and faces the cytolemma of the cell. On the cis side, vesicles are attached, separated from the transition zone of the EPS and containing newly synthesized and partially processed protein. In this case, the membranes of the vesicles are embedded in the membrane of the cis-surface. The trans sides are separated secretory vesicles And lysosomes. Thus, in the Golgi complex there is a constant flow of cell membranes and their maturation. Functions Golgi complex:
1. Accumulation, maturation and condensation of protein biosynthesis products (occurring in granular EPS).
2. Synthesis of polysaccharides and conversion of simple proteins into glycoproteins.
3. Formation of liponrotheids.
4. Formation of secretory inclusions and their release from the cell (packaging and secretion).
5. Formation of primary lysosomes.
6. Formation of cell membranes.
7. Education acrosomes- a structure containing enzymes located at the front end of the sperm and necessary for fertilization of the egg and destruction of its membranes.
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The sizes of mitochondria range from 0.5 to 7 microns, and their total number in a cell is from 50 to 5000. These organelles are clearly visible in a light microscope, but the information obtained about their structure is scarce (Fig. 3.5, A). An electron microscope showed that mitochondria consist of two membranes - outer and inner, each of which has a thickness of 7 nm (Fig. 3.5, b, c, 3.6, A). Between the outer and inner membranes there is a gap up to 20 nm in size.
The inner membrane is uneven and forms many folds, or cristae. These cristae run perpendicular to the surface of the mitochondria. There are mushroom-shaped formations on the surface of the cristae (oxisomes, ATPsomes or F particles), representing an ATP synthetase complex (Fig. 3.6) The inner membrane delimits the mitochondrial matrix. It contains numerous enzymes for the oxidation of pyruvate and fatty acids, as well as Krebs cycle enzymes. In addition, the matrix contains mitochondrial DNA, mitochondrial ribosomes, t-RNA and mitochondrial genome activation enzymes. The inner membrane contains three types of proteins: enzymes that catalyze oxidative reactions; ATP synthesate complex, which synthesizes ATP in the matrix; transport proteins. The outer membrane contains enzymes that convert lipids into reaction compounds, which then participate in the metabolic processes of the matrix. The intermembrane space contains enzymes necessary for oxidative phosphorylation. Because Since mitochondria have their own genome, they have an autonomous protein synthesis system and can partially build their own membrane proteins.
Functions.
1. Providing energy to the cell in the form of ATP.
2. Participation in the biosynthesis of steroid hormones (some parts of the biosynthesis of these hormones occur in mitochondria). Ste producing cells
roid hormones have large mitochondria with complex large tubular cristae.
3. Calcium deposition.
4. Participation in the synthesis of nucleic acids. In some cases, as a result of mutations in mitochondrial DNA, so-called mitochondrial diseases, manifested by widespread and severe symptoms. LYSOSOMES. These are membranous organelles that are not visible under a light microscope. They were discovered in 1955 by K. de Duve using an electron microscope (Fig. 3.7). They are membrane vesicles containing hydrolytic enzymes: acid phosphatase, lipase, proteases, nucleases, etc., more than 50 enzymes in total. There are 5 types of lysosomes:
1. Primary lysosomes, just separated from the trans-surface of the Golgi complex.
2. Secondary lysosomes or phagolysosomes. These are lysosomes that have connected with phagosome- a phagocytosed particle surrounded by a membrane.
3. Residual bodies- these are layered formations that form if the process of splitting phagocytosed particles is not complete. An example of residual bodies can be lipofuscin inclusions, which appear in some cells during aging, contain endogenous pigment lipofuscin.
4. Primary lysosomes can merge with dying and old organelles, which they destroy. These lysosomes are called auto-phagosomes.
5. Multivesicular bodies. They are a large vacuole, which, in turn, contains several so-called internal vesicles. Internal vesicles apparently form by budding inward from the vacuole membrane. The internal vesicles can be gradually dissolved by enzymes contained in the matrix of the body.
Functions lysosomes: 1. Intracellular digestion. 2. Participation in phagocytosis. 3. Participation in mitosis - destruction of the nuclear membrane. 4. Participation in intracellular regeneration.5. Participation in autolysis - self-destruction of a cell after its death.
There is a large group of diseases called lysosomal diseases, or storage diseases. They are hereditary diseases, manifested by a deficiency of a certain lysosomal pigment. At the same time, undigested products accumulate in the cytoplasm of the cell
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metabolism (glycogen, glycolinides, proteins, Fig. 3.7, b,c), which leads to the gradual death of the cell. PEROXYSOMES. Peroxisomes are organielles that resemble lysosomes, but contain enzymes necessary for the synthesis and destruction of endogenous peroxides - nonoxidase, catalase and others, up to 15 in total. In an electron microscope, they appear as spherical or ellipsoidal vesicles with a moderately dense core (Fig. 3.8). Peroxisomes are formed by separating vesicles from the smooth ER. Enzymes then migrate into these vesicles and are synthesized separately in the cytosol or in the granular ER
Functions peroxisomes: 1. They are, along with mitochondria, organelles for oxygen utilization. As a result, a strong oxidizing agent H 2 0 2 is formed in them. 2. Breakdown of excess peroxides using the catalase enzyme and, thus, protecting cells from death. 3. Breakdown of toxic products of exogenous origin with the help of peroxisomes synthesized in the peroxisomes themselves (detoxification). This function is performed, for example, by peroxisomes of liver cells and kidney cells. 4. Participation in cell metabolism: peroxisomal enzymes catalyze the breakdown of fatty acids and participate in the metabolism of amino acids and other substances.
