An ultrastructural study of the neuronal changes in the cardiac ganglia of the monkey (Macaca fascicularis) following unilateral vagotomy

This study describes at the ultrastructural level the neuronal changes in the intracardiac ganglia of the monkey (Macaca fascicularis) following unilateral vagotomy. One day after unilateral vagotomy, some intracardiac neurons showed an overall increase in electron density, with the dendrites darkening first. At three days after operation the dendrites appeared jet black and their intracytoplasmic organelles were hardly distinguishable except for some pale mitochondria. The perikarya of the affected neurons showed a slight increase in electron density. By five and seven days after operation, satellite cells and macrophages with engulfed dendritic processes and degenerated axons were commonly observed. The majority of the intracardiac neurons appeared normal, except for the presence of numerous macrophage (containing several phagosomes) in their vicinities. It is suggested that transneuronal degeneration occurred very rapidly in the heart of the primate, Macaca fascicularis, and that most of the affected neurons appeared to have 'recovered' one week after unilateral vagotomy.

Scanning electron microscopic study of epiplexus cells in the lateral ventricles of the monkey (Macaca fascicularis)

Scanning electron microscopic study of epiplexus cells in the monkey revealed their diverse morphological forms. These pleomorphic cells were round, bipolar or stellate in shape. Their cell body was smooth or displayed various types of microappendages: microvilli, filopodia, pseudopodia and it was sometimes ruffled. Examination of the ventricular surface of the choroidal epithelium showed that the cell membrane of some of the epithelial cells was ruptured. Within these ruptured cells one or two cellular elements could be found which resembled epiplexus cells. The inclusion cell sometimes appeared to be migrating out of the epithelial cell. It is speculated that these cells represent the precursors of epiplexus cells before their release into the ventricle lumen.

Isolation and culture of amoeboid microglial cells from the corpus callosum and cavum septum pellucidum in postnatal rats

In phase contrast and scanning electron microscopy, diverse structural forms of cells tenaciously adherent to glass coverslips were observed in the culture of the corpus callosum and cavum septum pellucidum from postnatal rats. In day 1 culture, many of the cultured cells were round, with well spread peripheral cytoplasm which appeared homogeneous. Cell organelles aggregated mainly around the reniform or round nucleus. Some cells showed spinous projections. In day 3-5 culture, the cells became irregular, sending out long branching pseudopodial processes; often they displayed a vacuolated cytoplasm. The cultured cells were highly phagocytic, as shown by their uptake of colloidal carbon particles and latex beads, in light microscopy and scanning electron microscopy, respectively. Cytochemical studies have shown that the cells were peroxidase-negative but were strongly positive for non-specific esterase, similar to the amoeboid microglial cells in the postnatal corpus callosum. On the basis of their structural features, both in phase contrast and scanning electron microscopy, experimental as well as cytochemical properties, it is concluded that the cells in the present culture are in fact amoeboid microglial cells which are active macrophages in the developing corpus callosum.

Light and electron microscopic and cytochemical identification of amoeboid microglial cells in the brain of prenatal rats

The development of amoeboid microglial cells was studied in embryonic rats extending from the twelfth day post-conception (E12) through late gestation. With the silver stain, amoeboid microglial cells in the supraventricular corpus callosum appeared at around E17 and their number increased dramatically to more than 20 times just before birth. Most of the cells were round with projecting fine cytoplasmic processes. The silver-stained preparations also showed that the supraventricular region was vascularised around E13. With the same method, it has been shown that islands, with an admixture of erythroblasts and amoeboid microglial cells, were a common feature in the embryonic brains. Electron microscopic study confirmed the existence of amoeboid microglial cells in E15-E18 rats. Among these, there were small dense cells with little cytoplasm containing predominantly free polyribosomes. At the cell surface, there were microvilli. Other cells were large and appeared to be actively involved in phagocytosing degenerating cells. Transitional forms were present. With growth, the amoeboid microglial cells further differentiated and their cytoplasm accumulated abundant lysosomal granules. Cytochemical study showed that amoeboid microglial cells in the embryonic brains were stained for non-specific esterase. The observations in the present study suggest that amoeboid microglial cells are formed following vascularisation of the brain tissues. The invading mesenchymal cells with haemopoietic potentiality not only develop into endothelial cells but also into extravascular small amoeboid microglial cells which have the features of monocytes, and which, with growth, will transform into the large macrophagic cells.

