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the science devoted to the study of the cell (seeCELL). Cytology studies the cells of multicellular animals, plants, nucleocytoplasmic complexes that are not differentiated into cells (coenocytes, syncytia, plasmodia), unicellular animals and plants, and bacteria. It is fundamental to several biological disciplines, inasmuch as cell structures form the basis of the structure, functions, and individual development of all living things. It is also an important part of animal histology, plant anatomy, protistology, and bacteriology.
Before the 20th century. The development of cytology is associated with the development of methods of investigating cells. A cellular construction was first detected in some plant tissues by the British scientist R. Hooke (1665) using a microscope. The works of the microscopists M. Malpighi (Italy), N. Grew (Britain), and A. van Leeuwenhoek (the Netherlands), among others, which showed that many plant tissues are constructed of cells, appeared by the end of the 17th century. Leeuwenhoek was also the first to describe erythrocytes (1674), unicellular organisms (1675, 1681), spermatozoa of vertebrate animals (1677), and bacteria (1683). The 17th-century researchers, who laid the foundation for the microscopic study of organisms, saw in the cell only a membrane enclosing a cavity.
The design of the microscope was somewhat improved in the 18th century, mainly through the development of better mechanical parts and light sources. The technique of investigation remained primitive, and mostly dry preparations were used.
The ideas on the role of cells in the structure of organisms were considerably expanded in the first few decades of the 19th century. The studies of the German scientists H. Link, J. Molden-hawer, F. Meyen, and H. von Mohl and the French scientists C. de Mirbel and P. Turpin, among others, confirmed the view of botanists that cells are structural units. It was found that cells are converted into conducting elements in plants. Lower unicellular plants were discovered. Cells came to be viewed as individual objects possessing vital properties. In 1835, Mohl for the first time observed plant cell division. The studies of the French scientists H. Milne-Edwards, R. J. H. Dutrochet, and F. Raspail and the Czech scientist J. Purkinje yielded extensive information by the mid-1830’s on the microscopic structure of animal tissues. Many investigators observed the cellular construction of various animal organs, and some of them drew an analogy between the elementary structures of animal and plant organisms, thereby paving the way for the general biological cell theory (seeCELL THEORY).
In the period 1831–33 the British botanist R. Brown described the nucleus as a cell constituent, a discovery that attracted the attention of investigators to the cell contents and provided criteria for comparing animal and plant cells, which was done, in particular, by Purkinje (1837). The German scientist T. Schwann, relying on the theory of cell development proposed by the German botanist M. Schleiden that emphasized the importance of the nucleus, formulated a general cell theory of the structure and development of animals and plants (1838–39), which was soon also applied to protozoans (the German scientist C. von Siebold, 1845–48). The cell theory greatly stimulated the study of the cell as the basis of all living matter.
The development of immersion objectives (water immersion in 1850, oil immersion in 1878), E. Abbe’s condenser (1873), and apochromats (1886) in microscopy proved to be major steps in the development of cytology. Various methods of fixing and staining tissues were introduced in the mid-19th century. Techniques of embedding tissue specimens were devised for preparing sections; hand razors were used at first to prepare the sections, but in the 1870’s special instruments, called microtomes, were designed for this purpose.
As the cell theory evolved, the cell contents rather than the membrane gradually acquired greater importance. The idea that different cells have similar contents was reflected in the widespread use of the term “protoplasm” (coined by Purkinje in 1839) by Mohl (1844, 1846) for the contents. Despite the view of Schleiden and Schwann that cells originate from structureless noncellular material, called cytoblastema, scientists became increasingly convinced in the 1840’s that cells multiply by dividing (the German scientists K. von Nägeli, R. Kölliker, and R. Remak).
Further impetus for the development of cytology came from the concept of cellular pathology advanced by the German pathologist R. Virchow (1858). Virchow viewed the animal organism as an aggregation of cells, each cell possessing all the attributes of life, and enunciated the principle “omnis cellula e cellula” (“every cell originates from a cell”). Arguing against the humoral theory of pathology, which reduced disease to the spoiling of body juices, such as blood and tissue fluid, he demonstrated that every disease results from the dysfunction of the vital activities of various cells. Virchow’s views led pathologists to study cells.