There are so-called peroxisomal diseases associated with defects in peroxisomal enzymes and characterized by severe organ damage, leading to death in childhood. NON-MEMBRANE ORGANELLS
RIBOSOMES. These are organiellae of protein biosynthesis. They consist of two ribonucleotide subunits - large and small. These subunits can join together, with a messenger RNA molecule located between them. There are free ribosomes - ribosomes not associated with the EPS. They can be single or in the form policy, when there are several ribosomes on one mRNA molecule (Fig. 3.9). The second type of ribosome is bound ribosomes attached to the ER.
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Function ribosomes Free ribosomes and polysomes carry out protein biosynthesis for the cell's own needs.
Ribosomes bound to the EPS synthesize protein for “export”, for the needs of the whole organism (for example, in secretory cells, neurons, etc.).
MICROTUBLES. Microtubules are fibrillar organelles. They have a diameter of 24 mm and a length of up to several microns. These are straight, long, hollow cylinders built from 13 peripheral filaments, or protofilaments. Each strand is formed by a globular protein tubulin, which exists in the form of two subunits - calamus (Fig. 3.10). In each thread, these subunits are located alternately. The filaments in the microtubule have a spiral course. Protein molecules associated with them move away from microtubules (microtubule associated proteins, or MAPs). These proteins stabilize microtubules and also connect them with other cytoskeletal elements and organelles. Protein is also associated with microtubules kiyezin, which is an enzyme that breaks down ATP and converts the energy of its breakdown into mechanical energy. At one end, kiesin binds to a specific organelle, and at the other, due to the energy of ATP, it slides along the microtubule, thus moving the organelles in the cytoplasm
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Microtubules are very dynamic structures. They have two ends: (-) and (+)- ends. The negative end is the site of microtubule depolymerization, while at the positive end they grow due to new tubulin molecules. In some cases (basal body) the negative end is anchored, as it were, and the decay stops here. As a result, there is an increase in the size of the eyelashes due to extensions at the (+) - end.
Functions microtubules are as follows. 1. Act as a cytoskeleton;
2. Participate in the transport of substances and organelles in the cell;
3. Participate in the formation of the spindle and ensure the divergence of chromosomes in mitosis;
4. Part of centrioles, cilia, flagella.
If cells are treated with colchicine, which destroys the microtubules of the cytoskeleton, the cells change their shape, shrink, and lose the ability to divide.
MICROFILAMENTS. This is the second component of the cytoskeleton. There are two types of microfilaments: 1) actin; 2) intermediate. In addition, the cytoskeleton includes many accessory proteins that link filaments to each other or to other cellular structures.
Actin filaments are built from the protein actin and are formed as a result of its polymerization. Actin in the cell is in two forms: 1) in dissolved form (G-actin, or globular actin); 2) in polymerized form, i.e. in the form of filaments (F-actin). In the cell there is a dynamic equilibrium between the two forms of actin. As in microtubules, actin filaments have (+) and (-) - poles, and in the cell there is a constant process of disintegration of these filaments at the negative pole and creation at the positive pole. This process is called treadmilling. It plays an important role in changing the aggregative state of the cytoplasm, ensures cell mobility, participates in the movement of its organelles, in the formation and disappearance of pseudopodia, microvilli, endocytosis and exocytosis. Microtubules create the framework of microvilli and also participate in the organization of intercellular inclusions.
Intermediate filaments- filaments having a thickness greater than that of actin filaments, but less than that of microtubules. These are the most stable cell filaments. Perform a supporting function. For example, these structures lie along the entire length of the processes of nerve cells, in the region of desmosomes, and in the cytoplasm of smooth myocytes. In cells of different types, intermediate filaments differ in composition. Neurofilaments are formed in neurons, consisting of three different polypentides. In neuroglial cells, intermediate filaments contain acidic glial protein. Epithelial cells contain keratin filaments (tonophila-mentes)(Fig. 3.11).
