Popular Science Monthly/Volume 26/April 1885/Structure and Division of the Organic Cell
|STRUCTURE AND DIVISION OF THE ORGANIC CELL.|
THE doctrine of the cell, as the unit of vegetable and animal structure, has been constantly varying in its details since its first proposal by Schleiden in 1837 and Schwamm in 1839. It was at first held that the cell was a microscopic vesicle, globular in its typical form, bounded by a firm membranous wall, and inclosing fluid or semi-fluid contents. In its interior lay a smaller vesicle called the nucleus, which occasionally held a minute mass called the nucleolus. The cell-wall was believed to be its active constituent, which selected materials from the surrounding fluid for cell-nutrition, and set up physical and chemical changes within its contents. At a later date Goodsir and Barry maintained that the nucleus was the active agent in these processes, and that self-division of the nucleus was the source of cell-division. It was also perceived that a cell-wall was by no means always present, and Leydig defined a cell as "a little mass of soft substance inclosing a nucleus." A more important step of progress was made about 1861, when Von Mohl, Brücke, Max Schultze, Beale, and others, propounded their views upon the subject. Brücke pointed out that the contents of cells frequently displayed spontaneous movement and contractile power; and Max Schultze declared that sarcode—the contractile substance which forms a large part of the bodies of the lower animals—was homologous with the contents of actively growing cells. Von Mohl had proposed the term protoplasm to designate the active substance of vegetable cells. This term was extended by Max Schultze to embrace all organic cells, and he defined the cell as a nucleated mass of protoplasm. At a still later period it was declared that a nucleus was not always present, and the cell was defined as "a structureless mass of protoplasm."
Such was the stage of the cell-doctrine reached in 1872, thirteen years ago. First the cell-wall had been considered the active element, then the nucleus, and finally the protoplasmic contents, while wall and nucleus came to be considered inessential elements. As Drysdale expressed it about that date, "a cell is like a gun-barrel, without a stock and a lock." Meanwhile Beale had persistently declared that there is no such thing as a cell, in the ordinary sense of the term; but that all organic bodies are made up of minute particles of living or germinal matter, which consume nutriment and increase internally, while their exterior portions lose vital activity, and become dead or formed material. These living particles not only grow, but divide, and thus set up new centers of growth, from which emanates new-formed material.
The division of the cell-protoplasm is, indeed, a most essential part of the life-process, and to it growth and differentiation of tissue are principally due. The cell, when furnished with nutriment, manifests individual growth for a short period. Then it separates into two or more new cells, each of which sets up an individual life. This separation takes place in several methods, of which the most common is by an equatorial constriction, which gradually deepens until it cuts the cell into two sections. Other methods are by the budding off of minute portions from the surface, or the transformation of the cell-contents into many minute germs, which are subsequently set free.
Such was the cell of thirteen years ago—"a structureless mass of protoplasm," which increased in size by nutrition, and in numbers by division. Such is the cell of most of the text-books of to-day. But the cell of science is a very different affair. Instead of being structureless, it is found to possess an intricate structure, while its division is far from being the simple process above indicated. The new cell-theory is, in fact, but five or six years old in its developed form, and it is as yet settled only in its main features. Its minor details need much further elucidation.
These new discoveries, which we shall briefly describe, are largely due to the increased power and clearness of definition of the microscope, and still more to new and improved methods of preparing organic sections for investigation, by the employment of stains, preserving agents, and other useful appliances. It is not every microscopist that is able to see the minute details of cell-structure lately announced. The careful preparation of material and exceedingly delicate manipulation required need years of practice, and the discoveries referred to are due to the first microscopists of the age, though the methods are now so simplified that any skilled observer, with a good instrument and proper care, may hope to successfully employ them.
The matter of which an organic cell is composed is found to be not simply a homogeneous, or slightly granular, mass of protoplasm. On the contrary, it appears to be traversed in every direction by delicate fibers, which form an intricate network or reticulum. The interstices of this network are occupied by a fluid or semi-fluid substance of homogeneous appearance, though occasionally containing a few small granules. The reticulum occurs not only in the outer cell, but also within the nucleus, and its fibers extend to and are apparently connected with the nucleolus. Within this latter the fibrous formation has not been traced. Some observers, indeed, declare that there is no nucleolus, but that it is simply a node of the intersecting fibers. But this view is not generally entertained, and late writers ascribe to the nucleolus an important function.
