Histology.RU: CYTOPLASMA  

Material taken from the site www.hystology.ru

The cytoplasm of a cell consists of a microscopically structureless basic substance - hyaloplasm, in which its specialized structures (organelles) that perform specific functions are dispersed.

Hyaloplasma- a substance of the cytoplasm of cells, heterogeneous in chemical composition. It contains proteins, nucleic acids, polysaccharides, amino acids, nucleotides, various enzymes and many other compounds involved in cell metabolism. Hyaloplasm is a medium that unites various cell structures and ensures their interaction. ATP, metabolic products, inclusions of glycogen blocks, fat droplets, pigments, etc. are concentrated in the hyaloplasm.

Organelles- cytoplasmic structures that perform specific functions in the cell. These include the plasmalemma, ribosomes, endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, mitochondria, centrioles. In addition to the above-mentioned organelles, the cytoplasm of cells contains a significant number of structures (fibrils, filaments, microtubules) that differ in morphology and functional significance, reflecting the specificity of differentiation characteristic of certain tissues.

Plasmolemma- cell membrane that performs restrictive, transport and receptor functions. It demarcates the cell from the surface and provides communication with the external environment. The plasmalemma provides mechanical communication between cells and intercellular interactions, contains cellular receptors for hormones and other signals from the environment surrounding the cell, transports substances into and out of the cell both along a concentration gradient - passive transfer, and with energy expenditure against the concentration gradient - active transfer.

The membrane consists of a plasma membrane, a supra-membrane complex - the glycocalex, and a submembrane musculoskeletal apparatus. The plasma membrane is based on a bimolecular layer of lipids, into which protein and glycoprotein molecules are completely immersed. It contains about 40% lipids, 60% proteins and up to 1% carbohydrates. In connection with the functional characteristics of cells of various tissues, the composition of the thlycoprotein supramembrane complex is specific. It contains up to 1% carbohydrates (hyaluronic, sialic acid, etc.), the molecules of which form long branching chains of polysaccharides associated with membrane proteins (Fig. 12 and 13). Enzyme proteins located in the glycocalex are involved in the final extracellular breakdown of substances. The products of these reactions enter the cell in the form of monomers. During active transport, the transport of substances into the cell (endocytosis) is carried out either by the entry of molecules in the form of a solution - pinocytosis, or by the capture of large particles - phagocytosis.

The process of phagocytosis consists of two phases: the interaction of the particle with the receptor of the cell plasmalemma and then its absorption as a result of the formation of pseudopodia. The initial interaction of the particle and the plasmalemma receptor causes a signal, as a result of which local accumulations of contractile proteins (actin, etc.) occur in the surface layer of the cytoplasm, leading to the formation of pseudopodia. This increases the area of ​​its contact with the particle, which causes further accumulation of contractile proteins. The process continues until the pseudopodia close over the particle, forming a phagosome.

Pinocytosis is the vesicular absorption of fluid containing low molecular weight solutions (lipoproteins, immune complexes, ferritin, hormones, etc.). A distinction is made between macropinocytosis, in which undulating folds of the cell surface capture droplets of solution visible under phase-contrast microscopy, and micropinocytosis, in which liquid is captured by minimal invaginations visible only under electron microscopy.

According to the mechanism of action, micropinocytosis is liquid-phase and absorptive. The first is non-selective: solutes are absorbed proportionally to their concentration in the liquid medium, and the membrane that absorbs them is not morphologically specialized. In the second case, the membranes of the bubble-shaped invaginations of the plasmalemma of the cell are covered on the outer surface with a thin local layer of glycocalex, and on the inner surface with bristles of fine hairs.

The amount of membrane internalized (immersed) during endocytosis can be large, especially during phagocytosis. Macrophages in vitro can internalize up to 18% of their plasmalemma per hour during phagocytosis.

Rice. 12. Scheme of the structure of the plasma membrane: proteins on the outer side of the layer are associated with polysaccharides, forming a layer of glycocalex.