CELL CENTER (Fig. 3.12). This is an organelle visible and visible under a light microscope, but its fine structure could only be studied with an electron microscope. In an interphase cell, the cell center consists of two cylindrical cavity structures up to 0.5 µm long and up to 0.2 µm in diameter. These structures are called centrioles. They form a diplosome. In a diplosome, the daughter centrioles lie at right angles to each other. Each centriole consists of 9 triplets of microtubules arranged in a circle, which are partially fused along their length. In addition to microtubules, ceptrioles include “handles” made of the protein dynein, which connect neighboring triplets in the form of bridges. There are no central microtubules, and centriole formula - (9x3)+0. Each triplet of microtubules is also associated with spherical structures - satellites. Microtubules diverge from the satellites to the sides, forming centrosphere.
Centrioles are dynamic structures and undergo changes during the mitotic cycle. In a nondividing cell, paired centrioles (centrosomes) lie in the perinuclear zone of the cell. In the S-period of the mitotic cycle, they are duplicated, and a daughter centriole is formed at a right angle to each mature centriole. The daughter centrioles initially have only 9 single microtubules, but as the centrioles mature, they turn into triplets. Next, pairs of centrioles diverge to the cell poles, becoming centers for organizing spindle microtubules.
The meaning of centrioles.
1. They are the center of organization of spindle microtubules.
2. Formation of cilia and flagella.
3. Ensuring intracellular movement of organelles. Some authors believe that the defining functions of cellular
the center has the second and third functions, since in plant cells there are no centrioles, however, a division spindle is formed in them.
CILIA AND FLANGELLA (Fig. 3.13). These are special movement organelles. They are present in some cells - sperm, epithelial cells of the trachea and bronchi, sperm ducts of a man, etc. In a light microscope, cilia and flagella look like thin outgrowths. An electron microscope revealed that at the base of cilia and flagella there are small granules - basal bodies, identical in structure to centrioles. From the basal body, which is the matrix for the growth of cilia and flagella, a thin cylinder of microtubules extends - axial thread, or axoneme. It consists of 9 doublets of microtubules, on which there are protein “handles” dynein. The axoneme is covered by the cytolemma. In the center there is a pair of microtubules surrounded by a special shell - coupling, or internal capsule. Radial spokes go from the doublets to the central coupling. Hence, the formula of cilia and flagella is (9x2)+2.
The basis of the microtubules of flagella and cilia is an irreducible protein tubulin. Protein "handles" - dynein- has an active ATPase: it breaks down ATP, due to the energy of which the doublets of microtubules are displaced relative to each other. This is how the wave-like movements of the cilia and flagella occur.
There is a genetically determined disease - Carth-Gsner syndrome, in which the axoneme lacks either dynein handles or a central capsule and central microtubules (fixed cilia syndrome). Such patients suffer from recurrent bronchitis, sinusitis and tracheitis. In men, due to sperm immobility, infertility is observed.
MYOFIBRILLS are found in muscle cells and myosymplasts, and their structure is discussed in the topic “Muscle tissue”. Neurofibrils are found in neurons and consist of neurotubules And neurofilaments. Their function is support and transport.
INCLUSIONS
Inclusions are unstable components of the cell that do not have a strictly constant structure (their structure can change). They are detected in the cell only during certain periods of vital activity or life cycle.
 |
CLASSIFICATION OF INCLUSIONS.
1. Trophic inclusions represent stored nutrients. Such inclusions include, for example, inclusions of glycogen and fat.
2. Pigment inclusions. Examples of such inclusions are hemoglobin in erythrocytes and melanin in melanocytes. In some cells (nerve, liver, cardiomyocytes) during aging, brown aging pigment accumulates in lysosomes lipofuscin, not believed to have a specific function and is formed as a result of wear and tear of cellular structures. Consequently, pigment inclusions represent a chemically, structurally and functionally heterogeneous group. Hemoglobin is involved in gas transport, melanin performs a protective function, and lipofuscin is the end product of metabolism. Pigment inclusions, with the exception of liofuscin inclusions, are not surrounded by a membrane.
3. Secretory inclusions are detected in secretory cells and consist of products that are biologically active substances and other substances necessary for the implementation of body functions (protein inclusions, including enzymes, mucous inclusions in goblet cells, etc.). These inclusions have the appearance of membrane-surrounded vesicles, in which the secreted product can have different electron densities and are often surrounded by a light, structureless rim. 4. Excretory inclusions- inclusions that must be removed from the cell, since they consist of end products of metabolism. An example is urea inclusions in kidney cells, etc. They are similar in structure to secretory inclusions.
5. Special inclusions - phagocytosed particles (phagosomes) that enter the cell by endocytosis (see below). Various types of inclusions are shown in Fig. 3.14.
the ability of cells to adhere to each other and to various substrates
Cell ADHESION(from Latin adhaesio- adhesion), their ability to adhere to each other and to various substrates. Adhesion is apparently determined by the glycocalyx and lipoproteins of the plasma membrane. There are two main types of cell adhesion: cell-extracellular matrix and cell-cell. Cell adhesion proteins include: integrins, functioning as both cell-substrate and intercellular adhesion receptors; selectins are adhesion molecules that ensure adhesion of leukocytes to endothelial cells; cadherins - calcium-dependent homophilic intercellular proteins; adhesion receptors of the immunoglobulin superfamily, which are especially important in embryogenesis, wound healing and immune response; Homing receptors are molecules that ensure that lymphocytes enter specific lymphoid tissue. Most cells are characterized by selective adhesion: after artificial dissociation of cells from different organisms or tissues, predominantly cells of the same type gather (aggregate) from the suspension into separate clusters. Adhesion is disrupted when Ca 2+ ions are removed from the medium, cells are treated with specific enzymes (for example, trypsin) and is quickly restored after removal of the dissociating agent. The ability of tumor cells to metastasize is associated with impaired adhesion selectivity.
