Cell Theory

Cell Theory, the classical doctrine of the microscopic structure of all plant and animal organisms from special elementary units - cells. According to this doctrine, these units, while composing the organism, are themselves to a certain degree independent living beings, corresponding morphologically to unicellular plants and animals. - In its essence, Cell Theory represents a simple expression of the factual composition of all higher organisms from areas called cells. The first indications of the cellular structure of organisms appeared in the field of botany. In one chapter of his major work "Micrographia," Robert Hooke (1635-1673) describes the microscopic structure of thin slices cut from cork. He observed in such slices the presence of countless tiny cavities representing chambers closed on all sides. Hooke called these chambers "pores" or "cells," and compared the structure of cork to that of honeycombs.-Marcello Malpighi (1628-1694) devoted much more attention to the same question and can largely be considered the founder of the factual side of Cell Theory. In his "Anatomy of Plants," Malpighi distinguishes in the plant body a whole range of different tissues, describing the parenchymatous tissue as consisting of numerous tiny "sacs" (utriculi), which correspond to the cells of modern biology. - The beginning of the 19th century was marked by the flourishing of plant anatomy, which was also reflected in the doctrine of cells. At this time, Moldenhawer isolates individual plant cells and vessels through maceration; Treviranus (1779-1864), and somewhat later Hugo von Mohl (1805-1872), establish the origin of vessels in plants through the fusion of several initially independent cells and reduce the various types of plant tissues to formations of one basic form, namely cells. Thus, as early as the 1830s, Meyen (Meyen) in his botany textbook ("Phytotomie") describes the cell as "a space surrounded on all sides by a plant shell," and states that plant cells may occur singly, forming separate individuals, as for example in lower algae and fungi, or join in masses to form the body of higher plants. However, Meyen recognizes in the plant body three different types of elementary organs: cells, spiral tubes, and vessels. At the same time, knowledge of the structure of the cell itself deepened: Robert Brown discovered the cell nucleus in many plants in 1833, and Dumortier (1832) and Morren (1836) found in unicellular algae the process of cell division.-On the basis of the factual material described above grew the views of Schleiden, who is commonly considered the founder of modern Cell Theory. In his work "Beitrage zur Phytogenesis," published in 1838, Schleiden first notes the importance of the question of the mode of origin of cells for the proper evaluation of "these peculiar little organisms." "Each cell," says Schleiden, "leads a dual life: a completely independent one, belonging to its own development, and a dependent one, insofar as it is an essential component of the whole plant." In these words, the main positions of classical Cell Theory are fully expressed. Curiously, the factual side of Schleiden's work does not at all correspond to the views he develops and has only historical interest. Schleiden finds that new cells are supposedly formed within already existing cells due to clumps of "granular mucus," in which first small nucleoli become isolated, then around them the nucleus of the future cell, and finally still later the cell itself. The cell in which a new generation of cells is laid down, according to Schleiden, stretches and dissolves. Despite such errors in observations, Schleiden's work was of great importance, as it formed the basis of the famous work of the zoologist Schwann "Microscopic Researches on the Accordance in the Structure and Growth of Animals and Plants" (1839), in which Cell Theory was first extended to the tissues of animal organisms.-Before Schwann, there were only separate, scattered indications of the presence of cells in the animal organism. Thus, Henle (1838) observed cellular structure in epithelia, and Purkinje (1837) describes the composition of many glands, epithelia, spleen, etc., as consisting of "grains," under which he obviously means cells. The guiding principle for Schwann's work is the idea of complete morphological equivalence of all animal and plant cells. Proceeding from this principle, Schwann conducted a microscopic analysis of the animal body. Therefore, he sees in the animal egg nothing other than a cell; the embryo also consists entirely of cells; in the same way, Schwann proves that all tissues of adult animals - epithelium, nails, claws, feathers, cartilage, bone, teeth, muscles, nerves, capillaries - are composed of cells or of products of their transformation. The equivalence of cells of different organs and organisms is recognized by Schwann not only on the basis of their morphological similarity, but also due to the uniformity of their mode of origin. This idea is very important, as it formed the basis of later views on the homology of plant and animal cells. It should be noted, however, that Schwann interpreted the reproduction of cells incorrectly. On the one hand, he recognized the intracellular origin of new cells, described by Schleiden, on the other hand, he observed the appearance of new cells between old ones, i.e., in the intercellular substance, thus endowed with all the properties of life. - In Schwann's work, the idea of the organism as a state of cells begins to emerge - an idea that later became widespread. Each cell, according to Schwann, possesses its own life, its own energy, so that the organism as a whole exists only as a result of the interaction of its constituent parts. The question of the basic vital forces of the organism, in Schwann's opinion, reduces to the question of the basic forces of individual cells.