Rice. 13. Hypothetical diagram of the plasma membrane (according to Bergelson).

Rice. 14. Diagram of cell contacts:

1 - simple contact; 2 - lock; 3 - tight contact; 4 - intermediate contact; 5 - desmosome; b - gap contact.

In accordance with the functional and morphological characteristics of tissues, the cell membrane forms their characteristic apparatus for intercellular contacts. Their main forms are: simple contact, tight contact, intermediate contact (or adhesion zone) and gap contact (Fig. 14).

Simple contact- the most common form of contact between two adjacent cells. With it, the cells are spaced from one another at a distance of 15 - 20 nm. The intercellular space corresponds to the supramembrane components of the cell membranes of contacting cells.

Tight (closed) contact. With it, the outer layers of the plasmalemma at the luminal surface of adjacent cells merge into one common structure and isolate the intercellular space from the environment external to the tissue. This type of junction is found between epithelial cells at their apical surface and forms membrane fusion zone(adhesion of their integral proteins), surrounding the tops of cells in the form of a belt. Membrane proteins are connected in the zone of tight closing contact with a system of thin cytoplasmic fibrils, oriented parallel to the cell surface along the adhesion zone.

A type of tight contact is desmosomes. They are characterized by the special development and differentiation of the supramembrane complex of adjacent cells. In point desmosomes, the distance between the membranes of two contacting cells is 22 - 35 nm. In the intercellular space, due to the supra-membrane complex, a fibrous substance is formed. In its central part a plate containing proteins and mucopolysaccharides is formed. It is connected to the plasma membranes of adjacent cells by transverse fibrils. Adjacent to the membranes of contacting cells are electron-dense zones of the cytoplasm with fibrils extending from them. Desmosomes provide mechanical communication between adjacent cells.

Gap contact characterized by the presence of insignificant intercellular space (up to 2 - 3 nm). This is a specialized area of ​​the plasma membranes of adjacent cells, ensuring the diffusion of ions and small molecules from one cell to another. With appropriate treatment of the tissue with an electron-dense substance, it is clear that the intercellular space is intersected by bridges with a diameter of 7 nm at a distance of up to 10 nm. In some cases, the presence of tiny pores is noted in the bridges. The corresponding materials were also obtained by freezing - by chipping. This suggests that the globular particle in the gap junction region stretches through the membrane lipid bilayer and protrudes into the intercellular gap, where it connects with the corresponding particle of the opposite membrane of the adjacent cell. The end-to-end connection of these particles forms units - connexons, through which a hydrophilic channel with a diameter of 1.5 - 2 nm runs from cell to cell, conducting ions and small molecules, supporting their electrical and metabolic interactions. The permeability of gap junctions is reliably proven by the passage of fluorescent dyes, amino acids, nucleotides and other substances from one cell to another during microinjection. Proteins, amino acids and other macromolecules do not pass through the gap junction.

Ribosomes are granules 15 - 35 nm in diameter. They are located freely in the cytoplasm or fixed on the membrane of the endoplasmic reticulum (granular endoplasmic reticulum). Free ribosomes are characteristic of the cytoplasm of undifferentiated cambial cells. Under light microscopy, the cytoplasm of cells rich in ribosomes is basophilic. Ribosomes are also present in the nucleus, where they ensure the synthesis of nuclear proteins (Fig. 15).

Ribosomes consist of two subunits - small and large. The small subunit is attached to a flattened region of the large subunit. Each of them contains a molecule of ribosomal RNA (r-RNA) and protein, which makes up 40 - 60% of the total mass of the ribosome. Located on the membranes of the endoplasmic reticulum of the cell cytoplasm, the ribosome is attached by a large subunit.

Ribosomes are involved in the assembly of protein molecules - the arrangement of amino acids into polymer chains in strict accordance with the genetic information contained in DNA.