See also:
Glycocalyx
GLYCOCALYX(from Greek glykys- sweet and latin callum- thick skin), a glycoprotein complex included in the outer surface of the plasma membrane in animal cells. Thickness - several tens of nanometers...
Agglutination
AGGLUTINATION(from Latin agglutinatio- adhesion), gluing and aggregation of antigenic particles (for example, bacteria, erythrocytes, leukocytes and other cells), as well as any inert particles loaded with antigens, under the action of specific antibodies - agglutinins. Occurs in the body and can be observed in vitro...
Plan I. Definition of adhesion and its significance II. Adhesive proteins III. Intercellular contacts 1. Cell-cell contacts 2. Cell-matrix contacts 3. Intercellular matrix proteins
Definition of adhesion Cell adhesion is the connection of cells leading to the formation of certain correct types of histological structures specific to those cell types. Adhesion mechanisms determine the architecture of the body—its shape, mechanical properties, and distribution of different cell types.
The Importance of Cell-Cell Adhesion Cell junctions form communication pathways, allowing cells to exchange signals that coordinate their behavior and regulate gene expression. Attachments to neighboring cells and the extracellular matrix influence the orientation of internal cell structures. The establishment and breaking of contacts, modification of the matrix are involved in the migration of cells within the developing organism and direct their movement during repair processes.
Adhesion proteins The specificity of cell adhesion is determined by the presence of cell adhesion proteins on the cell surface Adhesion proteins Integrins Ig-like proteins Selectins Cadherins
Cadherins exhibit their adhesive ability only in the presence of Ca 2+ ions. Structurally, classical cadherin is a transmembrane protein that exists in the form of a parallel dimer. Cadherins are found in a complex with catenins. Participate in intercellular adhesion.
Integrins are integral proteins of the heterodimeric αβ structure. Participate in the formation of cell-matrix contacts. The recognizable locus in these ligands is the tripeptide sequence –Arg-Gly-Asp (RGD).
Selectins are monomeric proteins. Their N-terminal domain has the properties of lectins, i.e., it has a specific affinity for one or another terminal monosaccharide of oligosaccharide chains. That. , selectins can recognize specific carbohydrate components on the surface of cells. The lectin domain is followed by a series of three to ten other domains. Of these, some influence the conformation of the first domain, while others take part in the binding of carbohydrates. Selectins play an important role in the process of transmigration of leukocytes to the site of damage to L-selectin (leukocytes) during an inflammatory response. E-selectin (endothelial cells) P-selectin (platelets)
Ig-like proteins (ICAMs) Adhesive Ig and Ig-like proteins are found on the surface of lymphoid and a number of other cells (for example, endothelial cells), acting as receptors.
The B-cell receptor has a structure close to that of classical immunoglobulins. It consists of two identical heavy chains and two identical light chains, connected by several bisulfide bridges. B cells of one clone have Ig of only one immunospecificity on their surface. Therefore, B lymphocytes react most specifically with antigens.
T cell receptor The T cell receptor consists of one α and one β chain connected by a bisulfide bridge. In alpha and beta chains, variable and constant domains can be distinguished.
Types of molecular connections Adhesion can be carried out on the basis of two mechanisms: a) homophilic - adhesion molecules of one cell bind to molecules of the same type of neighboring cell; b) heterophilic, when two cells have different types of adhesion molecules on their surface that bind to each other.
Cellular contacts Cell - cell 1) Simple type contacts: a) adhesive b) interdigitation (finger joints) 2) adhesive type contacts - desmosomes and adhesive bands; 3) contacts of a locking type - tight junction 4) Communication contacts a) nexuses b) synapses Cell - matrix 1) Hemidesmosomes; 2) Focal contacts
Architectural types of tissues Epithelial Many cells - little intercellular substance Intercellular contacts Connective Lots of intercellular substance - few cells Contacts of cells with the matrix
General scheme of the structure of cell contacts Intercellular contacts, as well as cell contacts with intercellular contacts, are formed according to the following scheme: Cytoskeletal element (actin or intermediate filaments) Cytoplasm Plasmalemma Intercellular space A number of special proteins Transmembrane adhesion protein (integrin or cadherin) Transmembrane protein ligand Same white on the membrane of another cell, or an extracellular matrix protein
Contacts of a simple type Adhesive junctions This is a simple bringing together of the plasma membranes of neighboring cells at a distance of 15 -20 nm without the formation of special structures. In this case, plasmalemmas interact with each other with the help of specific adhesive glycoproteins - cadherins, integrins, etc. Adhesive contacts are points of attachment of actin filaments.