The “membrane” period in the study of cells ended by the mid-19th century. In 1861 the work of the German scientist M. Schultze confirmed the view that the cell is “a globule of protoplasm surrounding a nucleus.” That same year, the Austrian physiologist E. Brücke, who considered the cell to be an elementary organism, demonstrated the structural complexity of protoplasm. A number of permanent constituents of protoplasm, organoids, were discovered in the last quarter of the 19th century: centrosomes by the Belgian scientist E. van Beneden (1876), mitochondria by the German scientist C. Benda (1897–98) in animals and the German scientist F. Meves (1904) in plants, and the Golgi apparatus by the Italian scientist C. Golgi (1898). The Swiss scientist F. Miescher discovered (1868) the presence of nucleic acid in cell nuclei. The karyokinetic division of cells (seeMITOSIS) was discovered, first in plants (the German botanist E. Strasburger, 1875) and then in animals (the Russian scientist P. I. Peremezhko, 1878; the German scientist W. Flemming, 1882). The theory of chromosome individuality was formulated, and the rule of the constant number of chromosomes was established (the Austrian scientist K. Rabl, 1885; the German scientist T. Boveri, 1887). The phenomenon of the reduction in the number of chromosomes during the development of sex cells was discovered, and fertilization was found to consist in the union of the nuclei of an egg cell and a spermatozoon (discovered in animals by the German zoologist O. Hertwig, 1875, and in plants by the Russian botanist I. N. Gorozhankin, 1880–83). In, 1898 the Russian cytologist S. G. Navashin discovered double fertilization in angiosperms, which consists in the union of a sperm -ucleus with an egg cell nucleus and the union of a second sperm nucleus with the nucleus of an endosperm-producing cell. Diploid (asexual) and haploid (sexual) generations were found to alternate in plant reproduction.
New advances in cell physiology were achieved. In 1882, I. I. Mechnikov discovered phagocytosis. The selective permeability of plant and animal cells was discovered and thoroughly investigated (the Dutch scientist H. De Vries; the German scientists W. Pfeffer and C. E. Overton). The membrane theory of permeability was formulated. Techniques of staining living cells were developed (the Russian histologist N. A. Khrzhonshchevskii, 1864; the German scientists P. Ehrlich, 1885, and Pfeffer, 1886). The reactions of cells to various stimuli were studied. Studies on different cells of higher and lower organisms strengthened the view that the structure of protoplasm is based on a single principle, despite all the structural and functional differences between the cells. Many investigators were dissatisfied with the cell theory, believing that cells contain even tinier living elementary units (for example, Altmann’s bioblasts, Wiesner’s plasomes, and Heidenhain’s protomeres). Speculative ideas regarding living submicroscopic units were also shared by some 20th-century cy-tologists, but new advances in cytology compelled most scientists to abandon these hypotheses and assume that life is the property of protoplasm, which is a complex heterogeneous system.
Advances in cytology at the end of the 19th century were summarized in a number of classical works that led to further developments in the field; these include E. B. Wilson’s The Cell in Development and Heredity (New York, 1928), M. Heidenhain’s Plasma und Zelle (1907), R. Höber’s Physikalische Chemie der Zelle und der Gewebe (1902), and M. Verworn’s Allgemeine Physiologie (1895).
First half of the 20th century. During the first decades of the 20th century, the dark-field condenser, used for viewing objects under a microscope with oblique illumination, came into use, enabling scientists to study the degree of dispersion and hydration of cell structures and detect some submicroscopic structures. The polarizing microscope made it possible to determine the orientation of particles in cell structures. Microscopy in ultraviolet rays, introduced in 1903, soon became an important means of studying the cytochemistry of cells, specifically, the nucleic acids. Fluorescence microscopy also came into use. In 1941 the phase-contrast microscope appeared, enabling scientists to distinguish colorless structures differing from one another only in optical density or thickness. The last two techniques proved to be exceptionally valuable for studying living cells.
New methods of cytochemical analysis were also devised, among them a technique for detecting deoxyribonucleic acid (the German scientists G. Feulgen and H. Rossenbeck, 1924). Micro-manipulators were designed to perform a variety of operations on cells, such as injection of substances into a cell, extraction and transplantation of nuclei, and infliction of local injury to cell structures. Of great importance was the technique of culturing tissues in vitro, first used by the American scientist R. Harrison in 1907. Interesting results were obtained by combining this method with time-lapse microphotography, which made it possible to project on a screen slow, normally imperceptible, changes in cells accelerated tens and hundreds of times.
The efforts of scientists during the first three decades of the 20th century were directed at elucidating the functions of the cell structures discovered in the last quarter of the 19th century. Specifically, the Golgi apparatus was found to participate in the production of secretions and other substances in granular form (the Soviet scientist D. N. Nasonov, 1923). N. K. Kol’tsov described (1903–11) individual organoids of specialized cells and “skeletal” elements in a number of cells. Structural changes were studied in different types of cell activity (secretory process, contraction, division, morphogenesis). The development of the vacuolar system in plant cells and the formation of starch in plastids were traced (the French scientist A. Guilliermond, 1911).