In addition to ribosomal RNA, the cell contains messenger RNA (i-RNA), which is synthesized on the DNA of the nucleus. The latter determines the order of alternation of nitrogenous bases in mRNA. mRNA carries information from the genome to the ribosomes of the cytoplasm, where the encoded message is translated into the amino acid sequence of the synthesized protein.

Protein is usually synthesized not on one ribosome, but on a group of ribosomes - a polyribosome (polysome). The ribosomes in a polysome are linked by an mRNA molecule, which passes along a series of ribosomes until all the information encoded in it has been read. Messenger RNA is associated with the small subunit of the ribosome, and the forming polypeptide chain is associated with the large one.

The third type of RNA in the cytoplasm is transfer RNA (tRNA), which carries amino acids to the ribosome. There is a special tRNA for each amino acid, and each
Rice. 15. Polyribosomes of reticulocytes: A - coated with platinum (magnification 100,000); B - positively stained with uronyl acetate (magnitude 400,000, according to RPH) carries a specific trinucleotide that can attach to a specific trinucleotide (codon) on the mRNA molecule. The sequence of codons on the mRNA molecule determines the sequence of tRNA attachment and, consequently, the sequence of alternation of amino acids in the forming polypeptide chain.

The ribosome creates the spatial relationships necessary for the interaction of t-RNA with mRNA and ensures the formation of polypeptide bonds between amino acids, which is catalyzed by the active site of one of the ribosomal proteins.

Endoplasmic reticulum- a system of tubes and flattened extensions called cisterns, which together create a membrane network in the cytoplasm of the cell. The endoplasmic reticulum is involved in synthesis processes, performs a transport function in the cell, contains enzymes and their substrates, which play an active role in cell metabolism. There are two types of endoplasmic reticulum: granular, to the outer surface of which ribosomes are attached, and agranular without ribosomes (Fig. 16).

The cisterns of the granular endoplasmic reticulum are especially numerous in cells that synthesize large amounts of protein as a secretory product. In such cells, the cisterns can be located in parallel clusters or form concentric systems. In such clusters, the lumen of the tanks is very narrow, the distance between them is only

Rice. 16. Electron micrograph of the agranular (A) endoplasmic reticulum of a hamster liver cell (according to Cartesi and Lond) and the granular (B) endoplasmic reticulum of a pancreatic cell.

Rice. 16, a. Scheme of the hypothesis of the passage of proteins through the membrane of the cytoplasmic reticulum:

1 - signal codon; 2 - i-RNA; 3 - signal peptide; 4 - tank cavity; 5 - signal peptidase; 6 - receptor protein.

35 nm. Tangential sections of the cisterns of the granular endoplasmic reticulum show that the ribosomes fixed on its membranes are also combined into polysomes and arranged in the form of rosettes, spirals, and loops on the outer surface of the membranes.

To explain the passage of synthesized proteins through the membrane into the tubule of the endoplasmic reticulum, the following hypothesis was created. Messenger RNA for secretory proteins contains a sequence of signal codons. The synthesis of signal peptides occurs on free ribosomes. When the signal peptide emerges from a channel on the larger subunit, the ribosome binds to ribosome receptor proteins on the membrane of the endoplasmic reticulum. Such proteins appear to be ribophorin I and ribophorin II, which are absent on the membranes of the smooth endoplasmic reticulum. In this case, the receptor proteins come together and a transmembrane channel is formed, located so that it is a continuation of the tubule on the large subunit of the ribosome. As a result of elongation of the polypeptide chain, the signal polypeptide moves into the tank and is cleaved off by signal peptidase localized on the inner surface of the membrane. The polypeptide chain of the synthesized secretory protein continues to move into the tank. When the synthesis of the protein molecule ends, the ribosome is separated from the membrane, and the channels are obliterated (Fig. 16, a).