Labelling of amoeboid microglial cells in the supraventricular corpus callosum following the application of horseradish peroxidase in the cerebrum and spinal cord in rats

Following an injection of horseradish peroxidase solution into the cervical or lumbosacral segments of the spinal cords, or into the parietal cortices, of postnatal (1-10 days), developing (15-20 days) and mature (2 months and older) rats, only postnatal rats demonstrated peroxidase-positive amoeboid microglial cells in the supraventricular part of the corpus callosum. The amoeboid microglial cells were bilaterally labelled following a unilateral injection into the parietal cortex or the spinal cord. They were, however, not labelled after 15 minutes, but were definitely labelled 1 hour after the injection of the solution into the spinal cord. Also, they were not labelled when the solution was injected into the central canal or the subarachnoid space. The results suggest an ascending diffusion of the injected horseradish peroxidase in the spinal cord via the wide interstitial spaces to reach the cerebrum where it is engulfed by the amoeboid microglial cells. An enhanced activity of endogenous peroxidase was also indicated by the observation of peroxidase product in the lysosomal granules of the amoeboid microglial cells.

Scanning electron microscopy of amoeboid microglial cells in the transient cavum septum pellucidum in pre- and postnatal rats

The cavum septum pellucidum in rats of different ages was studied by light and scanning electron microscopy. A reconstruction from serial paraffin sections showed that the cavum was a pyramidal shaped closed cavity which was bounded above by the corpus callosum and inferolaterally by the lateral septal nuclei. The first sign of the cavum formation was noted in the 20 days post conception rat where there was a loosening up of the neuropil beneath the corpus callosum deep to the longitudinal fissure. A variable number of amoeboid microglial cells, characterised by their abundant eosinophilic cytoplasm, was seen among the smaller immature cells in the wide interstitial spaces. A definitive cavity was formed in the 21 days post conception rat and it continued to grow until the fifth postnatal day when it gradually diminished in size to become slit-like by the fifteenth postnatal day. The scanning electron microscope showed that the wall of the cavum was composed of a feltwork of glial and nerve fibres. Two types of cells were present in the cavum: cells identified as glioblasts, and amoeboid microglial cells. The glioblasts were were characterised by having a smooth cell body with radiating long processes. The amoeboid microglial cells showed diverse forms of surface protrusions: blebs, filopodia and membrane ruffles similar to other tissue macrophages. They were either adherent to the walls of the cavum, clustered around the blood vessel which traversed the cavum, or floating freely in the lumen. It was suggested that the amoeboid microglial cells were probably derived from extravasated blood monocytes in response to the physical damage resulting from the formation of the cavum septum pellucidum in the developing brain.

An ultrastructural study of small granule-containing cells in the heart of the monkey (Macaca fascicularis)

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Light and electron microscopic demonstration of non-specific esterase in amoeboid microglial cells in the corpus callosum in postnatal rats: a cytochemical link to monocytes

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Influence of cortisone on amoeboid microglia and microglial cells in the corpus callosum in postnatal rats