In the growth and division of the cell the nucleus appears to be specially active, and the new doctrine known as karyokinesis relates principally to the peculiar metamorphoses of the nucleus during cell-division. Two phases of cell-life are now well marked. One of these is an active stage, during which transformation of the cell-contents rapidly takes place, and division follows. This is succeeded by a resting-stage, in which all activity of the nucleus ceases, the fibers grow less distinct, and a partly homogeneous condition results. This resting stage is, after a period, followed by a new period of activity.
The behavior of the cell-contents, when treated with carmine or other staining reagents, indicates that they are composed of at least two distinct substances. During the resting-stage this does not appear, for the whole cell takes the stain, though it deepens in the nucleus, and still more in the nucleolus. But during the active stage only the fibers take the stain, while the intermediate ground or basis substance remains clear and transparent. From this difference in behavior the name chromatin is proposed for the fibers, achromatin for the ground substance.
Flemming, one of the most skillful observers of these phenomena, distinguishes two forms of division—the direct and the indirect. The former—which may eventually prove to have no real existence—is a direct separation, first of the nucleolus, then of the nucleus, and finally of the cell. In the latter there is a peculiar metamorphosis of the nucleus. Flemming, from observation of the cells of Salamandra, describes the process as follows:
The resting-nucleus possesses a faintly-defined reticulum of fibrils, whose meshes hold a homogeneous ground substance, one or more nucleoli, and occasionally a few small granules. Possibly these latter are merely the nodes of the reticulum. When the active stage commences, the membrane of the nucleus disappears, as also the nucleolus and the granules. If the latter are merely nodes of the fibrillar network, we can understand their disappearance, for the fibrils lose their net-like reticulation, and become an irregular convolution, with no free ends. Around this fibrous nuclear mass appears a clear space, which separates it from the outer cell-substance. As thus arranged it forms what has been called the aster.
Soon the fibrous convolutions assume a wreath-like arrangement, with their bends irregularly directed toward a central space. Eventually the wreath loses its continuity, and breaks into a series of short, separate fibers, which form V-shaped loops. The bends of these loops are directed toward the center space, their openings outwardly. This arrangement forms the mother-star. Next there is shown a doubtful appearance, as if the fibers had split into two, or had become tubular. The loops are also compressed toward the equatorial plane of the nucleus, and lose their extension toward its polar region. After some further dubious movements, a rearrangement of the loops is found to have taken place, their bends being now turned outward, their openings inward toward the equatorial plane. They have also separated in this plane, so as to form two distinct masses, one on each side of the equator. If we consider the cell as a globe, and the equatorial plane as a circular disk dividing this globe into two hemispheres, then on each side of this disk lies a smaller circle of fibrous loops, which present something of the aspect of a circular basket, or of a partly opened daisy. The openings of these basket-like figures are turned toward each other, with the equatorial plane separating them. The converging looped ends of the fibers are turned outward.
This stage in the process of division of the nucleus is followed by a recession of the basket-figures. They retreat in the axial line of the cell until they reach the polar regions of the nucleus. Here a rearrangement of the fibrous loops takes place, their bends again become directed toward a central space, and two new stars, similar to the mother-star, are formed. The division of the chromatin, or fibrous substance of the nucleus, has become complete, and the whole new arrangement is known as the dyaster.
As the basket-like figures recede, there often appear in the interval between them delicate striæ, which cross the equator from pole to pole. This condition, which is most declared in vegetable cells and in segmenting ova, is known as the nuclear spindle. The lines of the striæ seem to be composed of achromatin. Other faint lines often radiate from the poles toward the surface of the cell, forming sun-like figures at the extremities of the nuclear spindle. Complete division is preceded by the appearance of a row of dots across the equatorial plane, which seem to be thickenings in the centers of the lines of the spindle. These thickenings are probably composed of chromatin, and form what is called the equatorial plate. They soon divide, the spindle separating in its center, while the thickenings appear like minute disks at the extremities of the nuclear striæ. Thus a double equatorial plate is formed, inclosing a narrow equatorial plane. This is the plane of cell-division. A furrow appears around the equator of the cell, which deepens, and extends inward between the equatorial plates. It continues to deepen until it finally meets in the center, and the cell is separated into two new ones.
