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cell theory[′sel ‚thē·ə·rē]
a fundamental biological generalization asserting the common origin and unity of the structural and developmental principle of the plant and animal worlds: the cell.
Cell theory asserts the unity of all living things and their evolutionary development. F. Engels called cell theory one of the three greatest discoveries ensuring the progress of natural science in the 19th century (The Dialectics of Nature, 1969, p. 168).
The discovery of the cell and the development of cell theory do not coincide historically. The English microscopist R. Hooke was the first to observe the cell structure of plants (in sections of cork and the stems of various living plants) under the microscope, describing his observations in his Micrographia (1665). The English botanist N. Grew believed that the cell wall was formed of interwoven fibers, as in textiles (from which the term “tissue” was derived in 1682).
In the 18th century, influenced by philosophy, the notion of the unity of the natural world became current in science. K. F. Wolff attempted to find something in common in the structure of plants and animals, but his notion of the commonality of the development of “vesicles,” “granules,” and “cells” was only the primitive forerunner of future cell theory, as were the ideas of the German scientist L. Oken on the construction of organisms from vesicles, or “infusoria.” At the beginning of the 19th century, progress in the microscopic study of plants made it clear that cells are not merely empty spaces in a continuous mass of plant matter but structures that have their own membranes and can be isolated from one another. In 1812, H. Link and J. Moldenhawer proposed that cells do not share a common wall but that each cell in fact has its own enveloping membrane. By the late 1820’s it became clear that almost all plant organs have a cellular structure, and in the textbook of the German botanist F. Meyen (1830) the cell figures as the common structural element of plant tissues. However, the cell was still assumed to be a chamber whose most important part was its membrane, its contents being of secondary significance.
The nucleus of a plant cell was described by R. Brown in 1831, but attention was first drawn to the nucleus by M. Schleiden, who considered it a cytoblast, a body that formed new cells. According to Schleiden, a nucleolus condenses from granular matter and the nucleus forms around the nucleolus; the cell, in turn, forms around the nucleus, which disappears in the process of cell formation. At about the same time, the work of the school of the Czech biologist J. Purkinje yielded a great deal of material on the microscopic structure of animal tissues; however, in his “granule theory,” Purkinje was unable to draw boundaries between the various granules (the term he applied to cells, nuclei, and sometimes even secretory inclusions).
Credit for formulating the cell theory belongs to T. Schwann, a student of the German biologist J. Müller. Schwann, who was familiar with Schleiden’s research, saw criteria in the nucleus for comparing the tissue structure of animal cells to that of plant cells. In 1839, Schwann published his Microscopic Investigations of the Correspondence in Structure and Growth of Animals and Plants (Russian translation, 1939). The title itself expresses the essence of the cell theory. However, Schwann continued to consider the membrane the chief component of the cell and accepted Schleiden’s false notion of the formation of new cells from structureless matter, or cytoblastema. The cell theory spread subsequently to include unicellular organisms, or protozoans, which were taken to be free-living cells (C. von Siebold, 1848).
The subsequent development of the cell theory is associated with the discovery of protoplasm and cell division. Toward the middle of the 19th century it became clear that the important part of the cell was its content, or protoplasm. In 1844, H. Mohl proposed a cellular model consisting of protoplasm and a nucleus. In 1858 the German pathologist R. Virchow published his Cellular Pathology, in which he extended cell theory to pathological phenomena and drew attention to the leading role of the nucleus in the cell, advancing the principle of cell formation by means of division (“each cell from a cell”).
Division was at first treated as a relacing of the nucleus and the cell body. Mitosis was discovered in the 1870’s and 1880’s to be the universal means of cell division, typical of all cellular organisms. Cell organoids were discovered at the end of the 19th century, and the cell ceased to be considered a simple lump of protoplasm. In addition, in the second half of the 19th century, a mechanistic treatment of the organism as the sum of its cells was outlined.
Modern cell theory proceeds from the unity of two concepts: (1) the division of multicellular organisms into individual cells and (2) the integrity of the organisms that is provided by the interaction of these cells. The more complex the organism, the more outstanding its integrity. In animals this integrity is made possible by neural and humoral systems; in plants, by direct cytoplasmic communication (by means of plasmodesmata and phytohormones). Investigations using the electron microscope have given strength to the basic propositions of cell theory. The universality of cell organoids in both plant and animal cells has been proved. It has been shown that there are organisms (procariotes) that have no formed nucleus (for example, bacteriophages, viruses, some bacteria, and algae). Because of the presence in them of DNA, some of these, such as bacteria and algae, are often called cells; however, it would be more correct to reserve the concept of the cell for organisms whose DNA is formed (as chromosomes) and is found only in the nucleus (eucariotes).
REFERENCESKatsnel’son, Z. S. Kletochnaia teoriia v ee istoricheskom razvitii. Leningrad, 1963.
Vermel’, E. M. Istoriia ucheniia o kletke. Moscow, 1970.
Z. S. KATSNEL’SON