Amoeboid microglia and typical microglial cells in the corpus callosum of rats were studied following a subcutaneous injection of cortisone at birth. The rats were killed 2, 5, 10 and 20 days after the injection. The most striking change in the corpus callosum was the great reduction in the number of amoeboid microglial cells as shown by the silver carbonate stain of Rio-Hortega. The change was sustained throughout the period studied. Ramified and typical microglial cells which do not normally appear until between the fifth and tenth postnatal day, were observed on the second postnatal day after the cortisone administration. Cell enumeration in semithin sections showed that the proportion of amoboid microglial cells was reduced to 50% of their normal value soon (2 days) after the cortisone injection. This proportion was further decreased and the cells were absent from the fifth postnatal day onwards. Typical microglial cells were developed prematurely and they constituted more than 2% of the total glial population 2 days after the cortisone injection. Another striking change noted in the semithin section was the increase in the compactness of the axons in the corpus callosum in the experimental animals. Electron microscopic observations were in general agreement with the light microscopy. The amoeboid microglial cells appeared less active; they contained small Golgi apparatus, accumulations of lipid droplets and sparse lysosomes. The cell outline was regular. The reduction in the number of amoeboid microglia after the cortisone injection was explained by the fact that the drug suppressed the production of their precursor cells, i.e. circulating monocytes, Moreover, it was suggested that cortisone probably interfered with the phagocytic activity of the amoeboid microglial cells which would normally undergo structural changes to become the quiescent microglia which were observed in the early postnatal animals.

A light microscopic demonstration of amoeboid microglia and microglial cells in the retina of rats of various ages

The retinae from rats of various ages were stained using the weak silver carbonate method of del Rio-Hortega. In newborn and early postnatal (3-day-old) rats, silver-impregnated round and amoeboidic cells displaying thick stout pseudopodial processes were distributed in the nerve fiber and ganglion cell layers. In 10-day-old rats, the cell bodies were rod-shaped bearing pseudopodia or fusiform showing slender and branching processes. The majority of the cells were found in the ganglion cell layer with occasional ones in the inner plexiforn layer. By 20 days of age, the cells became smaller and exhibited long branching processes. They were located predominantly in the inner plexiform or bipolar cell layer. In the adult animals, all the silver-impregnated cells appeared small and flattened with long branching processes. Based on their morphological features, it was concluded that they were in fact amoeboid microglia and microglial cells present in the developing retina which probably function as macrophages as described in the brain tissues.

Ultrastructure and mode of formation of epiplexus cells in the choroid plexus in the lateral ventricles of the monkey (Macaca fascicularis)

The epiplexus cells in the lateral ventricles of the adult monkey (Macaca Fascicularis) were examined in the electron microscope. The majority of the cells displayed ultrastructural features typical of tissue macrophages, while others resembled monocytes or microglial cells. These cells were characterized by an abundant cytoplasm containing vacuoles, a prominent Golgi complex and a variable number of electron-dense granules resembling lysosomes. The cell surface often showed pseudopodial or filopodial processes. Macrophages bearing similar features were also observed in the connective tissue stroma of the choroid plexus. In the same area, occasional circulating monocytes, which appeared to be in the process of migrating out of the vascular lumina, were seen. Furthermore, the present study showed the frequent occurrence of macrophages which appeared to be within the choroidal epithelial cells. Based on these observations, it was suggested that the epiplexus cells were derived from diapedesis and metamorphosis of the circulating monocytes in the core of the choroid plexus. It would appear, then, that the extravasated cells crossed the choroidal epithelium by an intracellular pathway to reach the ventricular lumen to become the macrophagic epiplexus cells. However, an alternative route of cell migration, i.e. through the choroidal epithelium by an intercellular pathway, was also considered.

Ultrastructure and peroxidase cytochemistry of macrophages present in the retina of postnatal rats

Electron microscopic study showed the presence of macrophages in the retina in postnatal rat. These phagocytic cells were localized predominantly in the nerve fiber layer between the ganglion cells and the inner limiting membrane. They were pleomorphic, some displayed typical features of tissue macrophages bearing prominent cytoplasmic inclusions, others resembled monocytes or microglial cells. Their cytoplasm contained a variable number of electron-dense granules of size 0.15 micrograms and these exhibited peroxidase activity.