While this is proceeding, new nuclei are forming at the nuclear poles. The fibers of the daughter-stars pass through a series of changes opposite to those above described. The ends of the loops unite until a wreath is formed. This wreath soon becomes an irregular convolution, which quickly assumes the reticular structure. Membranes form around the new nuclei. Nucleoli and granules reappear. The resting stage is regained. The original cell is replaced by two daughter-cells.
The above description, with its detailed account of the process of cell-division, is not accepted in all its particulars by other observers. There is great diversity of opinion about many points, which can not be settled without much further investigation. It is also very probable that much of the diversity of opinion arises from the fact that the cells of different organisms vary in their features of change, and that vegetable cells only distantly resemble animal cells in this particular.
Some of the unsettled questions are the following: Klein and Strasburger see little importance in the nucleolus. Klein doubts its existence. There is an open question whether it and the granules are not merely the nodes of the network. But the majority of observers speak of the nucleoli and granules as lying free in the ground-substance, in the intervals of the network. It is also a question whether or not the outer cell-substance is like the nucleus in structure. Klein holds that it is. Flemming has lately announced the discovery, in the resting-nucleus, of a very fine network, in connection with the coarser one already known. He also declares that the membrane surrounding the nucleus is really composed of minute flat plates of chromatin continuous with the fibrils of the network. These are separated by slight intervals, so that the membrane seems pierced by holes, which perhaps may be occupied by the transparent ground-substance. Others deny the existence of a nuclear membrane, and think that it is an optical illusion, caused by the arrangement of the fibers. Dr. Pfitzner has recently declared that the chromatin fibrils are not homogeneous in structure, but that they really consist of minute spherules of chromatin, held together by some other substance, probably achromatin.
Such are some of the questions to be yet settled. It would appear that the chromatin of the original nucleus becomes first regularly arranged around its center, then divides equatorially and recedes to its poles, where it forms new nuclei, while the achromatin-fibrils of the spindle may possess some chromatin, which collects upon their centers to form the equatorial plate. If Flemming's last observation concerning the formation of the membrane be correct, it may prove that the disks of the spindle-fibrils are the origin of the cell-wall, and that similar disks arise at the extremities of achromatin-fibrils in the new nuclei to form their membranes. And Pfitzner's observations would indicate that the fibrils are really composed of achromatin, upon which chromatin gathers either continuously or in separate spherules. In such a case the movements of chromatin would be along lines of achromatin; and we can comprehend the appearance of the lines of the achromatin-spindle, after the chromatin has aggregated at the poles, and also of the chrormatin-disks which are shown equatorially on these lines. The chromatin of the fibrils has aggregated at the poles and the equator of the nucleus, and left apparent intermediate lines of achromatin.
In vegetable-cell divisions Strasburger finds none of this regular process, but only a vague approach to it in the movements and aggregations of masses of chromatin. But the achromatin-striæ of the nuclear spindle, the equatorial plate, and the sun-like polar rays, are well declared. In some cases of abnormally rapid nutrition a threefold division takes place, and possibly a still greater number of new cells may be formed. The process of cell-budding may be similar to that above described, if we can judge from observations on the early transformation of the ovum. Here a nuclear spindle is formed, with its polar suns. This moves to the surface of the cell, and one of the poles is pushed out through its wall. Constriction takes place, and the new nucleus remains on the outer surface of the cell as the polar body, while the other nucleus retreats to the center of the ovum. The process is precisely analogous to ordinary cell-division, the difference being that one of the new nuclei retains around it all the substance of the original cell, while the other is destitute of it. Did this polar body become free, and grow by absorbing new nutriment, the resemblance to ordinary cell-budding would be complete. Frequently two or more polar bodies are thus formed ere fertilization of the ovum takes place. Possibly the cell buds off its male element and retains only its female. An analogous process takes place in the spermatozoa. It would seem as if the germinal cells were becoming specially male and female in energy ere combining to form the germ of a new individual.
Recently Mr. J. M. Macfarlane, of Edinburgh, has published an interesting paper, descriptive of vegetable-cell division. His observations were made on the cells of Spirogyra, a common fresh-water alga. The large nuclei of these cells seem specially adapted to observation. He found not only that the nucleolus was very distinct, but that it invariably contained a well-defined body, which he names the nucleolo-nucleus. He found this body in all plant-cells examined, and also in cerebellum-cells of animals. In staining with carmine the stain hardly affected the outer cell-substance, the nucleus took a somewhat deeper stain, the nucleolus was deeply colored, and the nucleolo-nucleus still more deeply. The density of the chromatin appeared to increase inwardly.