-In evaluating the composition of tissues from cells, Schwann correctly followed the presence of nuclei, considering that where there is a nucleus, the presence of the corresponding cell should also be assumed. However, even after the publication of Schwann's work, nuclei were not found in all cells for some time, and the most characteristic feature of the cell was still considered its shell, not its living contents. However, in 1844, Kolliker finally proved the cellular nature of blastomeres of the cleaving egg, tracing without interruption their transition through a series of generations into the true cells of the embryo. He was also the first to point out that the cell shell is most likely a secondary derivative of cells, and distinguished "proper cells," i.e., cells without shells, from "Umhullungszellen," i.e., cells clothed in a clear membrane.-In 1850, the botanist Alexander Braun further emphasized the importance of the cell contents, saying that the term "cell" is best understood as the contents of plant cells, not their surrounding shell. Remak (R. Remak; 1855) finally established the relationship between cells and their shells, saying that the cell shell can be imagined as the result of the thickening of the outer layer of the cell's basic substance, i.e., protoplasm. To this same scientist belongs the merit, as early as 1852, of refuting the doctrines of Schleiden and Schwann on the mode of origin of new cells. Remak categorically denies their origin from formless living substance, asserting that new cells always arise through the division of pre-existing cells, accompanied by the division of the nuclei of these latter.-To the same period belong the works of R. Virchow in the field of cellular pathology. Virchow finally buried the doctrine of formless living substance, or cytoplasm, and clothed the result of Remak's embryological research in the famous formula: omnis cellula e cellula, i.e., every cell arises only from a pre-existing cell. By the 1860s, Cell Theory can be considered fully formed; its meaning was as follows: every organism is a complex of units of a lower order, cells, forming a kind of cellular state and multiplying by division. The concept of the cell developed by this time finds its expression in the work of Schulze (M. Schulze; 1861): by cell is meant a spatially limited piece of living substance (protoplasm) with a nucleus, which in morphological and physiological respects possesses the properties of an elementary individual. In favor of the considerable independence of each cell at that time spoke a whole series of arguments, such as the comparison of tissue cells with free-living unicellular animals and plants, the unicellular nature of egg cells, the great independence of the behavior of leukocytes within the body of multicellular organisms, the ability of cells to multiply by division similar to the unicellular organism Protozoa. The independence of cells was also evidenced by observations of the death of individual cells in a multicellular organism, which does not affect the life of the organism as a whole.

At the present time, to this series of arguments can be added the brilliant achievements of tissue culture, which allows cells artificially isolated from a multicellular organism to live and reproduce in culture for an indefinitely long time. The morphological equivalence of the cell to an independent individual received a particularly clear formulation on the basis of the evolutionary theory, when all multicellular organisms, following the initiative of Haeckel (1866), began to be interpreted as a collection of phylogenetically equivalent individuals derived from colonies of Protozoa. On the same ground, the doctrine of multicellular organisms as 'cell states' also received particular development. The essence of this doctrine is that the formation of multicellular organisms from colonies of Protozoa was based on the gradual specialization of the colony's cells in different directions through physiological division of labor. This specialization leads to the differentiation in the bodies of higher organisms of several types of different cellular tissues. Similarly, as it were, as in a state the existence of individuals depends on the conditions of existence of other members of the state and the state as a whole, so to an even greater degree there exists a mutual dependence of the cells of the bodies of higher animals, which were once completely independent organisms. This phenomenon of the subordination of the life of originally independent elementary organisms to the requirements of the multicellular organism as a whole is designated by H. Spencer as 'integration': cells become integrating parts of a single higher-order organism. In approximately this form, the classical Cell Theory has been preserved to this day in its orthodox interpretation. However, during the past 50 years, several attempts have been made to introduce a number of essential changes into the Cell Theory. The most important of these attempts are the following. 1. Extension of the concept of elementary organisms to units of a lower order than cells. As early as 1861, Brücke in his article on elementary organisms expressed the hypothesis that 'cells themselves may consist of other, even smaller organisms, which stand to them in the same relation as the cells themselves to the whole organism.' In the 1880s, Altmann began to develop his theory of the granular structure of protoplasm, the main essence of which is that the cell cannot be considered that elementary morphological and biological unit to the activity of which the physiological functions of the entire multicellular organism can be reduced. On the contrary, the cell should be regarded in turn as a complex formation, namely as a colony of elementary living beings from the group of bacterial microorganisms. These elementary beings, called bioblasts by Altmann, are represented in the cell by an extremely large number of small grains or granules filling all the plasma. Bioblasts are characterized by the ability to assimilate, grow, and, what particularly brings them closer to independent organisms, the ability to reproduce by