Contacts of a simple type Interdigitation (finger-like connection) (No. 2 in the figure) is a contact in which the plasmalemma of two cells, accompanying each other, invaginates into the cytoplasm of first one and then the neighboring cell. Due to interdigitation, the strength of the cell connection and the area of their contact increases.
Contacts of a simple type are found in epithelial tissues, here they form a belt around each cell (adhesion zone); In nervous and connective tissues they are present in the form of pinpoint cell communications; In the cardiac muscle, they provide indirect communication from the contractile apparatus of cardiomyocytes; Together with desmosomes, adhesive junctions form intercalated discs between myocardial cells.
Contacts of the adhesion type Desmosome is a small round formation containing specific intra- and intercellular elements.
Desmosome In the region of the desmosome, the plasma membranes of both cells are thickened on the inside - due to the desmoplakin proteins, which form an additional layer. A bundle of intermediate filaments extends from this layer into the cytoplasm of the cell. In the region of the desmosome, the space between the plasmolemmas of contacting cells is somewhat expanded and filled with a thickened glycocalyx, which is penetrated by cadherins—desmoglein and desmocollin.
The hemidesmosome provides cell contact with the basement membrane. In structure, hemidesmosomes resemble desmosomes and also contain intermediate filaments, but are formed by different proteins. The main transmembrane proteins are integrins and collagen XVII. They connect to intermediate filaments with the participation of dystonin and plectin. The main protein of the intercellular matrix, to which cells are attached using hemidesmosomes, is laminin.
Adhesion belt The adhesive belt, (adhesion belt, belt desmosome) (zonula adherens), is a paired formation in the form of ribbons, each of which encircles the apical parts of neighboring cells and ensures their adhesion to each other in this area.
Cohesion belt proteins 1. The thickening of the plasmalemma on the cytoplasmic side is formed by vinculin; 2. The threads extending into the cytoplasm are formed by actin; 3. The coupling protein is E-cadherin.
Comparative table of contacts of the adhesion type Contact type Desmosome Connection Thickening on the side of the cytoplasm Adhesion protein, type of adhesion Threads extending into the cytoplasm Cell-cell Desmoplakin Cadherin, homophilic Intermediate filaments Hemidesmosome Cell-intercellular matrix Cohesion belts Cell-cell Dystonin and plectin Vinculin Integrin, Intermediate exact heterophilic filaments with laminin Cadherin, homophilic Actin
Contacts of the adhesive type 1. Desmosomes are formed between cells of tissues exposed to mechanical stress (epithelial cells, cardiac muscle cells); 2. Hemidesmosomes connect epithelial cells to the basement membrane; 3. Adhesive bands are found in the apical zone of single-layer epithelium, often adjacent to the tight junction.
Locking type contact Tight contact The plasma membranes of the cells are adjacent to each other closely, interlocking with the help of special proteins. This ensures reliable delimitation of two environments located on opposite sides of the cell layer. Distributed in epithelial tissues, where they form the most apical part of cells (lat. zonula occludens).
Tight junction proteins The main tight junction proteins are claudins and occludins. Actin is attached to them through a number of special proteins.
Contacts of the communication type Gap-like connections (nexes, electrical synapses, ephapses) The nexus has the shape of a circle with a diameter of 0.5 -0.3 microns. The plasmalemmas of contacting cells are close together and penetrated by numerous channels that connect the cytoplasms of the cells. Each channel consists of two halves - connexons. The connexon penetrates the membrane of only one cell and protrudes into the intercellular gap, where it joins with the second connexon.
Transport of substances through nexuses Electrical and metabolic connections exist between contacting cells. Inorganic ions and low molecular weight organic compounds - sugars, amino acids, and intermediate metabolic products - can diffuse through connexon channels. Ca 2+ ions change the configuration of connexons so that the lumen of the channels closes.
Communication-type contacts Synapses serve to transmit signals from one excitable cell to another. In a synapse there are: 1) a presynaptic membrane (Pre. M), belonging to one cell; 2) synaptic cleft; 3) postsynaptic membrane (Po. M) - part of the plasmalemma of another cell. Usually the signal is transmitted by a chemical substance - a mediator: the latter diffuses from Pre. M and affects specific receptors in Po. M.
Communication connections Type Synaptic cleft Signal transmission Synaptic delay Impulse speed Accuracy of signal transmission Excitation / inhibition Capacity for morphophysiological changes Chem. Wide (20 -50 nm) Strictly from Pre. M to Po. M + Below Above +/+ + Ephaps Narrow (5 nm) In any direction - Above Below +/- -
Plasmodesmata are cytoplasmic bridges connecting neighboring plant cells. Plasmodesmata pass through the tubules of the pore fields of the primary cell wall; the cavity of the tubules is lined with plasmalemma. Unlike animal desmosomes, plant plasmodesmata form direct cytoplasmic intercellular contacts, ensuring intercellular transport of ions and metabolites. A collection of cells united by plasmodesmata form a symplast.
