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Bible Encyclopedias
Cytology
1911 Encyclopedia Britannica
or (from hums, a hollow vessel, and Xeyos, science), the scientific study of the " cells " or living units of protoplasm, of which plants and animals are composed. All the higher, and the great majority of the lower, plants and animals are composed of a vast number of these vital units or " cells." In the case of many microscopic forms, however, the entire organism, plant or animal, consists throughout life of a single cell. Familiar examples of these " unicellular " forms are Bacteria and Diatoms among the plants, and Foraminifera and Infusoria among the animals. In all cases, however, whether the cell-unit lives freely as a unicellular organism or forms an integral part of a multicellular individual, it exhibits in itself all the phenomena characteristic of living things. Each cell assimilates food material, whether this is obtained by its own activity, as in the majority of the protozoa, or is brought, as it were, to its own door by the blood stream, as in the higher Metazoa, and builds this food material into its own substance, a process accompanied by respiration and excretion and resulting in growth. Each cell exhibits in greater or less degree "irritability," or the power of responding to stimuli; and finally each cell, at some time in its life, is capable of reproduction. It is evident therefore that in the multicellular forms all the complex manifestations of life are but the outcome of the co-ordinated activities of the constituent cells. The latter are indeed, as Virchow has termed them, " vital units." It is therefore in these vital units that the explanation of vital phenomena must be sought (see Physiology). As Verworn 1 said, " It is to the cell that the study of every bodily function sooner or later drives us. In the muscle cell lies the problem of the heart beat and that of muscular contraction; in the gland cell reside the causes of secretion; in the epithelial cell, in the white blood corpuscle, lies the problem of the absorption of food, and the secrets of the mind are hidden in the ganglion cell." So also the problems of development and inheritance have shown themselves to be cell problems, while the study of disease has produced a " cellular pathology." The most important problems awaiting solution in biology are cell problems.
Historical
The cell-theory ranks with the evolution theory in the far-reaching influence it has exerted on the growth of modern biology; and although almost entirely a product of the 19th century, the history of its development gives place, in point of interest, to that of no other general conception. The cell-theory - in a form, however, very different from that in which we now know it - was originally suggested by the study of plant structure; and the first steps to the formulation, many years later, of a definite cell-theory, were made as early as the later part of the 17th century by Robert Hooke, Marcello Malpighi and Nehemiah Grew. Hooke (1665) noted and described the vesicular nature of cork and similar vegetable substances, and designated the cavities by the term " cells." A few years later Malpighi (1674) and Grew (1682), still of course working with the low power lenses alone available at that time, gave a more detailed description of the finer structure of plant tissue. They showed that it consisted in part of little cell-like cavities, provided with firm cell-walls and filled with fluid, and in part of long tube-like vessels. A long time passed before the next important step forward was made by C. L. Treviranus, 2 who, working on the growing parts of young plants, showed that the tubes and vessels of Malpighi and Grew arose from cells by the 1 Allgemeine Physiologie, p. 53 (1895). I Vom inwendigen Bau der Geweichse (1806).
