Page images
PDF
EPUB

been operative at each successive evolutionary stage to determine the next step to be taken.

In the doctrine of nerve components as it is now being wrought out we have a concrete illustration of the correlation of structural and functional data and methods in the solution of some of the greater problems of vertebrate descent, and especially in the interpretation of the human nervous system, the culmination of that evolutionary history. We shall be able to present from our contributors illustrations of the practical workings of this principle in detail within a few months.

RECENT STUDIES ON THE FINER STRUCTURE OF THE

NERVE CELL.

By G. E. COGHILL,
Professor of Biology, Pacific University.

The finer structure of the nerve cell remains the object of study for numerous investigators. Interest centers here from many points of view. From the physiological view-point there is sought the correct differentiation of the protoplasm from the metaplasm and the determination of the relation which each of these, in its various aspects, holds to the activities of the cell. The pathologist demands, further, the structural basis and explanation of the various morbid activities as distinguished from each other and from the normal. And for the morphologist the subject presents a variety of problems which may be approached by both comparative and embryological methods.

In a field of so varied interests and in which at the same time there is much diversity of opinion, the results from various sources must undergo frequent critical analysis and synthesis. In no other way can the general trend of facts be discovered and progress in the subject as a whole be measured and masle useful. With this thought in mind I have undertaken to study such recent papers and monographs relating to the structure of the nerve cell as were available and to bring together synthetically the opinion of different authors under topics which appear to hold important place in the minds of investigators. The discussion is arranged according to the following plan:

Ground Substance and Neurofibrillae
The Moniliform (ondition of the Dendrites
Golgi's Endocellular Net
The Gemmules
Golgi's Pericellular Net
Intracellular Canaliculi
The Nucleus
The Nucleolus
The Centrosome
The Tigroid Substance and Chromatolysis

The Ground Substance and Neurofibrillae. The challenge which came to the neurone theory through the works of APÁTHY and BETHE has awakened new interest in the finer structure of the cytoplasm of the nerve cell. That there are structures in the properly fixed and stained neurone, especially of invertebrates, which accord with the neurofibrillae of these authors can no longer be doubted. But the exact relation which these fibrillae hold to the protoplasm of the living cell, to the inter-relation of neurones, and, therefore, to the conduction paths of the nervous system, is not satisfactorily explained. Is the neurofibril a protoplasmic thread or is it a derived substance of the protoplasm as Pugnat and others argue? If it is a derived substance, it may well pass beyond the limits of the cell and form extra-cellular nets in the neuropil, as BETHE and Apáthy and their followers describe. But if the fibril is protoplasmic its nature excludes the possibility of such nets since the limits of the cell would coincide with the limits of the protoplasm.

But if the fibril be protoplasmic, are there stuctural features of the cytoplasm by which the origin, development and final behavior of the fibril can be explained? Towards the answer of such leading questions, some of the following works contribute in a positive manner.

HOLMGREN ('99), in his monograph upon the spinal ganglion cells of Lophius, does not commit himself to a definite statement regarding the ultimate structure of the cytoplasm. He describes however, a certain radial appearance about the nucleolus in its migration from the nucleus into the cytoplasm which he considers suggestive of the alveolar structure. Yet his figures and descriptions in general do not seem to sustain this interpretation throughout. In his beautifully executed drawings (Taf. IX-X, Fig. 3) he represents the cone of origin as marked with stripes which are directed from the axone into the cell body. In the more distal part of the cone these stripes seem to form elongated and very narrow meshes. As the structure recedes into the cell body the meshes become shorter and broader, and especially is this true in the peripheral region of the cell where the meshes become very irregular in shape and size. Though HOLMGREN does not discuss this structure in detail as related to the structure of the cytoplasm his figures are strongly suggestive of the cytoplasmic reticulum as described by Hatai and others.

In a later work upon the structure of the nerve cell HOLMGREN (oo) finds more positive data on this feature of the neurone. He demonstrates a fibrillar structure which he considers identical with the fibrillar substance of FLEMMING. The fibrillae of the cytoplasm are

1

zone.

continuous with those of the axone, but none of them anastomose. They follow an undulating or somewhat spiral course through the cell body, with a tendency to be more nearly parallel in the peripheral

The interfibrillar substance is homogeneous.

