A Note on the Glial Fiber the Nature of So-Called Glial Fibers Has Been

A Note on the Glial Fiber by Hiroshi Hosokawa Department of Anatomy, University of Tokyo Faculty of Medicine, Hongo, Tokyo The nature of so-called glial fibers has been interpreted in two different ways. Several years after the discovery of the " Nerven- kitt " or " neuroglia " by V ir cho w (1846, '51), Clarke (1859) studied this tissue and stated that it was composed of cellular and fibrous elements which were independent of each other. D e i t e r s (1865), who teased small blocks of the brain fixed in chromic acid solution, found the dissociated " Zellaequivalente " or D e it e r s' cells, which clearly correspond to the astrocytes of today's termino- logy, and stated that these elements were furnished with long, fibrous processes extending radially from the cell body. Apparently he took the fibrous structures of the glial tissue for protoplasmic or cellular expansions. Golgi (1894), the inventor of the epoch- making silver impregnation method, thought in the same way that the glial as well as the nerve cells have long cellular processes. On the other hand, Ranvier (1883) dissociated blocks of the spinal cord in 33 per cent alchohol, stained the sediments with picrocarmin, and was led to the conclusion that the fibrous elements of the glial tissue had -essentially nothing to do with the cellular elements or the De it e r s' cells, although the former were found sometimes to penetrate through the cell body of the latter. We i g e r t (1895) studied the glial fibers with his new staining method, and insisted strongly upon their extracellular nature. Ac- cording to him, these fibers were morphologically as well as chemi- tally quite different structures from the glial cells, representing intercellular, extraplasmic fibers just like the collagenic or elastic fibers in the ordinary connective tissues. Thereafter Kölliker 1) This study was supportcd by a research grant from the Education Ministry of Japan. 2) Dedicated to Prof. Dr. T. Oga w a for his Sixtieth Birthday. 315 316 Hiroshi Hosokawa (1896) proposed a compromise that, whereas the glial fibers were continuous with the cytoplasm of glial cells in the embryonal stages, they became gradually emancipated and remained independent from the latter. According to H e 1 d ('03, '09), glial fibers represent the differen- tiated product of the protoplasm, and they remain permanently wrapped in a thin sheath of cytoplasm continuous with the cell body of the astrocytes. H e 1 d assumed further that the astrocytic processes formed three-dimensional syncytial lattice works every- where in the central nervous system. So, he thought, the glial fibers were intraplasmic structures, not necessarily found in relation to nuclei. H e 1 d's syncytium theory was supported and extended by Bauer ('53). Two different opinions about the glial fibers, both the extracellular and the intraplasmic interpretations, have found supporters until recently among histologists and pathologists. T a f t and Ludlum ('29), Wilke ('51), Wilke and Kircher ('52) and others supported the extracellular theory, while Schmidt ('42) and Bair at i ('58) believed that the intraplasmic interpretation was the correct one. Recent advances in electron microscopy have revealed very clearly that the central nervous system is filled with protoplasmic cells and processes, leaving very narrow intercellular or extraplasmic spaces of only 100-200A in width, where no extraplasmic fibers or fibrils are shown, although fine fibrils called glia filaments are sometimes seen in the cytoplasm of fibrous astrocytes. Thus it may be said that the extracellular theory of the glial fiber has been proved to be erroneous. As stated by Palay ('58), they must be understood to be nothing but the cytoplasmic extensions of the glial cells, although it is still difficult to exclude completely the possibility of extraplasmic emancipation of glial fibers, especially in some histopathological cases (B ielschowsk y, '35). At the same time, however, it is also true that the so-called glial fibers present characteristic properties somewhat different from the ordinary cytoplasm of the astrocytes. So the question remains as to whether or not some special cytoplasmic differentia- tion is really concerned with the formation of glial fibers. If so, glial fibers are strictly speaking not the synonym of the mere fine fibrous processes of astrocytes. Wei g e r t (1895) stressed the differ- ence of glial fibers in the specific staining method introduced by A Note on the Glial Fiber 317 himself. By means of roentgenographic study, Wilke and K i r- c h e r ('52) maintained that glial fibers seemed to be identical in composition with the fibrin. The physical and chemical properties of glial fibers have been subjected to detailed investigations by Schmidt ('42) and Bairati ('58). Bairati stated that the double refracting substance found in glial fibers is apparently a kind of scleroprotein, keratin. According to