latter becoming elongated and attached end to end, the intervening walls breaking down; a conclusion afterwards confirmed by Hugo von Mohl (1830). It was not, however, until the appearance of Matthias Jakob Schleiden's paper Beitrage zur Phytogenesis (1838) that we have a really comprehensive treatment of the cell, and the formulation of a definite cell-theory for plants. It is to the wealth of correlated observations and to the philosophic breadth of the conclusions in this paper that the subsequent rapid progress in cytology is undoubtedly to be attributed. Schleiden in this paper attempted to solve the problem of the mode of origin of cells. The nucleus (vide infra) of the cell had already been discovered by Robert Brown (1831), who, however, failed to realize its importance. Schleiden utilized Brown's discovery, and although his theory of phytogenesis is based on erroneous observations, yet the great importance which he rightly attached to the nucleus as a cell-structure made it possible to extend the cell-theory to animal tissues also. We may indeed date the birth of animal cytology from Schleiden's short but epoch-making paper. Comparisons between plant and animal tissues had already been made by several workers, among others by Johannes Muller (1835), and by F. G. J. Henle and J. E. Purkinje (1837). But the first real step to a comprehensive cell-theory to include animal tissues was made by Theodor Schwann. This author, stimulated by Schleiden's work, published in 1839 a series of Mikroskopische Untersuchungen fiber die Obereinstimmung in der Structur and dem Wachstum der Tiere and Pflanzen. This epoch-making work ranks with that of Schleiden in its stimulating influence on biological research, and in spite of the greater technical difficulties in the way, raised animal cytology at one blow to the position already, and so laboriously, acquired by plant cytology. In the animal cell it is the nucleus and not the cell-wall that is most conspicuous, and it is largely to the importance which Schwann, following the example of Schleiden, attached to this structure as a cell constituent, that the success and far-reaching influence of his work is due. Another feature determining the success of Schwann's work was his selection of embryonic tissue as material for investigation. He showed that in the embryo the cells all closely resemble one another, only becoming later converted into the tissue elements - nerve cells, muscle cells and so forth - as development proceeded; just as a similar mode of investigation had enabled Treviranus to trace the origin from typical cells of the vascular tissue in plants more than 30 years previously. And just as Treviranus showed that there was a union of cells to form the vessels in plants, so Schwann now showed that a union of cells frequently occurred in the formation of animal tissues.
So great was the stimulus given to cytological research by the work of Schleiden and Schwann that these authors are often referred to as the founders of the cell-theory. Their theory, however, differed very greatly from that of the present time. Not only did they suppose new cells to arise by a sort of " crystallization " from a formative " mother liquor " or " cytoblastema " (vide infra), but they both defined the cell as a " vesicle " provided with a firm cell-wall and with fluid contents. The cell-wall was regarded as the essential cell-structure, which by its own peculiar properties controlled the cell-processes. The work of Schleiden and Schwann marks the close of the first period in the history of the cell-theory - the period dominated by the cell-wall. The subsequent history is marked by the gradual recognition of the importance of the cell-contents. Schleiden had noticed in the plant cell a finely granular substance which he termed " plant slime " (Pflanzenschleim). In 1846 Hugo von Mohl applied to this substance the term " protoplasm "; a term already used by Purkinje six years previously for the formative substance of young animal embryos. Mohl showed that the young plant cell was at first completely filled by the protoplasm, and that only later, by the gradual accumulation of vacuoles in the interior, did this substance come to form a thin layer on the inner surface of the cell-wall. Mohl also described the spontaneous movement of the protoplasm, a phenomenon already noted by Schleiden for his plant slime, and originally discovered by Bonaventura Corti in 1772 for the cells of Chara, and rediscovered in 1807 by Treviranus. Not only was attention thus gradually directed to the importance of the cell-contents, but observations were not lacking, even in the plant kingdom, tending to weaken the importance hitherto attached to the cell-wall. Among these may be mentioned Cohn's observation that in the reproduction of Algal forms the protoplasm contracts away from the cell-wall and escapes as a naked " swarm spore." Similarly in the animal kingdom instances began to be noted in which no membrane appeared to be present (Kolliker, 1845; Bischoff, 1842), and for some time it was hotly debated whether these structures could be regarded as true cells. As a result of the resemblance between the streaming movements in these apparently naked cells (e.g. lymphocytes) and those seen in plant cells, R. Remak was led (1852-1853) to apply Mohl's term " protoplasm " to the substance of these animal cells also. Similarly Max Schultze (1863) and H. A. de Bary (1859), as a result of the study of unicellular animals, came to the conclusion that the substance of these organisms, originally termed " Sarcode " by F. Dujardin, was identical with that of the plant and animal cell. Numerous workers now began to realize the subordinate position of the cell-wall (e.g. Nageli, Alexander Braun, Leydig, Kolliker, Cohn, de