division. Hence follows Altmann's assertion that every grain originates only from another grain (omne granulum e granulo). The granules can differentiate and specialize in different directions, performing various life functions of the cell. According to Altmann's original idea, the intergranular substance is dead, and the cells themselves arose from colonies of free-living micrococci immersed in a lump of mucus secreted by them, or zoogloe, which subsequently turned into the intergranular substance. - It is not difficult to see that Altmann, in his theory, extended the stereotyped view of the cellular structure of organisms and of the organism as a cell state to the cell itself. As for the factual substantiation of Altmann's doctrine, besides the widespread occurrence of granular inclusions in very different cells, only a small amount of data speaks in its favor. It is true that in cells there are several types of granules that have the ability to reproduce by division (chlorophyll grains of plants, chondriosomes), but their number is very small, and Altmann's viewpoint currently has few supporters. Altmann's theory apparently influenced M. Heidenhain in his doctrine of the composition of each organism from so-called biosystems. Heidenhain introduces the following changes into the Cell Theory. He states that multicellular organisms consist of a whole series of structural systems of different orders, each system being characterized by the ability to reproduce by division or fission. These systems in descending order are approximately as follows: 1) muscles, tendons, nerves, parts of the skeleton, glandular organs; 2) cells, primitive muscle bundles, nerve fibers; 3) nuclei; 4) chromosomes, centrosomes, myofibrils, neurofibrils, etc.; 5) chromomeres, centrioles, chromatophores. Some of these systems, for example cells, correspond in their past to independent free individuals of Protozoa, while most systems are not homologous but only analogous to living individuals in their mode of reproduction by division. Each biosystem, or as Heidenhain also calls it, histosystem, consists of systems of a lower order, which with respect to it are subordinate parts, or histomers, and with respect to units of an even lower order are themselves histosystems. An essential feature of Heidenhain's histosystems is the heterogeneity of their histomers, in contrast to the Cell Theory, according to which the entire organism is interpreted as an aggregate of homogeneous units, cells. Heidenhain's histomers also possess this property, distinguishing them from Altmann's granules or micrococci. As can be seen from the list given, Heidenhain does not consider the cell the last link in his histological ladder, interpreting it as a complex histosystem. All histosystems are united by the fact that at the basis of the lowest-order histomers are, according to Heidenhain's assumption, the smallest metamicroscopic bodies, protomers, which are the ultimate formal elements of all living things; these are probably simply large molecules or groups of molecules possessing basic life properties. Thus, the cornerstone of Heidenhain's theory is not the cell and the cellular composition of organisms, but the concept of living substance consisting of the smallest protomers. To complete the distinction from the Cell Theory, Heidenhain recognizes as living substance not only the protoplasm of the body cells but to a large extent also various intercellular substances; the latter, in his opinion, also possess life properties and are designated by a special term 'metaplasm,' and are not merely a product of the secretory activity of cells. - Heidenhain's views are based much more on speculative concepts than on observed facts. With some strain, Heidenhain's reasoning could perhaps be brought closer to modern views on the micellar structure of protoplasm; but in any case, Heidenhain's doctrine currently has only historical interest. 2. Recognition of the secondary nature of the division of multicellular organisms into cells. Among the scientists defending this viewpoint are primarily Sedgwick (1894) and Whitman. They consider the cellular structure of higher organisms as a certain physiological condition for the existence of a large and multinucleated plasma body, but deny any phylogenetic significance to it. By this, the aforementioned scientists do not recognize the origin of multicellular organisms from colonies of unicellular organisms, and thus deny the interpretation of the organism as a cell state. Sedgwick bases his reasoning on the initial stages of development of Peripatus (from the primary tracheates), during which the division of the embryo into cells is very weakly expressed and the cells communicate with each other through wide bridges. The widespread occurrence in animal tissues of so-called intercellular bridges also speaks against the presence of sharp boundaries between individual cells and in favor of a certain integrity of the entire plasma mass forming the multicellular organism. - Driesch (1907), guided mainly by theoretical considerations as well as his experiments on the development of echinoderms, states that 'all attempts to conceive of the organism as a simple collection of cells are erroneous.' The same thoughts are echoed in the words of some botanists, for example de Bary, who says that 'the plant forms cells, and not cells - the plant.' Finally, Lillie's experiments on the development of the egg of the worm Chaetopterus show that under certain conditions (under the influence of lithium chloride) the worm's egg gives a fully differentiated larva, formed however by a single common plasma mass in which lie numerous nuclei. From this Lillie concludes that the process of dividing the body into cells is not necessary as such either for growth or for the early stages of embryonic differentiation, but is in Metazoa only a secondary, additional factor. Dobell particularly dwells on the question of cellular structure in 1911 in his interesting article on 'the principles of protistology.'