Focal Cell Contacts Focal contacts are contacts between cells and the extracellular matrix. Transmembrane focal contact adhesion proteins are various integrins. On the inside of the plasmalemma, actin filaments are attached to integrin with the help of intermediate proteins. Extracellular ligands are proteins of the extracellular matrix. Found in connective tissue
Intercellular matrix proteins Adhesive 1. Fibronectin 2. Vitronectin 3. Laminin 4. Nidogen (entactin) 5. Fibrillar collagens 6. Type IV collagen Antiadhesive 1. Osteonectin 2. tenascin 3. thrombospondin
Adhesion proteins using the example of fibronectin Fibronectin is a glycoprotein built from two identical polypeptide chains connected by disulfide bridges at their C-termini. The polypeptide chain of fibronectin contains 7-8 domains, each of which contains specific centers for binding different substances. Due to its structure, fibronectin can play an integrating role in the organization of intercellular substances and also promote cell adhesion.
Fibronectin has a binding center for transglutaminase, an enzyme that catalyzes the reaction between glutamine residues of one polypeptide chain and lysine residues of another protein molecule. This allows cross-linking of fibronectin molecules with each other, collagen and other proteins using covalent cross-links. In this way, the structures that arise through self-assembly are fixed by strong covalent bonds.
Types of fibronectin The human genome contains one gene for the fibronectin peptide chain, but alternative splicing and post-translational modification result in several forms of the protein. There are 2 main forms of fibronectin: 1. Tissue (insoluble) fibronectin is synthesized by fibroblasts or endothelial cells, gliocytes and epithelial cells; 2. Plasma (soluble) fibronectin is synthesized by hepatocytes and cells of the reticuloendothelial system.
Functions of fibronectin Fibronectin is involved in a variety of processes: 1. Adhesion and proliferation of epithelial and mesenchymal cells; 2. Stimulation of proliferation and migration of embryonic and tumor cells; 3. Control of differentiation and maintenance of the cell cytoskeleton; 4. Participation in inflammatory and reparative processes.
Conclusion Thus, the system of cell contacts, cell adhesion mechanisms and the extracellular matrix plays a fundamental role in all manifestations of the organization, functioning and dynamics of multicellular organisms.
Intercellular contacts Plan
I. Definition of adhesion and its significance
II. Adhesive proteins
III. Intercellular contacts
1.Cage-to-cage contacts
2.Cell-matrix contacts
3.Intercellular matrix proteins Adhesion Determination
Cell adhesion is the connection of cells leading to
formation of certain correct types of histological
structures specific to these cell types.
Adhesion mechanisms determine the architecture of the body - its shape,
mechanical properties and distribution of different cell types. The importance of intercellular adhesion
Cell junctions form communication pathways, allowing cells
exchange signals that coordinate their behavior and
regulating gene expression.
Attachments to neighboring cells and the extracellular matrix influence
orientation of the internal structures of the cell.
The establishment and rupture of contacts, matrix modification are involved in
migration of cells within a developing organism and guide them
movement during repair processes. Adhesive proteins
Specificity of cell adhesion
determined by the presence on the surface of cells
cell adhesion proteins
Adhesion proteins
Integrins
Ig-like
squirrels
Selectins
Cadherins Cadherins
Cadherins exhibit their
adhesive ability
only
in the presence of ions
2+
Ca.
Classic in structure
cadherin is
transmembrane protein
existing in form
parallel dimer.
Cadherins are found in
complex with catenins.
Participate in intercellular
adhesion. Integrins
Integrins are integral proteins
αβ heterodimeric structure.
Participate in the formation of contacts
cells with matrix.
Recognizable locus in these ligands
is tripeptide
sequence –Arg-Gly-Asp
(RGD). Selectins
Selectins are
monomeric proteins. Their N-terminal domain
has lectin properties, i.e.
has a specific affinity for something or
another terminal monosaccharide
oligosaccharide chains.
Thus, selectins can recognize
certain carbohydrate components
cell surfaces.
The lectin domain is followed by a series of
three to ten other domains. Of these, one
influence the conformation of the first domain,
and others take part in
binding of carbohydrates.
Selectins play an important role in
the process of transmigration of leukocytes into
site of damage due to inflammation
L-selectin (leukocytes)
reactions.
E-selectin (endothelial cells)
P-selectin (platelets) Ig-like proteins (ICAMs)
Adhesive Ig and Ig-like proteins are located on the surface
lymphoid and a number of other cells (for example, endothelial cells),
acting as receptors. B cell receptor
The B cell receptor has
structure close to the structure
classical immunoglobulins.