Bary, &c.), but it is to Max Schultze above all that the credit is due for having laid the foundation of the modern conception of the cell - a conception often referred to as the proto-plasmic- theory in opposition to the cell-theory of Schleiden and Schwann. Max Schultze showed that one and the same substance, protoplasm, occurred in unicellular forms and in the higher plants and animals; that in plants this substance, though usually enclosed within a cell membrane, was sometimes naked (e.g. swarm spores), while in many animal tissues and in many of the unicellular forms the cell-membrane was always absent. He therefore concluded that in all cases the cell-membrane was unessential, and he redefined the " cell " of Schleiden and Schwann as " a small mass of protoplasm endowed with the attributes of life " (1861). In the same year the physiologist Briicke maintained that the complexity of vital phenomena necessitated the assumption for the cell-protoplasm itself of a complex structure, only invisible because of the limitations of our methods of observation. The cell in fact was to be regarded as being itself an " elementary organism." By this time too it was realized that the formation of cells de novo, postulated by Schleiden's theory of " phytogenesis," did not occur. Cells only arose by the division of pre-existing cells, - as Virchow neatly expressed it in his since famous aphorism, omnis cellula e cellula. It was, however, many years before the details of this " cell-division " were laid bare (see Cell-Division below).
General Morphology of the Cell
In its simplest form the cell is a more or less spherical mass of viscid,translucent and granular protoplasm. In addition to the living protoplasm there is present in the cell food-material in various stages of assimilation, which usually presents the appearance of fine granules or spherules suspended in the more or less alveolar or reticular mesh-work of the living protoplasm. In addition there may be more or less obvious accumulations of waste material, pigment, oil drops, &c. - products of the cell's metabolic activity. All these relatively passive inclusions 1 are distinguished from the living protoplasm by the term " metaplasm " (Hanstein), or " paraplasm " (Kupffer), although in practice no very sharp distinction can be drawn between them. The cell is frequently, but by no means always, bounded by a cell-wall of greater or less thickness. In plants this cell-wall consists of cellulose, a substance closely allied to starch; in animals only very rarely is this the case. Usually the cell-wall, when this is present, is a product of the cell's secretive activity; sometimes, however, it appears to be formed by an actual conversion of the surface layer of the protoplasm, and retains the power of growth by " intussusception " like the rest of the protoplasm. Even when a limiting membrane is present, however, evidence is steadily accumulating to show that the cell is not an isolated physiological unit, but that, in the vast majority of cases, there is a proto 1 The Chromoplastids of the vegetable cell come under a different category of cell-inclusions; see Plants: Cytology. plasmic continuity between the cells of the organism. This continuity, which is effected by fine protoplasmic threads (" cell-bridges ") piercing the cell-wall and bridging the intercellular spaces when these are present, is to be regarded as the morphological expression of the physiological interdependence of the various - often widely separated - tissues of the body.2 It is probable that it is the specialization of this primitive condition which has produced the cell-elements of the nervous system. In many cases the cell-connexions are so extensive as to obliterate cell-boundaries. A good example of such a " syncytial " tissue is provided by the heart muscle of Vertebrates and the intestinal musculature of Insects (Webber).3 In all multicellular, and in the great majority of unicellular, organisms the protoplasm of the cell-unit is differentiated into two very distinct regions, - a more or less central region, the nucleus, and a peripheral region (usually much more extensive), the cell-body or cytoplasm. This universal morphological differentiation of the cell-protoplasm is accompanied by corresponding chemical differences, and is the expression of a physiological division of labour of fundamental importance. In some of the simpler unicellular organisms, e.g. Tetramitus, the differentiated protoplasm is not segregated. Such forms are said to have a " distributed " nucleus, and among the Protozoa correspond to Haeckel's " Protista." It is probable that among plants the Bacteria and Cyanophyceae have a similar distributed nucleus. In all the higher forms, however, the segregation is well marked, and a " nuclear membrane " separates the substance of the nucleus, or "karyoplasm " from the surrounding " cytoplasm." Within the nuclear membrane the karyoplasm is differentiated into two very distinct portions, a clear fluid portion, the " karyolymph," and a firmer portion in the form of a coarser or finer " nuclear reticulum." This latter is again composed of two parts, the " linin reticulum," 5 and, embedded in the latter and often irregularly aggregated at its nodal points, a granular substance, the " chromatin," 6 the latter being the essential constituent of the nucleus. In addition to the chromatin there may be present in the nucleus one or more, usually spherical, and as yet somewhat enigmatical bodies, the " nucleoli." In addition to the nucleus and cytoplasm, a third body, the " centrosome," has often been considered as a constant cell-structure. It is a minute granule, usually lying in the cytoplasm not far from the nucleus, and plays an important part in cell-division and fertilization (see below).
