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A NOTE ON THE SIGNIFICANCE OF THE FORM
AND CONTENTS OF THE NUCLEUS IN THE
By SHINKISHI HATAI.
(From the Neurological Laboratory of The University of Chicago.)
With Plates III and IV.
In the course of an investigation on the growth changes in the nerve cells of the white rat, the writer noticed that the nucleus very often had a peculiar shape; the shape being similar to an amoeba exhibiting pseudopodia-like processes along one side.
Careful observation revealed the fact that this peculiar form of the nucleus occurs normally during the period of the early growth and in attempting to explain it we meet several interesting questions of histological as well as physiological importance. The results here reported form a part of a series of observations on the growth changes in the developing nerve cells.
For this investigation, the intrauterine embryos of cat, pig and white rat were used. The present description and drawings are, however, based entirely on preparations from white rat.
Unless otherwise mentioned, the description also applies to the cat and pig. The tissue was preserved in a normal salt solution saturated with HgCl; in CarnoY's solution and in GRAF's chrom-oxalic mixture. For the microchemical test for P. and Fe., however, the tissue was fixed advantageously with 95 % alcohol. The paraffine sections were made 3 to 6 micra in thickness, and were stained with HEIDENHAIN's iron-haematoxylin alone or sometimes followed by 1% aqueous solution of eosin and BIONDI-Ehrlich's tricolor stain. Other sections were stained with toluidin blue and eosin.
General characters of the spinal ganglion cells.—The spinal ganglion cells in embryos of the white rat, 10 to 13 mm. long, present a bipolar shape in most cases; one of the processes contains a large amount of the cytoplasm and is recog. nized easily because it stains more deeply than the other. The second process arises from the opposite side of the cell-body and stains faintly. It contains a very small amount of cytoplasm and is hard to distinguish from the surrounding structures. Only the former process is shown in the figures. The relation of these processes to the spinal cord will be described in a future paper.
Hereafter, in this paper, the term process means the former branch, rich in cytoplasm.
The nucleus is very large compared with the cell-body and presents a more or less oval shape. It contains a large number of the minute granules among which two different forms may be distinguished, not only by their size but also by their staining reactions with iron-haematoxylin; one form of the granules stains deep black while the other presents a grey tint. The granules which stain an intense black are very much larger in size and occur most abundantly along the nuclear wall and its vicinity; the faintly staining granules, on the other hand, are very small in size and appear to form a fine network in the nucleus. This network is most condensed around the larger granules of the former group. The large granules are identified with the basophile substance and the small granules with linin or oxyphile substance. This grouping is by no means satisfactory, for by using the Biondi-Ehrlich stain, as well as by applying the microchemical tests, it has been noticed that among basophile granules (larger form) there are several different kinds which stain with different intensities and similarly there are several different kinds of the oxyphile granules. So far, therefore, as color reactions are concerned, the oxyphile and basophile granules grade into one another and no sharp distinction can be drawn between the two. This fact is extremely important in connection with the present investigation and will be discussed more fully later on.
Among the large granules, sometimes one and in some
cases more than one, can be distinguished as exclusively composed of the basophile substance, but in many cases, the large granules contain both basophile and oxyphile substances. When this occurs and the basophile surrounds the oxyphile substance then such a granule may be regarded as a nucleolus of the adult nerve cells.
Enlarged granules of this nucleolar type are shown in Figs. I and 2. The position of these granules is not constant but they lie in some cases along the nuclear membrane and in others they occupy the center of the nucleus (Fig. 1).
Shape of nucleus.—Changes in the shape of the nucleus and the alterations of its position in the cell have been noted by a number of observers. The phenomena have been observed especially under experimental and pathological conditions. In the normal condition, however, they have been reported by only a few investigators. Several investigators observed a pocket formation along the nuclear surface in the spinal ganglion cells of fish. These invaginations are repeated several times in one nucleus, some of them being deeper than the others, and thus the nucleus presents pseudopodia-like processes. Such an appearance is rather common in the nuclei of the nerve cells in spinal ganglia and ventral horn of lower vertebrates (HOLMGREN), but on the other hand, it is rarely visible in the cells of the higher mammalia, and when it occurs it is not so conspicuous as in the lower forms. The present writer had the opportunity to examine a large number of the preparations of the nerve cells of the white rat at different ages, but failed to find the pseudopodia-like processes of the nuclei in animals of one day or older. As a rule, after the age of one day the shape of the nuclei is constantly ovoid or spherical and does not show pseudopodia-like processes.