The agranular (smooth) endoplasmic reticulum usually does not form cisterns, but consists of anastomosing tubes. It is associated with the synthesis and breakdown of glycogen, with lipid metabolism, in particular with the synthesis of steroid hormones. Therefore, the smooth endoplasmic reticulum is very developed in cells that produce steroid hormones (interstitial cells of the testis, cells of the adrenal cortex, corpus luteum of the ovaries). The administration of barbiturates, insecticides, carcinogens and other drugs to experimental animals causes hypertrophy of the smooth endoplasmic reticulum in liver cells. This adaptive response of liver cells increasing their ability to metabolize and eliminate drugs underlies drug tolerance during long-term drug use. Thus, the agranular endoplasmic reticulum is involved in the detoxifying function of the liver.

Mitochondria are present in almost all eukaryotic cells. Their main function is to provide the chemical energy necessary for the biosynthetic and motor activity of cells. The products of the breakdown of carbohydrates entering the mitochondrion in the form of pyruvates, amino acids and fatty acids are oxidized in the mitochondria to C0 2 and H 2 O. The energy released in this case is used for the synthesis of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate. The reaction of ATP formation is called phosphorylation, ATP provides energy for almost all life processes. In this case, ATP is broken down into phosphate and ADP. The latter is reabsorbed by the mitochondrion and phosphorylated. To ensure the processes of oxidation, phosphorylation and other reactions, more than 50 enzymes are present in mitochondria.

At the light-optical level, mitochondria look like threads or short rods, less often - grains. Their average length is 2 - 6 microns, width 0.2 microns. They are usually distributed throughout the cytoplasm, but sometimes they can be concentrated in those areas of the cell where the energy requirement is greatest. For example, near the propulsion apparatus (Fig. 17).

Rice. 17. Mitochondria of intestinal prismatic epithelial cells:

1 - core: 2 - brush-like border; 3 - mitochondria; 4 - basement membrane

Rice. 18. Scheme of the general organization of mitochondria:

1 - outer membrane; 2 - internal membrane; 3 - invagination of the inner membrane - cristae; 4 - places of protrusions, view from the surface.

Rice. 19. Electron micrograph of a section of pancreatic mitochondria.

Mitochondria have a characteristic ultrastructure. Outside, the mitochondrion is surrounded by a smooth-contoured outer mitochondrial membrane 7 nm thick. At a distance of 8 - 10 nm from the outer one lies the inner mitochondrial membrane. It has numerous folds - mitochondrial cristae, increasing the area of ​​the inner membrane. Between the outer and inner membranes there is a membrane space of low electron density. The space bounded by the inner mitochondrial membrane is filled with a homogeneous or fine-grained mitochondrial matrix. Enzymes of the tricarboxylic acid cycle are localized in the matrix, in which pyruvates, as well as the breakdown products of proteins and lipids, are oxidized to CO 2 and H 2 O. The inner membrane of mitochondria contains enzymes of the respiratory chain. Mitochondrial subunits or elementary particles are located on the inner membrane of mitochondria. They are spherical particles with a diameter of 9 nm, connected to the membrane by a stalk 3 - 4 nm wide and 5 nm long. The subunits contain sets of enzymes responsible for phosphorylation (Fig. 18, 19, 20).

Mitochondrial granules are located in the mitochondrial matrix. They can be free or associated with cristae. Their size ranges from 25 to 120 nm depending on the type of cells and their functional state. The high density usually hides the internal structure of the granules, but thin sections show that they are separated by very thin septa. The function of granules has not yet been fully established, but it is believed that they are binding sites for divalent cations, in particular Ca ++, and thus participate in maintaining the constancy of their content in the hyaloplasm surrounding the mitochondrion.

Mitochondria have their own DNA and RNA. In sections of mitochondria, DNA molecules appear as branching threads of varying thickness, surrounded by a more transparent area of ​​the matrix. When the destroyed mitochondria are distributed over the surface of the water, the released DNA looks like a thread 4 nm thick, 5 mm long, closed in the form of a circle. The circular shape of mitochondrial DNA is very similar to the DNA of viruses and bacteria. The mitochondrial matrix also contains ribonucleoprotein particles - ribosomes with a diameter of 10 - 15 nm, messenger and transfer RNA, as well as all the necessary enzymes for the synthesis of DNA, RNA and protein. However, due to the small information contained in the mitochondrial genome, they cannot synthesize all of their components and the synthesis of most enzymes is provided by the nuclear genome.