The number and shape of chromosomes were found to be species specific, a fact later used to classify plants and animals and to clarify phylogenetic relationships within lower taxonomic units (karyosystematics). It was discovered that tissues possess different classes of cells, distinguished by a multiple size-to-size ratio of their nuclei (the German scientist W. Jacobi, 1925). The multiple increase of nuclei entails a corresponding increase (by endomitosis) in the number of chromosomes (the German scientist L. Geitler, 1941). Studies of the effect of agents that disrupt the mechanism of division and the chromosomal apparatus of the cell, such as penetrating radiation, colchicine, acetonaphthene, and trypaflavine, led to the development of methods of obtaining polyploid forms artificially, thereby making it possible to breed a number of valuable plant varieties. The use of the Feulgen reaction helped answer the thorny question of whether there is a homologue of a deoxyribonucleic acid-containing nucleus in bacteria (the Soviet scientist M. A. Peshkov, 1939–43; the French scientist B. Delaporte, 1939; the British scientist C. Robinow, 1942) and blue-green algae (the Soviet scientists Iu. I. Polianskii and Iu. K. Petrushevskii, 1929).
In addition to the membrane theory of permeability, some scientists advanced the phase theory, in which considerable importance in the distribution of substances between the cell and extracellular fluid is attached to the dissolution and binding of the substances in the protoplasm (the Soviet scientists D. N. Naso-nov, V. Ia. Aleksandrov, and A. S. Troshin). The study of the reaction of protoplasm to various physical and chemical agents led to the discovery of the phenomenon of paranecrosis and the formulation of the denaturation theory of injury and excitation (Na-sonov and V. Ia. Aleksandrov, 1940), whereby reversible changes in the structure of protoplasm proteins are leading factors in these processes. The localization of several enzymes in the cell was established with the help of newly developed cytochemical reactions using histological preparations. On the basis of the work of the American scientists R. Bensley and N. Hoerr, who used the technique of cell homogenization (grinding) and differential centrifugation, researchers, beginning in 1934, extracted individual components from cells—nuclei, chloroplasts, mitochondria, and microsomes—and studied their chemical and enzymic compositions. However, major advances in understanding the functions of cell structures were not achieved until the advent of the modern period of cytology, after the 1950’s.
The rediscovery of Mendel’s law in 1900 had an enormous impact on cytology. The study of the processes that occur in the nuclei of sex and somatic cells helped clarify the facts obtained by research on the hereditary transmission of characters and to formulate the chromosome theory of heredity. The study of the cytological basis of heredity evolved into a separate branch of cytology—cytogenetics (see).
Modern cytology. The birth of the modern era of cytology dates to the 1950’s. The advent of new research techniques and the advances made in related branches of science stimulated the rapid development of cytology and led to a blurring of the boundaries between cytology, biochemistry, biophysics, and molecular biology. The use of the electron microscope (its resolving power is 2–4 angstroms, compared to the maximum resolution of the light microscope of about 2,000 angstroms) resulted in the emergence of submicroscopic cell morphology and brought the visual study of cell structures to the macromolecular level. Hitherto unknown structural details of previously discovered cell organoids and nuclear structures were observed. New ultramicroscopic cell components were discovered: the plasma, or cell, membrane, which separates the cell from the surrounding medium; the endoplasmic reticulum; ribosomes, which synthesize proteins; lysosomes, which contain the hydrolytic enzymes; peroxysomes, which contain the enzymes catalase and urokinase; and microtubules and microfilaments, which help maintain the shape and assure the motility of cell structures. Dictyosomes, which are elements of the Golgi apparatus, were found in plant cells. Ultramicroscopic elements and features characteristic of specialized cells were detected along with the common cell structures. Electron microscopy revealed the special significance of membranes in the construction of the various cell components. Submicroscopic studies made it possible to divide all known cells and consequently all organisms into two groups: the eucaryotes (tissue cells of all multicellular organisms and unicellular animals and plants) and procaryotes (bacteria, blue-green algae, actinomycetes, and rickettsiae). Procaryotes are primitive cells, differing from eucaryotes in their lack of a typical nucleus, nucleolus, nuclear membrane, typical chromosomes, mitochondria, and Golgi apparatus.