It consists of two identical
heavy chains and two identical
light chains connected between
several bisulfide
bridges.
B cells of the same clone have
surface Ig only one
immunospecificity.
Therefore, B lymphocytes are the most
react specifically with
antigens. T cell receptor
The T cell receptor consists of
from one α and one β chains,
bisulfide connected
bridge.
In alpha and beta chains you can
highlight variable and
constant domains. Types of molecular compounds
Adhesion can be carried out on
based on two mechanisms:
a) homophilic – molecules
single cell adhesion
bind to the molecules
same type of neighboring cell;
b) heterophilic, when two
cells have on their
different types of surfaces
adhesion molecules, which
communicate with each other. Cell contacts
Cell - cell
1) Simple type contacts:
a) adhesive
b) interdigitation (finger
connections)
2) clutch type contacts –
desmosomes and adhesive bands;
3) locking type contacts –
tight connection
4) Communication contacts
a) nexuses
b) synapses
Cell - matrix
1) Hemidesmosomes;
2) Focal contacts Architectural fabric types
Epithelial
Many cells - few
intercellular
substances
Intercellular
contacts
Connecting
Lots of intercellular
substances – few cells
Cell contacts with
matrix General diagram of the structure of cellular
contacts
Intercellular contacts, as well as contacts
cells with intercellular contacts are formed by
following diagram:
Cytoskeletal element
(actin- or intermediate
filaments)
Cytoplasm
A number of special proteins
Plasmalemma
Intercellular
space
Transmembrane adhesion protein
(integrin or cadherin)
Transmembrane protein ligand
The same white one on the membrane of another cell, or
extracellular matrix protein Simple type contacts
Adhesive connections
It's a simple rapprochement
plasma membranes of neighboring cells on
distance 15-20 nm without
special education
structures. Wherein
plasma membranes interact
with each other using
specific adhesive
glycoproteins - cadherins,
integrins, etc.
Adhesive contacts
represent points
actin attachment
filaments. Simple type contacts
Interdigitation
Interdigitation (digital
connection) (No. 2 in the figure)
represents a contact when
in which the plasmalemma of two cells,
accompanying
Friend
friend,
invaginates into the cytoplasm first
one and then the next cell.
Behind
check
interdigitations
increases
strength
cell connections and their area
contact. Simple type contacts
Found in epithelial tissues, here they form around
each cell has a girdle (adhesion zone);
In nervous and connective tissues they are present in the form of punctate
cell messages;
Provides indirect communication in the heart muscle
contractile apparatus of cardiomyocytes;
Together with desmosomes, adhesive contacts form intercalated discs
between myocardial cells. Coupling type contacts
Desmosomes
Hemidesmosomes
Belt
clutch Coupling type contacts
Desmosome
The desmosome is a small round structure
containing specific intra- and intercellular elements. Desmosome
In the region of the desmosome
plasma membranes of both cells with
the inner sides are thickened -
due to desmoplakin proteins,
forming an additional
layer.
From this layer into the cytoplasm of the cell
a bunch of intermediates comes off
filaments.
In the region of the desmosome
space between
plasma membranes of contacting
cells are slightly expanded and
filled with thickened
glycocalyx, which is permeated
cadherins – desmoglein and
desmocollin. Hemidesmosome
The hemidesmosome provides cell contact with the basement membrane.
The structure of hemidesmosomes resembles desmosomes and also contain
intermediate filaments, however, are formed by other proteins.
The main transmembrane proteins are integrins and collagen XVII. WITH
they are connected by intermediate filaments with the participation of dystonin
and plectin. The main protein of the intercellular matrix to which cells
attached via hemidesmosomes - laminin. Hemidesmosome Clutch Belt
Adhesive belt (adhesion belt, girdle desmosome)
(zonula adherens), - paired formations in the form of ribbons, each
of which surrounds the apical parts of neighboring cells and
ensures their adhesion to each other in this area. Proteins of adhesion belts
1. Thickening of the plasmalemma
from the cytoplasm
formed by vinculin;
2. Threads extending into
cytoplasm formed
actin;
3. Cohesion protein
E-cadherin acts. Contact comparison table
clutch type
Contact type
Desmosome
Compound
Thickenings
from the outside
cytoplasm
Coupling
protein, type
clutch
Threads,
departing to
cytoplasm
Cell-cell
Desmoplakin
Cadherin,
homophilous
Intermediate
filaments
Dystonin and
plectin
Integrin,
heterophilic
with laminin
Intermediate
filaments
Vinculin
Cadherin,
homophilous
Actin
Hemidesmosome Cell Intercellular
matrix
Belts
clutch
Cell-cell Coupling type contacts
1. Desmosomes are formed between tissue cells,
exposed to mechanical stress
(epithelial
cells,
cells
cardiac
muscles);
2. Hemidesmosomes connect epithelial cells with
basement membrane;
3. Adhesive bands are found in the apical zone
single-layer epithelium, often adjacent to dense
contact. Locking type contact
Tight contact
Plasmolemmas of cells
adjacent to each other
closely, engaging with
using special proteins.
