Microscopical Researches/CLASS I. Isolated, independent cells

by Theodor Schwann

CLASS I.

Isolated, independent Cells.

By the above term we understand cells which either float free in fluids, or, at least, are moveable, though lying in close contact. Such cells, therefore, possess the highest degree of individuality. This class includes the cells of lymph, blood, and the various secretions. The ovum might be placed at the head of this class in a system of general anatomy; but the plan of the present work required that it should be discussed previously.

1. Lymph-corpuscles. Accordig to Vogel’s description (Physiologisch-pathologische Untersuchungen über Eiter, &c. Erlangen, 1838), the lymph-corpuscles appear to be cells, although he does not express the fact in words. For example, after the corpuscles have been exposed to the action of acetic acid, a nucleus is brought into view, the production of which I do not suppose to be referrible to a separation into envelope and nucleus, but believe it to have been previously formed, and rendered visible solely in consequence of the greater degree of transparency acquired by the envelope, i. e. the cell-membrane, and its contents, from the action of the acid upon them. One of the nuclei, amongst the lymph-corpuscles, delineated in the above-mentioned work (fig. 4 b) appears to contain a nucleolus in its centre. I have not made any researches myself upon this subject. The mode of production of the lymph-corpuscles has not as yet undergone investigation. They are probably formed in the lymph-plasma, which serves as their cytoblastema, in accordance with the general law before laid down. We cannot as yet decide the question whether the nuclei are present before the cells, and whether the latter are first formed around them; perhaps the small granules which Vogel delineates from lymph are young nuclei.

2. Blood-corpuscles. C.H. Schultz was the first who proved the blood-corpuscles to be vesicles.^[1] He relied especially upon the manner in which they were acted on by water, whereby they lose their colouring matter, swell, and become round, and under which circumstances he frequently saw the nucleus roll about within the round and very transparent vesicle. The last fact would of itself be sufficiently conclusive. I have not as yet observed this fact; on the contrary, in most instances the nucleus decidedly adheres to the internal surface of the wall of the vesicle, eccentrical as in all cells, though it may probably also sometimes become detached. The fact, however, of the blood-corpuscles becoming swollen and round, renders their cellular nature highly probable. If the envelope (hülle) of the blood-corpuscle were not a flattened vesicle, it might indeed lose its colour and swell in water, but it would retain its flat form, like a sponge when filling with fluid. The circumstance of the nucleus remaining on the wall during the swelling of the blood-corpuscle in water is no accidental appearance; for even in the round blood-corpuscles of a chick, forty-eight hours after the commencement of incubation, when they were not as yet flattened, I found that the nuclei, which were also circular, were not placed in the centre, but lay eccentrical upon the internal surface of the wall. The cellular nature of the blood-corpuscle, and the signification of its separate parts scarcely appear to admit of doubt when regarded in connexion with the whole of this investigation. It is a flattened cell furnished with a cell-nucleus, which is fixed to a spot on the internal surface of the cell-membrane. The size of the cell as compared with the nucleus is not the same in all corpuscles; that of the nucleus is much more constant. The nucleus of some blood-corpuscles of frogs which had swollen in water, also appeared to me in some instances to be hollow. It also loses its flatness in water, but retains its oval figure. I have never distinctly observed nucleoli in it; occasionally only I thought I perceived something of the kind, for instance, in the blood-corpuscles of a salamander ; it was not, however, sufciently evident to permit of my asserting their presence. Cell-contents must certainly exist; for if the cell-walls lay immediately upon one another, the corpuscle must be as much thinner on the margins beside the nucleus as the thickness of the nucleus amounts to. If it be assumed that the cell-membrane alongside the nucleus may be so much thicker as thereby to produce the almost level side surfaces, the cell-membrane must in such case have a thickness equal to the half of that of the corpuscle; but it would then be sufficiently thick to allow of a double outline being distinguished when it was swollen by water ; observation, however, does not detect any such appearance. The red colouring matter forms the cell-contents. It is difficult to decide whether the cell-membrane and nucleus are also coloured, but it is in some degree probable that they are so, since otherwise the centre of the corpuscle where the nucleus lies must appear white, whilst it in fact exhibits a paler red colour. The colouring matter of the blood-cells is not contained in granules, as it is in most kinds of pigment, but in a state of solution. If the lymph-corpuscles be cells, their transformation into the blood-corpus- cles may at least be conjectured as taking place by their becoming flattened and absorbing colouring matter. Those blood-corpuscles in which the envelope (hülle) is smaller in proportion to the nucleus, a fact often observed in the frog, are probably younger cells. I have made no observations upon the formation of the blood-corpuscles in the germinal membrane. According to C. H. Schultz (System der Cirkulation, p. 33), the blood-corpuscles in the chick are formed round the yelk-globules. (?) The latter are first present, and form the nucleus of the blood-corpuscles; they become surrounded with a delicate membrane. The vesicle then dilates, and at length becomes flattened. This description accords excellently with the fundamental laws previously developed, and shows that as early as 1836 Schultz had discovered the pre-existence of the nucleus of the blood-corpuscle, the formation of the blood-vesicle around it, and the gradual expansion of that vesicle.