Mitochondria have a limited lifespan (the half-life of the mitochondria of liver cells is 8 days, cardiac muscle - 6 days, neurons - 31 days). The loss of mitochondria is replenished due to their division. In this case, a septum grows from the inner membrane of the mitochondria until it meets the opposite side of the inner membrane. The outer membrane grows concentrically into the septum and the mitochondria are divided into two daughter ones.

Golgi complex(lamellar complex) on preparations treated with silver nitrate or osmium tetroxide looks like a network of intertwining dark lines. In some

Rice. 20. Electron micrograph of the crista: mitochondria of the heart muscle of a bull (magnitude 40000). Particles carrying electrons are visible.

Rice. 21. Golgi complex (3) in the nerve cells of the spinal ganglion. Impregnation with osmium (magnitude 400, according to Almazov and Sutulov):

1 - nucleus with nucleolus; 2 - cytoplasm.

In cells it is localized near the centrioles, in others it surrounds the nucleus, and in epithelial cells it is usually located between the nucleus and the apical surface of the cell (Fig. 21).

Electron microscopy shows that the main component of the Golgi complex is membrane-surrounded flattened sacs, or cisterns, stacked one above the other. The tanks are curved so that a convex (outer) and concave (inner) surface can be distinguished in the stack (Fig. 22). A separate collection of cisternae is called a dictyosome. Features of the ultrastructure of the Golgi complex are associated with its main function of condensation and removal of secretions. Protein secretions are synthesized on ribosomes associated with the granular endoplasmic reticulum, enter the reticulum tubules and are transported to the Golgi zone. The cisternae of the endoplasmic reticulum on the surface facing the Golgi complex usually lack ribosomes. Small protrusions of this surface, filled with protein secretion, break off and form transport vesicles that flow into the external cisterns of the dictyosome. In addition to cisterns and transport vesicles, the Golgi complex also includes condensing vacuoles and secretory granules. According to the currently most generally accepted concept, the participation of the Golgi complex in the secretion process is as follows. Transport vesicles merge to form cisterns on the outer (forming) surface of the dictyosome. As new tanks form, the old ones move towards the inner (maturing) surface. The cisterns swell, turning into condensing vacuoles. The latter, as a result of condensation of their contents, can turn into secretory granules. The membranes of the tanks transform as they move towards the maturing surface, becoming similar to the plasmalemma. Therefore, the shell of secretory granules easily merges with the plasmalemma and the secretion enters the lumen of the gland. Lysosomes are formed in the Golgi complex in the same way as secretory granules (Fig. 23).

However, there is another hypothesis that is currently quite widespread. According to its supporters, tubes and cisternae with a smooth surface, located near the maturing surface of the Golgi complex and giving a reaction to acid phosphatase, represent a specialized system for transferring acid hydrolases from the granular endoplasmic reticulum directly to lysosomes, bypassing the Golgi complex. This system was named GERL (Golgi-associated endoplasmic reticulum from which lysosomes are formed). The authors also associate the formation of peroxisomes, condensing vacuoles, and autophagosomal membranes with GERL.

In addition to the removal of protein secretions, the Golgi complex takes part in the synthesis of polysaccharides and their attachment to protein. During the synthesis of glycoproteins, some oligosaccharides are incorporated into polypeptides when they are synthesized on ribosomes, while others are added later to the formed polypeptide chains when they reach the Golgi complex. In addition to participating in

Rice. 22. Electron micrograph of the Golgi complex (arrows indicate small vacuoles).

Rice. 23. Fusion of the lysosome with the plasmalemma (according to Daems).