This ensures
reliable separation of two
environments located in different
sides from the layer of cells.
Distributed
in epithelial tissues, where
make up
most apical part
cells (lat. zonula occludens). Tight junction proteins
The main proteins of dense
contacts are claudins and
occludins.
Through a series of special proteins to them
actin is attached.
Gap joints (nexes,
electrical synapses, ephapses)
The nexus has the shape of a circle with a diameter
0.5-0.3 microns.
Plasma membranes in contact
cells are close together and penetrated
numerous channels,
which bind cytoplasm
cells.
Each channel consists of two
half are connexons. Connexon
penetrates the membrane with only one
cells and protrudes into the intercellular
the gap where it joins with the second
connexon. Structure of the ephaps (Gap junction) Transport of substances through nexuses
Between contacting
exists by cells
electrical and
metabolic connections.
Through connexon channels they can
diffuse
inorganic ions and
low molecular weight
organic compounds –
sugars, amino acids,
intermediate products
metabolism.
Ca2+ ions change
configuration of connexons -
so that the lumen of the channels
closes. Communication type contacts
Synapses
Synapses serve to transmit signals
from one excitable cell to another.
In a synapse there are:
1) presynaptic membrane
(PreM) belonging to one
cage;
2) synaptic cleft;
3) postsynaptic membrane
(PoM) – part of the plasmalemma of another
cells.
Usually the signal is transmitted
chemical substance - mediator:
the latter diffuses from PreM and
affects specific
receptors in PoM. Communication connections
Found in excitable tissues (nervous and muscle) Communication connections
Type
Synapty
cheskaya
gap
Conducted
no
signal
Synaptic
I'm delayed
Speed
impulse
Accuracy
transfers
signal
Excitation
/braking
Ability to
morphophysiol
ogic
changes
Chem.
Wide
(20-50 nm)
Strictly from
PreM to
PoM
+
Below
Higher
+/+
+
Ephaps
Narrow (5
nm)
In any
directed
II
-
Higher
Below
+/-
-Plasmodesmata
They are cytoplasmic bridges connecting adjacent
plant cells.
Plasmodesmata pass through the canaliculi of the pore fields
primary cell wall, the cavity of the tubules is lined with plasmalemma.
Unlike animal desmosomes, plant plasmodesmata form straight
cytoplasmic intercellular contacts providing
intercellular transport of ions and metabolites.
A collection of cells united by plasmodesmata form a symplast. Focal cell contacts
Focal contacts
represent contacts
between cells and extracellular
matrix.
Transmembrane proteins
focal contact adhesion
are various integrins.
From the inside
plasma membranes to integrin
actin attached
filaments using
intermediate proteins.
Extracellular ligand
extracellular proteins act
matrix.
Found in the connective
fabrics Intercellular proteins
matrix
Adhesive
1. Fibronectin
2. Vitronectin
3. Laminin
4. Nidogen (entactin)
5. Fibrillar collagens
6. Type IV collagen
Anti-adhesive
1. Osteonectin
2. tenascin
3. thrombospondin Adhesion proteins as an example
fibronectin
Fibronectin is a glycoprotein built
of two identical polypeptide chains,
connected by disulfide bridges
their C-termini.
The fibronectin polypeptide chain contains
7-8 domains, on each of which
there are specific centers for
binding of different substances.
Due to its structure, fibronectin can
play an integrating role in the organization
intercellular substance, as well as
promote cell adhesion. Fibronectin has a binding site for transglutaminase, an enzyme
catalyzing the reaction of combining glutamine residues with one
polypeptide chain with lysine residues of another protein molecule.
This allows cross-linking of molecules by cross-linking covalent bonds.
fibronectin with each other, collagen and other proteins.
In this way, structures arising through self-assembly
fixed by strong covalent bonds. Types of fibronectin
There is one peptide gene in the human genome
fibronectin chains, but as a result
alternative
splicing
And
post-translational
modifications
Several forms of protein are formed.
2 main forms of fibronectin:
1.
Fabric
(insoluble)
fibronectin
synthesized
fibroblasts or endothelial cells,
gliocytes
And
epithelial
cells;
2.
Plasma
(soluble)
fibronectin
synthesized
hepatocytes and cells of the reticuloendothelial system. Functions of fibronectin
Fibronectin is involved in a variety of processes:
1. Adhesion and spread of epithelial and mesenchymal
cells;
2. Stimulation of proliferation and migration of embryonic and
tumor cells;
3. Control of differentiation and maintenance of the cytoskeleton
cells;
4. Participation in inflammatory and reparative processes. Conclusion
Thus, the system of cell contacts, mechanisms
cell adhesion and extracellular matrix plays
a fundamental role in all manifestations of the organization,
functioning and dynamics of multicellular organisms.