3. Mucus-corpuscles. The mucus corpuscles have already been described as cells, in consequence of their resemblance to the cells of epithelium. They are round globules, enclosing a nucleus, which is eccentrical. We already know this to be the elementary form of most animal and vegetable cells, and the presence and characteristic position of the nucleus, therefore, warrant us in concluding that in this instance also the globule is a cell, although an especial cell-membrane cannot be distinguished. Giuiterbock discovered that the nucleus of the mucus-corpuscle has the peculiar property of splitting into two or three smaller corpuscles when acted upon by acetic acid, and that the enclosing or cell-membrane is gradually dissolved in the same acid. Vogel, indeed, attributes this property to such mucus-corpuscles alone as have been secreted by a morbid action, and to pus-corpuscles. But I have been informed by Henle that the true mucus-corpuscles (of which, according to him, only a very small quantity exist in healthy mucus,) exhibit the same peculiarity, and that those which are not affected by the acid are true epithelial cells. As I have never observed any other cell-nuclei to be similarly acted on by acetic acid, the fact marks the distinction between mucus and pus-corpuscles and all other cells, and, according to Henle, even the youngest epithelial cells do not possess this property, so that the mucus-corpuscles differ distinctly from them. It appears to be a characteristic of all cell-nuclei that they not only are insoluble, but do not even become transparent in dilute acetic acid. These, therefore, are peculiar cells, which are formed in the fluid of mucus as their cytoblastema, in the same manner as the yelk-cells in the fluid of the yelk-ball. They become more abundant, when the cytoblastema obtains a greater degree of "plasticity" as the result of irritation of the mucous membrane; and as on the other hand the secretions in the normal condition possess but a very small amount of plastic force, and some — the urine and bile, for instance — have not any; we accordingly find in them but a very few cells, or indeed none at all, save some cast-off epithelium. I have not investigated the question whether the nucleus exist before the cell in the mucus-corpuscles, or upon what the division of these nuclei by means of acetic acid depends.

