Cytolysis

Cytolysis, destruction of cell structure, incompatible with its vital activity. This phenomenon, long known, has been studied extremely poorly, mainly on simple and plant cells and especially on the eggs of certain echinoderms, for example, Arbacia. Cytolytic agents are extremely diverse: mechanical and thermal effects, hypotonic and hypertonic environments, lipid solvents, electrolytes, electric currents, etc. The mechanism of cytolysis has been studied extremely little. In recent years, in connection with the development of colloidal concepts of protoplasm structure, it has been suggested that cytolysis is the result of protoplasm coagulation, one of the visible manifestations of which is the formation of vacuoles inside the cytolysed cell. When the contents of a crushed cell flow out, depending on the properties of the protoplasm and the ionic composition of the environment, a membrane often forms around the droplets of protoplasm, playing the role of a cell membrane. The formation of the latter is explained by some authors as the surface condensation of surface-active colloids of the protoplasm, as occurs, for example, in the formation of films in protein solutions and other surface-active substances. However, Heilbrunn, to whom a number of new works on this question belong, considers the formation of a fresh surface of protoplasm as the result of protoplasm coagulation, designating the latter as a surface precipitation reaction. From this point of view, cytolysis should be considered as the same precipitation reaction, but occurring not in the external environment relative to the cell, but inside the cell itself; the visible manifestation of this reaction is the formation of vacuoles inside the cell. Experiments have shown that this reaction occurs only in the presence of calcium ions, and therefore the release of these ions inside the cell, caused by the action of cytolytic agents, must precede cytolysis. It has long been suggested that cytolysis is associated with the destruction of protein-lipid complexes inside the cell, which may also be accompanied by the release of Ca++ ions, for example, under the action of lipid solvents. The next stage of cytolysis, according to Heilbrunn, is the interaction of Ca++ ions with pigment granules of the egg, as a result of which a substance is formed, which he named ovotrombin by analogy with blood thrombin; in both cases, this author sees a significant similarity in the mechanism of action. Finally, the interaction of ovotrombin with substances of the protoplasm (presumably protein) leads to a surface precipitation reaction. In addition to the similarity with blood clotting, this author also finds a significant similarity between the exit of pigment granules during egg cytolysis and the picture of blood hemolysis, insisting on the existence of common links in the mechanism of all three phenomena. Without mentioning that hemolysis of a blood corpuscle should be strictly distinguished from cytolysis, since the structural disturbances in both cases are of a different nature, the generalization of the mechanism of egg cytolysis at present should be considered at least premature. In the egg, the hypothetical ovotrombin is contained in the pigment granules, from which it is released under the action of a cytolytic agent (either directly or through interaction with calcium ions). The presence of substances similar to ovotrombin in cells of other species has not yet been proven, and yet the surface precipitation reaction is attributed to universality. Similarly, the analogy with phenomena accompanying blood clotting (action of Ca++ ions) does not yet allow both processes to be identified with each other. Nevertheless, the views presented above represent one of the few attempts at a theoretical approach to the phenomenon of cytolysis. Cytolysis is often characterized by a significant increase in the volume of the egg, especially under the action of salts of some polyvalent metals, and depending on the number of vacuoles formed and the general appearance of the cell, light and dark cytolysis are distinguished, according to Lebedev. The picture of cytolysis described above is an extreme expression of this process, which in a much more moderate form probably plays an important role in the physiological processes of the cell. A characteristic feature of such cytolysis is its reversibility, illustrated by the observations of Chambers. Puncturing the egg with a micropipette during colorimetric determination of intracellular pH causes rapidly disappearing acidification, which can be judged by the behavior of the indicator introduced into the cell. This acidification is the only visible manifestation of cytolysis. Cytolysis is also accompanied by a significant increase in oxidative processes in the egg, for example, when the ionic composition of the equilibrated environment is disturbed. This phenomenon, as in the case of egg activation, is probably associated with a violation of the structure of the egg's surface layer, which facilitates the access of enzymes to the substrate. Some observations also suggest that cytolysis should affect the oxidation-reduction potential of the cell. Indeed, the above-mentioned acidification of the cell during cytolysis should shift the redox potential of the cell in the positive direction. On the other hand, reducing substances released during cytolysis can shift the value of this potential in the negative direction. This may also explain the known inconsistency of the few facts known so far. Thus, the reducing ability of an amoeba significantly increases during cytolysis, while in the eggs of echinoderms, some authors find no change in the redox potential (I. Needham and D. Needham), while others (Chambers and others) find a strong shift in the positive direction, which they explain by the exit of reducing substances from the egg outward. These experiments should be carried out under identical conditions, since the indicated discrepancies may depend on the nature of the cytolytic agent and the degree of cytolysis. It is interesting to note in this connection that during cytolysis of the egg, the anaerobic potential in an acidic environment does not become more negative compared to the aerobic one, which indicates a possible destruction of the cell's oxido-reductase during cytolysis. Of the physiological phenomena in which reversible cytolysis plays an important role, mention should be made of egg activation (see Parthenogenesis), the initial stage of which is apparently the surface cytolysis of the egg. Many data speak in favor of this, in particular the fact that a cytolysed egg behaves like an activated one in terms of increased oxidative processes, mitogenetic radiation, permeability, etc. This kind of physiological cytolysis probably plays no less important a role in other types of cellular activity due to the close connection that exists between structural changes in the cell and the chemical processes occurring in it.