The basic position of Dobell states that 'an individual of the simplest organisms is in the same measure a complete and perfect individual as is any multicellular organism.' Meanwhile, researchers of the simplest organisms, under the influence of the cell theory, ordinarily considered Protozoa exclusively as cells, without paying attention to their individuality. 'If the expressed thought is correct,' Dobell continues further, 'then it is by no means possible to homologize the organism of the simplest with the cell of Metazoa, just as it is impossible to equate the whole (i.e., the simplest as an individual) with parts, which are the cells in the body of multicellular organisms.' Under the name 'cell' are usually mixed three completely equivalent concepts, since this name is given 1) to the whole organism ('cells' of the simplest); 2) to a part of the organism (cells of tissues in Metazoa and Metaphyta) and 3) to the whole organism in potency (fertilized egg). In connection with this, Dobell proposes to retain the name 'cell' only for the cells of multicellular organisms. As for the evaluation of the significance of the simplest organisms, Dobell says that there are 2 sorts of organisms. The plasma body of some is divided by partitions into sections, 'cells,' arranged around nuclei; in others the body (mono- or multinucleate) does not show such a division. The first possess cellular structure, while the latter, i.e., Protozoa, should logically be called acellular. Just as it would be incorrect to call an unsegmented worm, for example ascaris, unisegmented, so too it would be erroneous to call an acellular organism Protozoa unicellular. Dobell also finds it impossible to designate the fertilized egg with the term 'cell,' since it in potency already contains a whole organism, whereas a cell is always only a part of an organism. On the contrary, the first two blastomeres, into which the egg divides, can already be considered true cells. Dobell proposes to simply call the egg 'egg' (ovum), recognizing its acellular structure. The classical hypothesis about the origin of Metazoa and Metaphyta from colonies of protists with subsequent differentiation of individuals making up this colony into tissue cells of different structure and with different functions has long been subjected to doubt (e.g., from the side of Delage in the introduction to his large multi-volume guide to zoology). Along with this hypothesis, another is put forward, asserting that 'multicellular' organisms developed from multinucleate 'unicellular' organisms by differentiation of nuclei with subsequent differentiation of protoplasm around them; this led to the formation in the Metazoa organism of separate cells, more or less connected with each other. As an example of transitional forms illustrating this hypothesis, one can point to Cnidosporidia. They are invariably classified as Protozoa, but in reality during the development of their spores we observe differentiation of individual nuclei with adjacent areas of protoplasm into cells that form the cyst, spore shell, and spines. It would be unlikely to assume that such spores developed from colonies of unicellular organisms. It is much more probable to see here the division of a unicellular (according to Dobell, 'acellular') organism into cells. And if this process took place in this case, it could also have led to the emergence of true Metazoa and Metaphyta, at least of a known part of them. One should not also forget that life processes proceed in a living organism not only in cells but also in that internal medium which bathes all cells and tissues, connecting them with each other. The organism as a whole is qualitatively distinct from the sum of all its constituent cells. Therefore, the life of isolated organs or tissue cultures cannot be equated to the life of a whole organism possessing the ability to independently maintain its existence. The internal medium regulating the life of the organism as a whole (intercellular juice, lymph, blood plasma) has a complex chemical composition, representing a balanced solution of electrolytes, proteins, enzymes, and hormones. This internal medium of the organism is not taken into account by the classical cell theory, and therefore there can be no doubt that our modern views on the structure of higher organisms from cells differ sharply from the views of the era not only of Schleiden-Schwann but also of Virchow-Haeckel.