Improved techniques for isolating cell components and the use of methods of analytical and dynamic biochemistry (precursors tagged with radioactive isotopes, autoradiography, quantitative cytochemistry using cytophotometry, cytochemical techniques for electron microscopy, use of antibodies tagged with fluorochromes to detect the site of individual proteins under a fluorescence microscope, the hybridization on sections and smears of radioactive DNA and RNA to identify the cells of nucleic acids) led to a more precise determination of the chemical makeup of cells and the clarification of the functions and biological role of many cell constituents. This required extensive coordination of research in cytology with research in biochemistry, biophysics, and molecular biology. The discovery of DNA not only in the nucleus but in the cytoplasmic elements of the cell (mitochondria, chloroplasts) and, according to some data, in the basal corpuscles as well was of great importance for the study of the genetic role of the cell. Nuclei and mitochondria are transplanted to evaluate the role of the nuclear and cytoplasmic genetic apparatus in determining the hereditary properties of the cell. Hybridization of somatic cells is a promising method for studying the gene composition of individual chromosomes (seeSOMATIC CELLS, GENETICS OF). Substances were found to penetrate into cells and cell organoids by means of special transport systems that render biological membranes permeable. Electron-microscopic, biochemical, and genetic studies have increased the number of supporters of the hypothesis of the symbiotic origin of mitochondria and chloroplasts that was advanced at the end of the 19th century (seeSYMBIOGENESIS).
PRINCIPAL OBJECTIVES OF MODERN CYTOLOGY. The principal goals of modern cytology include the continued study of microscopic and submicroscopic cell structures, the chemical organization of cells, the functions of cell structures and their interactions, the ways in which substances penetrate into cells and may be isolated from cells and the role of membranes in these processes, the reactions of cells to neural and humoral stimuli of the macroorganism and to stimuli from the surrounding medium, the perception and conduction of excitation, the interaction between cells, the reactions of cells to injury, and the repair of injuries and the adaptation of cells to environmental factors and injurious agents. Cytology is also conducting further research in the reproduction of cells and cell structures, the transformation of cells in the course of morphophysiological specialization (differentiation), the nuclear and cytoplasmic genetic apparatus of the cells and of changes therein that cause hereditary diseases, the relationship between cells and viruses, the transformation of normal cells into malignant ones, cell behavior, and the origin and development of the cell system. In addition to the theoretical aspects, cytology is also working toward a solution of several very important biological, medical, and agricultural problems. A number of branches of cytology have emerged, depending on the objects being studied and the methods of study used; they include cytogenetics, karyosystematics, cytoecology, radiation cytology, oncological cytology, and immunocytology.
Among the specialized research institutions in the USSR in cytology are the Institute of Cytology of the Academy of Sciences of the USSR, the Institute of Cytology and Genetics of the Siberian Division of the Academy of Sciences of the USSR, and the Institute of Genetics and Cytology of the Academy of Sciences of the Byelorussian SSR. Many biological, medical, and agricultural institutions have special cytological laboratories. Research in cytology is coordinated by the Scientific Council on Problems in Cytology of the Academy of Sciences of the USSR. Among the journals published are Tsitologiia (Academy of Sciences of the USSR) and Tsitologiia i genetika (Academy of Sciences of the Ukrainian SSR). Cytological studies are also published in the journals of the allied disciplines.
More than 40 cytological journals are published throughout the world. Multivolume international publications are issued periodically, such as Protoplasmatologia (Vienna) and the International Review of Cytology (New York). The International Society of Cell Biology regularly holds congresses. The International Cell Research Organization and the European Cell Biology Organization appoint special working panels and study groups to deal with individual cytological problems, organize training courses on the principal topics and cytological methodology, and facilitate the exchange of information.
Biology and soil biology departments in Soviet universities offer courses in general cytology, while many universities also offer courses on different aspects of cytology. Cytology is also included in courses on animal histology, plant anatomy, embryology, protistology, bacteriology, physiology, and pathological anatomy that are taught in agricultural, pedagogical, and medical schools.
REFERENCESKatsnel’son, Z. S. Kletochnaia teoriia v ee istoricheskom razvitii. Leningrad, 1963.
Rukovodstvopo tsitologii, vols. 1–2. Moscow-Leningrad, 1965–66.
De Robertis, E., W. Nowinski, and F. Saez. Biologiia kletki. Moscow, 1973. (Translated from English.)
Brown, W. V., and E. M. Bertke. Textbook of Cytology. Saint Louis, 1969.
Hirsch, G. S., H. Ruska, and P. Sitte. Grundlagen der Cytologie. Jena, 1973.
V. IA. ALEKSANDROV