In the synthesis of the carbohydrate part of the secretory glycoproteins of glandular cells, the Golgi complex plays an important role in the synthesis of glycoproteins of the plasmalemma (glycocalyx).

The morphology of the Golgi complex may depend on the intensity of the secretion process. When the organelle is relatively inactive, the cisternae are continuous, closely spaced, and the same width throughout the dictyosome. In actively secreting cells, the cisternae profiles are shorter, their membranes are often fenestrated, and the width of the lumen increases from the forming surface to the maturing one. In the area of ​​the Golgi complex, in addition to smooth ones, there are also bordered vesicles. In secretory cells they are associated with the recycling of membranes from the plasmalemma back to the Golgi complex.

Lysosomes- bodies with a diameter of 0.2 - 0.5 microns, bounded by a membrane and containing about 50 different enzymes, mainly hydrolytic, active at acidic pH values ​​(phosphatases, glycosidases, proteases, lipases, sulfatases, etc.). The organelle received its name because the enzymes contained in it are capable of causing lysis (dissolution) of all cell components. Under normal conditions, this usually does not happen, since the enzymes contained in lysosomes are isolated from substrates and are therefore inactive. About 20% of enzymes are embedded in the lysosome membrane and 80% are located in its mucopolysaccharide complex (Fig. 24).

The function of lysosomes is the intracellular enzymatic breakdown of both exogenous substances that enter the cell as a result of endocytosis, and endogenous ones (removal of organelles and inclusions during normal renewal or in response to altered functional activity). Sometimes the permeability of cell lysosome membranes may increase and their enzymes are released

Rice. 24. Scheme of the functioning of lysosomes and intracellular proteolysis (according to De-Duve):

1 - phagocytosed particle; 2 - micromolecules; 3 - macromolecules pinocytosed by the cell; 4 - phagosome; b - ergastoplasma; 6 - lysosomes; 7 - fusion of ligosome and phagosome; 8 - proteolysis of particles and macromolecules (9); 10 - excretion of proteolysis residues; 11 - proteolysis in the lysosome with the formation of a phagocytic vacuole.

into the cytoplasm. Then dissolution (autolysis) of the cell occurs. This is observed under experimental conditions, pathology and in some cases of normal functioning of the organ (involution of the mammary gland during cessation of lactation, involution of the uterus after childbirth, resorption of the tail of amphibians during metamorphosis, etc.) Depending on the activity of lysosomes in the processes of intracellular digestion and on the nature of the object, subject to hydrolytic cleavage, the contents of lysosomes are very heterogeneous. There are: primary lysosomes, phagolysosomes (or heterophagosomes), autophagosomes and residual, or residual, bodies.

Primary lysosomes are small bodies with homogeneous contents. They represent a reserve of hydrolytic enzymes that are not yet involved in digestion.

Phagolysosomes (heterophagic vacuoles) are formed from the fusion of the primary lysosome with the phagosome. In this case, the hydrolytic breakdown of the contents of the latter begins.

Autophagosomes arise during intracellular renewal or during internal cell restructuring associated with a decrease in physiological activity. Then some of the organelles are removed by autophagy. The organelles to be destroyed are surrounded by a membrane, forming an autophagic vacuole. With the latter, lysosomes merge, pouring out their hydrolytic enzymes. The nature of the membrane has not been fully elucidated. Apparently, these are the membranes of the smooth endoplasmic reticulum, or SER.

As the contents are digested, phagolysosomes decrease in size and turn into residual, or residual, bodies filled with granules of indigestible material of various sizes and densities. The residual bodies may subsequently coalesce into accumulations of lipofuscin or wear pigment.