By examining the nuclei of spinal ganglion cells of embryos (10 to 13 mm.), the following appearances have been observed:
As is shown in the figures, the nucleus of the embryonic spinal ganglion cells lies, as a rule, at one side of the cell body; that is it lies eccentrically. Such an eccentric location of the
nucleus also occurs in the cells of the adult animal. The shape of the nuclei is somewhat oval, the longer diameter being perpendicular to the long axis of the protoplasmic process. On one side towards the center of the cell, the outline of the nucleus is more or less wavy. In some cases, the wavy outline is not very marked (Fig. 5) but in most cases, it is conspicuous and one is lead to compare it to the pseudopodia-like process of an amoeba (Figs. 1, 2, 4).
The nuclei showing the pseudopodia-like processes have been observed by several investigators; in the egg nuclei of the insects and Coelenterata by KORSCHELT ('89); in the spinning gland cells of a Swedish caterpillar and also in the spinal ganglion cells of the fishes, frogs, etc., by HOLMGREN ('95-'00); in the developing ovum of the Nassa by HOFFMANN ('02); in the nuclei of the ventral horn cells of various vertebrates by KolSTER ('01); and the same thing is also shown in the illustrations accompanying a large number of papers in which, however, the authors do not describe this interesting phenomenon. Before going on to a further discussion of this appearance, I shall describe more in detail the histological characters of the wavy outline together with the structure of the adjoining part of the cell body which contains the centrosome.
The neuclear membrane which covers the pseudopodia is not completely continuous but is composed of separate portions when seen in thin sections ; in other words, the surface of the nuclear membrane towards the cytoplasm is porous. A disappearance or dissolution of the membrane on this side of the nucleus has been observed by HOLMGREN, and PUGNAT ('98), but in the case of the white rat, it is always porous in character. This is clearly shown in Fig. 2. In many cases, however, the local dissolution of the nuclear membrane is not as conspicuous as in Fig. 2, but the outline appears varicose in structure owing to an accumulation of basophile granules around the pores (Figs. 2, 5).
The nuclear membrane which lies towards the protoplasmic process is of uneven thickness. The thicker portions stain much more deeply than the rest of the membrane with the basic dyes (Figs. I, 5). This indicates that the thicker portions contain an accumulation of the nucleoproteid. This statement is supported by the fact that the preparations tested for iron show this area deeply stained (Fig. 2). The accumulation of the nucleoproteid along the part of the nuclear membrane which turns towards the process is a highly interesting phenomenon since it bears on the problem of the cell metabolism. This point will be discussed later on in detail. An accumulation of the nucleoproteid is frequently visible along the outer surface of the nuclear membrane as is shown in Fig. 3.
These pseudopodia-like processes of the nucleus are intimately related to the rays of the centrosome. The centrosome in nerve cells has been described by several investigators, and in the nerve cells of the white rat in both adult and young it has been described by the present writer ('01). Although some investigators deny the existence of the centrosome in the nerve cell, the structure is so definite and so clear that in properly prepared sections, its presence can not be disputed. The centrosome is especially clear in the case of the embryonic cells and every minute feature of the organ may be distinguished. As a rule, the centrosome lies very near the nucleus and in the concavity formed by it (Figs. 1, 2, 3, 4, 5). The centrosome is composed of two minute central corpuscles surrounded by still more minute granules (centrosphere). These granules arrange
) themselves in straight lines which run from the center towards the periphery radially (astral lines). The astrosphere is clearly distinguished from both surrounding cytoplasm and centrosphere, since it stains very lightly with iron-haematoxylin owing to a lack of the Nissl granules. By overstaining with acid dyes, however, astrosphere stains a more intense red than the surrounding substance. The minute structure of the ceno trosome in the nerve, cells of the white rat has been reported already by the writer ('01) and therefore, with the exception of the astral structure, it need not be further described here. The astral rays which start from the centrospere run radially towards all parts of the cell body. Those rays which runs towards the nucleus extend not only as far as the nucleus but penetrate its