4. Pus-corpuscles. We are entitled to consider the pus-corpuscles as cells, by the same arguments which we applied to those of mucus. Vogel, indeed, regards them as identical with those mucus-corpuscles which, according to his view, are morbidly secreted, but which Henle believes to be normal. They are similarly affected by acetic acid, and cannot therefore be young epithelial cells, in which, according to Henle, the splitting of the nucleus does not take place under similar circumstances; indeed, that property appears to be confined entirely to the nuclei of the mucus and pus-corpuscles. Vogel states that the nuclei of pus-corpuscles are concave. The pus-corpuscles are thus peculiar cells which are formed in the serum of pus,—i. e. in cytoblastema, exuded during inflammation, in increased quantity, and of anomalous composition,— precisely in the same manner that mucus-corpuscles originate in mucus, and, indeed, as all cells form in their cytoblastema, in accordance with the fundamental law already laid down. According to the observations of H. Wood, they appear to be earliest formed upon the surface of the granulations, and for the reason that their cytoblastema, the pus-serum, is constantly exuding freshest at that part, and therefore possesses in that situation the greatest amount of plastic force, as we have already observed in reference to the formation of new yelk-cells on the outside and in the neighbourhood of the vitelline membrane. It is, however, probable that the pus-cells pursue an independent growth for a period, as we have seen to be the case with respect to those yelk-cells which were far removed from the vitelline membrane. It is also most likely that the nuclei of the pus-cells are their first formed part, but I have no investigations on the subject. The more healthy the pus, the greater is its plastic force, and the greater the number of cells which are formed in it, so that in healthy pus the quantity of serum is very small in comparison with the number of cells.

I cannot state whether the oil-globules which are present in certain secretions, such as milk and chyle, are contained in cells or not. I have not been able to detect anything indicating that they are so in milk; and, according to the theory of the secretions, which will be communicated at a subsequent stage of the work, there does not appear to be any necessity why they should be so. The low grade of development held by the class of cells now under consideration, in which those elementary formations retain their greatest degree of individuality, is indicated by the fact that it presents so very few modifications. The mucus-, pus-, and lymph-corpuscles are small round cells with a nucleus attached to their walls. According to Henle, mucus- and pus-corpuscles cannot be distinguished in any way from one another, and those of lymph differ from them only inasmuch as their nucleus is more round and granulous, and does not crumble under the action of acetic acid. No difference exists between them in the form of the entire cell. The blood-corpuscles present a higher degree of development in this class. In them we not only find very characteristic cell-contents, the red colouring matter, but the form of the cell also undergoes an important alteration, inasmuch as it becomes flattened. As this flattening takes place in cells which float free in a fluid, it cannot be explained as the result of mechanical causes, but must manifestly be regarded as a peculiar stage of development of these cells. The nucleus is persistent in all these cells, whilst in those more highly developed it usually disappears at some subsequent period. Throughout this class the cytoblastema is a fluid; and it is present in greater quantity than we shall find to be the case in the next class. If the egg be included in this class, we have yet another peculiarity in the cells to be added to the above; viz. that not only have the separate yelk-cells cell-contents consisting of distinct granules, but that the development of the yelk-cells within the yelk considered as one cell, is a formation of cells within cells, and in some of these cells even a second enclosure takes place. This peculiarity, however, is one which may almost be said to stand in inverse ratio to the importance of the tissue. It is most frequent, perhaps indeed universal, in vegetables, occurs more rarely in animals, as in the egg, crystalline lens, cartilage, and so on, and appears to be altogether absent in the higher structures, as areolar tissue, muscle, &c. We have already discussed the other peculiarities of the cells of the egg. In the following class we shall not only find a greater change in the form of the cells. from flattening, but we shall also become acquainted with many other different modifications of them.

1. [This is clearly an oversight as Hewson not only demonstrated their vesicular nature, and called them vesicles, but accurately described their becoming “changed from a flat to a spherical shape,” on the addition of water to the blood, and the falling of the nucleus “from side to side in the hollow vesicle, like a pea in a bladder.” See ‘Philosophical Transactions,’ 1773, vol. lxiii, Part II; or, ‘Experimental Inquiries,’ Part III, being ‘a Description of the Red Particles of the Blood,’ &c., &c. (published after his death), edited by Magnus Falconar, London, 1777; also the very valuable republication of Hewson’s Works by the Sydenham Society, edited by George Gulliver, Esq., where the reader is particularly referred to pp. 220, 221.—Trans.]