Peroxisomes- spherical bodies surrounded by a membrane measuring 0.2 - 0.5 microns. They somewhat resemble lysosomes, but do not contain hydrolytic enzymes. Peroxisomes are characterized by the presence of amino acid oxidases and catalase, an enzyme that destroys peroxides. In those animal species whose cells contain urate oxidase, a crystalloid (nucleoid) is present in the peroxisomes of the liver and kidneys. In species lacking urate oxidase (birds, humans), there is no crystalloid in the peroxisomes. The structure of the crystalloid varies among species and also depends on the type of cell. For example, in rat liver peroxisomes, the crystalloid consists of hollow tubes arranged in such a way that they form a honeycomb shape. In peroxisomes of the proximal part of rat kidneys, two types of nucleoids were found: cylindrical inclusions with a diameter of 85 - 140 nm and tubular crystals up to 3 μm in length. In hamster liver peroxisomes, the nucleopd has a plate shape.

In addition to peroxisomes of the liver and kidneys, membrane-bound bodies with a diameter of 0.15 - 0.25 µm, devoid of a nucleoid - microperoxisomes, were found in various types of epithelium.

Peroxisomal catalase: may play a protective role by breaking down hydrogen peroxide, which is toxic to cells. Peroxisomes are also associated with cholesterol metabolism, as they are especially numerous in cells involved in cholesterol metabolism and steroid synthesis: liver, adrenal glands, ovaries and interstitial cells of the testes. It was also noted that the introduction of substances that lower cholesterol levels in the blood causes a sharp increase in the number of liver peroxisomes.

Centrosome (cell center) located near the nucleus and Golgi complex. At the light-optical level, it is represented by two granules - centrioles, surrounded by a light structureless zone of the cytoplasm - the centrosphere. The latter passes into the radiant sphere, that is, into the zone of radially diverging fibrils that are on the verge of microscopic vision (Fig. 25).

With electron microscopy, centrioles are visible in the form of cylinders 300 - 500 nm in length and 150 nm in diameter, the wall of which is formed by nine groups of microtubules. Each group contains 3 microtubules, 25 nm in diameter. In a group, microtubules are arranged in a chain oriented to the radius of the centriole at an angle of 40°. The microtubule farthest from the periphery of the structure is designated subunit A, the other two, according to their position, B and C. Microtubule A consists of 13 tubulin protofilaments. Microtubules B and C - from 10 - 11.

Before cell division, centrioles duplicate, but preexisting centrioles do not divide. Subsidiary

Rice. 25. Cellular center of an epithelial cell: A, B, C - microtubules of the centriole.

The centriole is formed anew on a specific area of ​​the pre-existing centriole, but being separated from it by a narrow space. Initially, on the lateral surface of the old centriole, a ring-shaped accumulation of dense material is formed with the same diameter as the mature centriole, but devoid of microtubules - the procentriole. Dense material continues to attach to the free edge of the procentriole, and triplets of microtubules appear in the homogeneous, previously dense material. The forming centriole elongates in a direction perpendicular to the mother centriole.

The function of centrioles is to induce the polymerization of proteins - tubulins with the formation of microtubules. In interphase, they participate in the formation of microtubules in the cell framework. During mitosis, centrioles induce the formation of spindle microtubules. Centrioles serve as the basal bodies of cilia.

Satellites, which are granular foci of microtubule discharge, and additional microtubules can be associated with the centriole.

During the formation of cilia, centrioles (basal bodies) can also form at a distance from existing centrioles. Most of them develop around dense spherical bodies called deuterosomes, or procentriole organizers, which in turn develop by condensation of small aggregates of filamentous material. Multiple procentrioles can appear around a single organizer. The forming centrioles quickly elongate and acquire a microtubular internal structure. When the mature centriole reaches the length, it separates from the organizer and moves to the cell surface, where it induces the polymerization of the nine microtubule doublets of the developing cilium.