him, " it appears likely that neuroglia cells, being of ectodermal origin, undergo a partial cytomorphosis as epidermal cells do." Glial fibers in relation to the morphology of astrocytes Ordinarily the astrocytes are classified into two major types, the protoplasmic and the fibrous. The protoplasmic astrocytes are furnished with thick, branched processes which are entirely protoplasmic. The nuclei are large and somewhat oval in shape. They are abundant in the gray matter. On the other hand, the fibrous astrocytes are found mainly in the white matter. The cell body is smaller, enclosing an oval or spherical nucleus. Characteristically they are supplied with long, slender processes which do not show extensive branching. These fibrous expansions are demonstrated by silver impregnation and other methods as fine fibers or fibrils arranged in lattice works throughout the central nervous system. In addition to the typical protoplasmic and fibrous astrocytes, it is well known that there are many transitional forms between these two. As a matter of fact, it is sometimes very difficult to draw a clear line between protoplasmic and fibrous astrocytes. For the purpose of making the morphology of astrocytes more intelligible, the author examined carefully a great number of astrocytes in his preparations. The materials employed comprised the brain and spinal cord of man and various animals. The preparations were stained mainly with the Cajal's silver method for macroglia as well as Hortega's silver carbonate methods. Astrocytes of various forms were examined, sketched, and photographed (Figs. 10- 17). A series of transitional astrocytes thus appeared to " bridge " between the typical protoplasmic astrocytes on the one hand and the fibrous one on the other. Some types in the series are shown diagrammatically in Figure 1. Types P and F represent typical 318 Hiroshi Hosokawa Fig. 1. Diagrammatic illustration of the transitional series of astrocytes, which are arranged according to the grades of fibrization. Typical protoplasmic (P) and fibrous (F) astrocytes are on the extremities, atypical protoplasmic (P', P") and fibrous ones (F", F') being situated in between. protoplasmic and fibrous astrocyte respectively. The transitional forms in between are shown as types P', P", F", and F', the number of apostrophes indicating the grades of deviation from the typical forms. When the series P-P" is examined, it will be noted that the changes include the following points : 1) The branching of processes decreases. 2) The processes become thinner and straighter. 3) The argyrophilia of the protoplasmic processes increases, and (as if a kind of condensation of the protoplasm takes place here) the granular appearance in the typical protoplasmic processes is superseded by the dense, fibrous appearance. A Note on the Glial Fiber 319 These changes progress further in the series F"-F. Here the cellular processes exhibit distinct fibrous appearance, and the protoplasmic condensation occurs even in the perinuclear cell body, the granular portion being gradually reduced. The condensation in the perikaryon takes place at first along the surface or borders of the cell body. Then it goes on apparently through the cell body (Fig. 2). In the extremely fibrous astrocytes the cytoplasm is almost lost to sight, and often the nucleus seems to be free or naked in the meshes of the glial fibers (Figs. 16, 17). Fig. 2. An astrocyte from the white substance of the human spinal cord. Cajal's silver stain for macroglia. Drawn by camera lucida. The condensedor dark-staining protoplasm of the fibrous processescontinues and passes through the cell body. Thus it is clear that the most important change in the series of the astocytes lies in the transformation of the protoplasmic processes into fibrous ones. This phenomenon or tendency to become fibrous processes may be called fibrization. Neither the nature nor the mechanism of this transition is clear, although it is doubtless that some metabolic factors are concerned with this phenomenon in the so-called gliosis, where the fibrization is accerelated excessively. If Bair a t i's opinion is right, it is possible that a kind of keratinization or cornification plays a very important role in this process. Granules like keratohyalin in the corium were, however, 320 Hiroshi Hosokawa not encountered in the astrocytes. Is it a kind of keratinization similar to that in the nail and hair, and why does it occur more markedly in the white matter ? The author does not mean to imply that the individual astrocytes change their shape and are transformed in one or the other direction in the series mentioned. This is rather an attempt to establish a morphological standardization of astrocytes, which arose from the observation of the varieties of astrocytes in the central nervous system. It may be useful for detailed analysis of astrocytes in various parts of the brain and spinal cord, as well as for analysis of pathological changes in the astrocytes.

Load more