Cell-differentiation
Both among unicellular and multicellular individuals the cell assumes the most varied forms and performs the most diverse functions. In all cases, however, whether we examine the free-living shapeless and slowly creeping Amoeba, or the striped muscle cell or spermatozoon of the Metazoa (fig. 1, b and c), the constant recurrence of cytoplasm and nucleus show that we have to deal in each case with a cell. The variation in the form and structure of the cell is an expression of that universal economic law of nature, " division of labour," with its almost invariable accompanying " morphological differentiation "; the earliest and most fundamental example being in the differentiation of the cell-protoplasm into cytoplasm and nucleus. In multicellular individuals the division of labour to which the structural complexity of the organism is due is between the individual cell-units, some cells developing one 2 Cf. Pfeffer's classical experiments on the physiological significance of cell-continuity in plant tissues (fiber den Einfluss des Zellkerns auf die Bildung der Zellhaut, 1896). The recent work in physiology on the influence substances secreted by certain tissues and circulating in the blood-stream exert upon other and widely different tissues, should not be lost sight of in this connexion.
The influence this protoplasmic continuity may have upon our conception of the cell as a unit of organization is referred to below (Present Position of the Cell-theory). 4 A term (from Kapvov, kernel) suggested by Flemming to replace Strasburger's hybrid term " nucleoplasm " (1882). The earlier workers, e.g. Leydig, Schultze, Briicke, de Bary, &c., restricted the term protoplasm to the cell-body - the " Cytoplasm " of Strasburger, an example still followed by 0. Hertwig.
From linusn, a thread, Schwarz, 1887.
From xpwµa, colour, Flemming, 1879.
aspect, some another, of their vital attributes. Thus one cell specializes in, say, secretion, another in contractility, another in receiving and carrying stimuli, and so forth, so that we have the gland cell, the muscle cell, and the nerve cell, each appropriately grouped with its fellows to constitute the particular tissue or organ - gland, muscle or brain - which has for its function that of its constituent cells. In unicellular animals we also find division of labour and its accompanying morphological differentiation, but here there is no subdivision of the protoplasm of the organism into the semi-autonomous units which so greatly facilitate division of labour in the Metazoa; instead, division of labour must be between different regions of protoplasm in the single cell. The sharply defined character of this regional differentiation in the Protozoa, and the surprising structural complexity it may produce, sufficiently clearly show that although multicellular structure has greatly facilitated regional differentiation in the Metazoa, it is by no means essential to this process (see below, Present Position of the Cell-theory). It is not within the scope of this article to attempt a comprehensive review of the variety in structural complexity to which this division of labour among the cells of the Metazoan and the regional differentiation of the cell-bodies of the Protozoa has given rise. Some indication of the wealth of variety may be best given by taking a general survey of cell-modifications, grouped according to the cell-attributes the expression of which they facilitate.