From the outer cell-wall fibers pass inward to the nuclear membrane, which they probably penetrate, and become continuous with the fibers of the nucleus. These, in their turn, seem to connect with what appears to be a membrane of the nucleolus. Inside the latter there is no evidence of fibrous structure.
In cell-division the first stage is the aggregation, on opposite sides of the nucleus, in a line with the long axis of the cell, of a quantity of pale, slightly granular protoplasm. This is perhaps derived from the peripheral layer, and travels inward along the fibers, since minute thickenings, of similar appearance, show themselves upon these fibers. The nucleolus swells out into two protuberances, in the same axial line, joined by a bridge of denser matter. This change is, perhaps, connected with the division of the nucleolo-nucleus, since subsequently two of the latter are visible, while the nucleolus returns to its former state. At the same time it is found to have considerably increased in size. The next visible change occurs in the nucleus, whose contents aggregate at the nuclear poles, push through the membrane, and combine with the outer aggregation of protoplasm to form two dark amœboid lumps. From these polar masses fibers run inward and outward, though the external fibers have become loose and flaccid.
In the next stage the nuclear membrane disappears. The spindle of fibers which runs inward to the nucleolus is bordered by two darker strands, possibly the remnants of the membrane. This composes the nuclear barrel. The nucleolus divides by a dumb-bell-shaped constriction, similar to what appears in the division of amœbæ. It resembles what Flemming calls "direct division." The two halves of the divided nucleolus—each containing one of the nucleolo-nuclei—now move outward toward the poles, a new line of fibers forming between them as they separate. These bodies almost seem to have a repulsive energy, for the polar masses recede before them. The connecting lines also spread outward centrally, so that the nuclear barrel becomes considerably elongated and widened. It resembles a barrel with thick, narrow ends and widely swelled-out middle. Eventually the nucleoli reach the polar masses, into which they penetrate, while the substance of the latter spreads inward so as to inclose them. The rudiments of new nuclei are thus formed, between which extend rows of fine fibrillar lines, much separated centrally. The "nuclear barrel," with the nucleolus in its center, has thus been succeeded by the "nuclear spindle," with no central mass.
There now appears a row of dots, stretching across the equator of the spindle. This quickly separates into a double row. At the same time the lines of the spindle are sundered centrally, and the dots seem to be minute disks at their extremities. In this manner a double "equatorial plate" is formed, inclosing the circular equatorial plane of the cell. Around the cell-wall, in this plane, is seen a faint ring, which pushes inward, and develops into a new wall of cellulose. It extends into the space between the two equatorial plates, and continues to grow inward until the center is reached, when it forms the dividing wall of the two new cells. As it grows, the spindle, which had previously been swelling outward, begins to contract, until it becomes a narrow series of lines, reaching from the poles to the new cell-wall. Meanwhile the polar masses secrete new membranes, and assume the condition of nuclei of the new cells. So rapid is the process that the nucleolo-nucleus often again divides ere the nucleus has completed its division, and the nucleoli again divide ere the new cells are formed. Hence the new nucleus often has two nucleoli. After complete division the lines of the nuclear spindle are still apparent. They may, by splitting, give rise to the fibers of the new cells.
Such is a recent description of the process of division in the plant cell. In the cells of some plants, however, there is a preliminary step of change which does not appear in Spirogyra. In these cases division begins with a massing of the nuclear contents in the equatorial region. The nucleus has a spindle-shape, with a dark mass in its center, and clear areas reaching to its poles. This mass splits, and its two halves retreat to the poles. The further steps of division are as above.
Thus, so far as now appears, the process of cell-division in the plant is closely analogous to, though not identical with, that of the animal. It seems, indeed, a more primitive stage of the phenomenon. The division of the nucleolus, so marked in the plant, has not been observed in the animal, and may be, in the latter, suppressed or hastened, like many of the developmental changes in the higher animals. On the other hand, the peculiar movements of the chromatin-fibrils of the animal cell have no direct counterpart in the plant. They seem to present a distinct step forward in cellular evolution, and yield the idea that the animal cell is a more advanced organism than that of the vegetable. It certainly seems to hasten or suppress embryo changes which are well marked in the latter, and to clearly display advanced stages of development which are only vaguely outlined in the latter.