Cilia and flagella- organelles of cell movement. Their structure is similar and they differ from each other mainly in length. The cilium is an outgrowth of the cytoplasm 6 - 10 µm long and 200 nm thick, covered with plasmalemma. Inside the outgrowth there is an axoneme in the form of microtubules. Unlike the centriole, the axoneme of cilia and flagella consists of two central microtubules and nine paired peripheral ones. In pairs (doublets), microtubule A is distinguished, located somewhat closer to the central axis of the cilium, and microtubule B, which has the shape of a crescent and partially covers microtubule A. From microtubule A to microtubule B of the neighboring doublet, two “handles” stretch, consisting of the protein dynein, which has the activity of ATPase, an enzyme that causes the breakdown of ATP and the release of energy. The central pair of microtubules is connected by bridges. The direction of movement of the cilia is always perpendicular to the bridge. Basal bodies, which lie at the base of cilia and flagella, are centrioles. Often at the base of the cilium lies a pair of centrioles at right angles to each other. The basal body continues into the axoneme. In this case, the A and B microtubules of the triplets of the basal body continue into the A and B microtubules of the axoneme. The flagellum axoneme is constructed in the same way as the cilium axoneme. The length of the flagellum of protozoa is about 150 microns, and in the sperm of some species the flagellum is several times longer. Cilia and flagella cannot contract. Their movement is based on the mechanism of microtubules sliding relative to each other. At the same time, the eyelash bends.

Microtubules, microfibrils, microfilaments. Microtubules are associated with maintaining and changing cell shape, participating in the formation of the cytoskeleton. By forming the spindle, they ensure the movement of chromosomes during mitosis. Microtubules are associated with the directed movement of cytoplasmic bodies (for example, there is evidence of their participation in the movement of mitochondria, synaptic vesicles in neurons, melanosomes in melanophores). They are part of centrioles, cilia and flagella.

Microtubules have the appearance of a hollow cylinder built from 13 longitudinally oriented filaments. The latter consist of rounded tubulin subunits measuring 4–5 nm, which gives the filaments a distinct shape. The outer diameter of the microtubules is 25 nm, the lumen diameter is 15 hm.

Along with microtubules, microfibrils perform a supporting function in cells. These are filamentous formations

Rice. 26. Glycogen in liver cells. Carmine staining using the Vesta method (magnitude 900):

1 - liver cells; 2 - cytoplasm with grains and lumps of glycogen; 3 - nucleus with nucleolus; 4 - sinusoidal (dilated) blood capillary.

Rice. 27. Fatty inclusions in liver cells. Staining with osmic acid - safranin (magnitude 900):

1 - liver cells (a - lipoid granules in the cytoplasm; b- nucleus): 2 - capillary with red blood cells.

Rice. 28. Pigment inclusions in melanocytes. Total unstained preparation (magnitude 400):

1 - nucleus of a pigment cell; 2 - cytoplasm with pigment grains - melanin. with a diameter of 10 nm. They consist of protein subunits. The proteins in different tissues are different. In the epithelium these are keratins. Bundles of microfibrils in the epithelium are called tonofibrils. In cells of mesenchymal origin (fibroblasts), microfibrils consist of the protein vimetin, in muscles - desmin and skeletin.

Microfilaments 6 nm thick are present in large quantities in the cortical layer of cells and form bundles in their cytoplasm. They consist of contractile proteins, mainly actin. Myosin is also found in the cytoplasm of blood cells of granulocytes, fibroblasts, neurons and other cells.

Cellular inclusions. In the cytoplasm of cells of various tissues and organs, in accordance with the specificity of metabolism, various substances are naturally synthesized and accumulated in the form of their characteristic metabolic products - inclusions. They are trophic - associated with protein, carbohydrate and fat metabolism, secretory, pigmentary, inclusions of vitamins, etc. Not being a permanent component of the cytoplasm, inclusions reflect the patterns of metabolism of the corresponding tissues and organs. Thus, egg cells are characterized by protein inclusions of a certain chemical characteristic. The accumulation of glycogen inclusions in liver cells corresponds to the laws of the digestion process (Fig. 26). Fatty inclusions physiologically accumulate in fat cells of connective tissue (Fig. 27). Pigment cells of the skin epidermis contain melanin inclusions (Fig. 28). Vitamins and much more accumulate in the cells of various organs.