(a) Structural Complexity facilitating Movement
One of the most striking, and hence earliest described, of the fundamental attributes of protoplasm is its power of spontaneous movement. This is seen in the walled cell of plant tissue and in b a c 11111 a and b from Schafer's Essentials of His elegy, b+yI permission of Longmans, Green & Co. FIG. 1. - Types of Cells. a, Fat-cell enclosing a huge fat-globule.
b, Part of a Mammalian " striated " muscle-cell (diagrammatic).
c, Spermatozoa of mouse and bird.
the naked cell-body of Amoeba. In the latter case the streaming movements of the naked protoplasm are accompanied by the formation of " pseudopodia," and result in the highly characteristic " amoeboid " creeping movement of this and similar organisms (e.g. lymph corpuscles of the blood). 1 In these examples the whole protoplasm participates in the movement, - there has been no division of labour, and there is, therefore, no visible morphological differentiation. In many cells, movement (either of the entire body or of the surrounding medium) is by means of slender whip-like processes of the protoplasm flagella or cilia. These represent modified pseudopodia, and in the formation of the motile gametes of some of the lower forms, e.g. Myxomycetes (de Bary,1859), Rhizopods (R. Hertwig, 1874), &c., the actual conversion of a pseudopodium into a flagellum can be witnessed. These vibratile processes may be either one or few in number, and are then large in size and move independently of one another; or they may be very numerous, covering the free surface of the cell (fig. 2, a); they are then very small and move strictly in unison. In the former case they are termed " flagella," in the latter " cilia." In some cases the flagellum is accompanied by an undulating membrane (e.g. Trypanosoma among the protozoa and in many spermatozoa), and it may be situated either at the front end (Euglena) or hind end (spermatozoa) of the body during motion. The cilia may form a 1 The formation of pseudopodia and accompanying changes in form of Amoeba were observed as early as 1755 by Raesel von Rosenhof, who named it on this account the " little Proteus." uniform coating to the free surface of the cell, as in ciliated epithelium (fig. 2, a) and many infusoria, or the cilia may be variously modified and restricted to special regions of the body, e.g. the " undulating membrane " of the peristomial region in many infusoria, the swimming combs of the Ctenophora, From A. Gurwitsch, Morphologie and Biologie der Zelle, by permission of Gustav Fischer.
FIG. 2. - Types of Cells. a, Ciliated epithelial cells. (After Heidenhain.) b. Mucus-secreting " goblet "-cells. (After Gurwitsch.) and the flame cells of the Platyelmia. In one group of infusoria (Hypotricha), the cilia, " cirri," have attained a high degree of differentiation, and reach a considerable size. Both cilia and flagella spring directly from the cell-protoplasm, piercing the cell-membrane, when this is present. At the point where they become continuous with the cell-body there is usually a deeply staining " basal granule." In some cases the flagella are in direct connexion with the centrosome (see below, Celldivision), e.g. Trypanosoma and spermatozoa, in some cases even while the centrosome is functioning in mitosis (e.g. insect spermatogenesis, Henneguy 2 and Meves 3 (fig. 3).
In the ability of Amoeba to contract into a spherical mass, and in the presence in its protoplasm of the contractile vacuole, we see another type of spontaneous movement - contractilityof the protoplasm. In the " musculo-epithelial " cells of Hydra, From 0. Hertwig, Allgemeine Biologie, by permission of Gustav Fischer.
FIG. 3. - Spermatocytes of Bombyx mori, showing the precocious appearance of the spermatozoon flagellum and its relation to the centrosome. (After Henneguy.) the elongated basal portion of the cell alone possesses this contractility. In the higher Metazoa the whole cell - muscle cell - is specialized for contractility, and shows, as a result of its specialization, a distinct fibrillation. This fibrillation is foreshadowed in the contractile regions of many Protozoa, e.g. 2 " Sur les rapports des cils vibratiles avec les centrosomes," Archives d'anatomie microscopique (1898).