But the fibrils of this net, it is important to notice, are to HOLMGREN the neurofibrillae. In both vertebrates and invertebrates they may enter or leave the neurone at any point in the periphery of the perikaryon or of the processes. Furthermore, Holmgren says, “die wabige, pseudowabige oder spongioplasmatische Structur, wie man sie auch nennen will, die ich bei Lophius, die RAMÓN-Y-CAJAL, LENHOSSÉK, VAN GEHUCHTEN U. A. beschrieben haben, nur einem accidentellen Aussehen der resp. Zellen entspricht, im besten Falle durch einen gewissen physiologischen Zustand hervorgerufen." That is to say, the “filare Substanz" of FLEMMING is resolved by HOLMGREN into the neurofibrillae of Apáthy and BETHE, and the structural parts of the ground substance, such as granules, trabeculae etc., of many authors are interpreted as functional or artificial modifications of FLEMMING'S amorphous "interfilare Substanz."

KOLSTER's monograph upon the nerve cell of Petromyzon contains an interesting demonstration of certain features of the ground substance. KOLSTER has made an exhaustive study of unstained preparations of the nerve cell mounted in media of various refractive indices, and also of unstained osmic acid preparations. In none of his mounts made by these methods has he found anything akin to a fibrillar or reticular structure. But in cells .preserved in FLEMMING'S solution for several months or even for more than a year, and subsequently stained in saffranin and differentiated in a 20% tannin solution followed by absolute alcohol, he discovers very fine lines running through all the cytoplasm. These lines are made up of a single row of dark red granules which the author treats as microsomes. of his figures these lines seem to form a net with relatively large, irregularly shaped meshes, but the author believes that anastomoses between the lines are relatively rare. In thick sections these granules appear large, but thinner sections show that these relatively large granules are made up of short rows of exceedingly small granules running in all directions. The whole structure, then, is resolved into a network of microsomes in linear arrangement. Furthermore, KOLSTER demonstrates that this net is concentrated into a dense granular mass directly around the centrosphere, where a granular effect is given in unstained preparations. From this central mass rather thick rays extend in different directions, and these fray out into the fine lines of

In some

microsomes. KOLSTER interprets this structure as different from anything that had been previously described, but it would seem to be indistinguishable from the neurosomic net met with by other authors.

According to BOCHENEK (01) a fibrillar net may be beautifully demonstrated in the nerve cells of Helix by the gold chloride method. In the body of the cell the meshes of the net are triangular or polygonal but in the axone hillock they elongate. The fibrillae which course independently through the axone are continuous with the fibrillae of the net. The author finds it impossible, however, to distinguish between the motor and sensory fibrils. He finds, also, that the net in Helix is much more dense than it is in the nerve cells of Lumbricus. He interprets this as indicating a higher state of organization in Helix, which would be, in respect to the degree of differentiation of the intracellular net, intermediate between Lumbricus and the vertebrates. BOCHENEK treats this net as “un réseau protoplasmatique.”

VAN GEHUCHTEN, in his earlier work ('97) upon the internal organization of the nerve cell, took the position that the ground substance of the cytoplasm consists of a fibrillar net suspended in an amorphous fundamental substance. This net extends into the process in the form of elongated meshes which superficially give the appearance of distinct fibrils. These two elements are considered by VAN GEHUCHTEN “le véritable protoplasme de la cellule nerveuse.

The net or organized part he homologizes with the “masse filaire” of FLEMMING. In a later work ('98), published jointly with NELIS, VAN GEHUCHTEN quotes FLEMMING as coinciding perfectly with this interpretation as given in 1897. The authors, however, proceed at once to modify their interpretatation in certain details. They found certain spinal ganglion cells of the rabbit which, because of the absence of chromatic substance in the peripheral region, gave excellent advantages for studying the achromatic structure. In these cells, by the use of toluidin blue and erythrosin, they found the net to be made up of granules united together by thin trabeculae of the same substance. But they add: “Le reseau protoplasmatique qui existe indubitablement dans toutes nos preparations diffère totalement des fibrille courtes, flexueuses, irréguliéres et independantes decrétes et figurées par FLEMMING." He believes also that the net differs clearly from the net described by DogieL and that it is much more regular than that figured by MARINESCO.

In regard to the cone of origin, also, the authors have changed their views slightly. Instead of the comparatively thick and regular

« PreviousContinue »