There is another cell-theory extant to which some allusion must be made, as it indicates a final stage in cell-evolution in advance of that here indicated. It is known that in many cases elongations of the fibrous network extend outward from the cell. These have been seen in epithelial cells, joined so as to form a connecting link between two cells. It is well known also that numerous delicate fibrils extend beyond the walls of nerve-ganglion cells, probably as outer continuations of the internal network. It is supposed that these fibrils aggregate into bundles, and that thus the nerve-fibers, which run to all parts of the surface of the body, originate. This seems to be the cell connection of the sensory nerves, while the motor nerves leave the cells each as a single fiber. The nerve terminations of muscles present a similar condition, since they probably penetrate the cells as minute fibrils.
C. Heitzmann, a skilled microscopist, now of New York, has long maintained, and has recently reiterated, a theory which declares that this fibrillar extension is not confined to epithelial and ganglion cells, but is common to all the cells of the body, and that intimate interconnections between all the cells and tissues are thus made. Even the bony structure he declares to be everywhere permeated by tine channels, in which run fibrils of protoplasm, connecting the granular and nuclear masses throughout the whole substance. He, indeed, denies the existence of separate cells, and claims that the body is simply a vast reticulum, with nuclear masses as nodes of the network. Instead of being composed of numerous separate amœboid cells, it is a single complex amœba.
This bioplasson theory is not accepted by microscopists generally, and it certainly goes too far in denying the existence of distinct cell structures. It may be possible that it indicates a final stage in the process of cell-evolution. Distinct isolated cells undoubtedly exist in the blood and lymph fluids of the body. But in the more solid tissues this isolation is, in some cases at least, replaced by an interconnection of cells through the medium of inosculating fibrils. And it is quite possible that this fibrillar extension becomes so declared in extreme cases as to produce the appearances described by Heitzmann. The basis or ground-substance of the outer cell of osseous tissue may be converted, by deposition of lime-salts, into bony matter, through which the fibrils extend from the nuclei in open channels. If this theory be correct, the original cell becomes a nuclear center of active protoplasm and an outer region whose ground-substance is converted into bone, while its protoplasmic fibrils extend until they join similar fibrils of other cells, thus converting the whole mass into a living network whose interspaces are occupied with bone.
In other tissues a similar condition may exist, the bony matter of the osseous ground-substance being represented by other inactive material proper to the tissue. Perhaps every phase of differentiation exists, from the completely isolated corpuscles of the liquid tissues to the complete and extended reticular structure described as existing in bone.
This theory naturally leads to some probable speculative views. If, as seems evident, the nerve-fibers originate in such extensions of the intercellular network, possibly the fibrils of individual cells have a conductive or nerve function, as also the contractile or muscle function which some writers ascribe to them. Their extension from cell to cell would indicate nerve communication, and it may be that the undoubted nerve and muscle function of many low animals, in which no nerves and muscles have been discovered, may be due to these interlacing fibrils. And the widely extended nerve-system of the higher animals, by which the whole body is made one interrelated unit, may be but a final outgrowth of the fibrillar-cell system. The fibril reticulum of the isolated cell becomes the nerve reticulum of the complex body, which is virtually converted into a single cell, with its intricate network of fibers.
- Within the last few years research into the conditions of plant-cells has led to the interesting discovery that these cells are very generally connected by fine fibrils of protoplasm, in a manner somewhat similar to that which Heitzmann declares to be the general rule in animals. Possibly this may prove to be a universal condition. Mr. Walter Gardiner, in a memoir read before the Royal Society, April 26, 1883, says: "Although I am aware of the danger of rushing to conclusions, I can not but remark that when these results — which were foreshadowed by Sachs and Haustein when they discovered the perforation of the sieve-plate — are taken in connection with those of Russow, it appears extremely probable that, not only in the parenchymatous cells of pulvini, in phlœm parenchyma, in endosperm-cells, and in the prosenchymatous bast-fibers, is continuity established from cell to cell, but that the phenomenon is of much wider if not of universal occurrence." This condition, so commonly present in plants, has as yet not been widely traced in animals, but may eventually prove to be equally general, as Heitzmann declares. The connecting protoplasmic fibril may be the embryo stage of the nerve-fiber, and may serve to bring every cell in the body within the range of nerve influence.