Use of carbon labeling to demonstrate the role of blood monocytes as precursors of the 'ameboid cells' present in the corpus callosum of postnatal rats

Cells with features suggestive of ameboid motion and phagocytic properties are observed in the rat corpus callosum during the first few days of life. These cells, hereafter referred to as 'ameboid cells', have been investigated in several ways. An electron microscopic study of the corpus callosum in 5- to 7-day-old rats indicated that most 'ameboid cells' were typical macrophages, but some displayed features of monocytes, while others appeared to be transitional between the two types. These observations raised the possibility that blood monocytes were the precursors of 'ameboid cells'. This possibility was tested by injecting a suspension of carbon particles into the circulation of rats of various ages to label and trace monocytes. Within 15 minutes after injection, carbon particles were seen between cells in blood smears as well as in the lumen of capillaries, but not between cells and axons in corpus callosum. By a half hour, a few of the circulationg monocytes, and with time, up to half of them, contained carbon particles. Five days after injection, carbon particles were observed in cells of the corpus callosum identified as 'ameboid cells' of the monocytic and macrophagic type. Such carbon-containing cells were seen in many of the animals injected at the age of 0-1 day, in few of those injected at 3-5 days, and in none of the older animals. Since free carbon had not been observed in corpus callosum spaces, it was concluded that 'ameboid cells' did not pick up carbon locally. The alternative was that blood monocytes, after ingesting carbon particles in the circulation, migrated to the corpus callosum and settled as 'ameboid cells'. In the hope of obtaining a direct confirmation of this conclusion, blood cells obtained from carbon-injected Lewis rats were centrifuged in a Percoll gradient to obtain a fraction which contained 70-80% monocytes, less than 2% granulocytes, and 20-30% lymphocytes. Carbon was present in up to half of the monocytes and 1% of the granulocytes, but not in the lymphocytes; and it was calculated that over 99% of the carbon-labeled cells were monocytes. The cell fraction was then introduced into the blood circulation of 2- to 3-day-old syngeneic Lewis rats, and the animals were sacrificed 5 days later. Occasional carbon-labeled cells appeared not only in liver, spleen and connective tissue, but also in the corpus callosum, where they were identified as 'ameboid cells' of the monocytic and macrophagic type. Even though such cells were infrequent, their presence conclusively demonstrated that at least some 'ameboid cells' of the corpus callosum were derived from circulating blood monocytes.

Cytochemical localization of peroxidase in amoeboid cells in the corpus callosum in postnatal rats

Cytochemical study of the "amoeboid cells" in the corpus callosum in the early postnatal rat brain showed that a few of them exhibited peroxidase activity. The enzyme activity was localized in a variable number of cytoplasmic granules measuring 0.2 micron and this resembles that of monocyte-derived macrophages.

Ultrastruct and origin of epiplexus cells in the telencephalic choroid plexus of postnatal rats studied by intravenous injection of carbon particles

Epiplexus cells in the telencephalic choroid plexus of postnatal rats were shown with the electron microscope to be of two kinds, one with monocyte features including an indented nucleus with coarse chromatin, numerous polyribosomes, long profiles of rough endoplasmic reticulum, a well-developed Golgi apparatus, lysosomes, microtubules and coated vesicles, long cytoplasmic process and filopodia; the other with the addition of highly vacuolated cytoplasm. In order to clarify the origin of these cells, rats were given two intravenous injections of carbon particles. Shortly after the second carbon injection (1-4 days) none of the epiplexus cells were tagged with carbon. Five and six days after the second carbon injection, however, a variable number of epiplexus cells were labelled with intracytoplasmic carbon. Their number decreased later and by 9 days hardly any were seen. The present evidence suggests that the carbon-labelled epiplexus cells are derived from circulating monocytes which have ingested the carbon particles while in the blood. The labelled cells then cross the walls of subependymal blood vessels, penetrate the multilayered subspendyma and ependymal lining and enter the lumen of the lateral ventricle.

Transformation of monocytes into amoeboid microglia in the corpus callosum of postnatal rats, as shown by labelling monocytes by carbon particles

Two successive intravenous doses of carbon suspension were given at 24 hourly intervals into six days old rats. These animals were killed at intervals ranging from 1 to 9 days after the second injection. The corpus callosum and neighbouring structures were examined for cells containing ingested colloidal carbon particles in their cytoplasm. Twenty four hours after the second injection, a variable number of carbon-labelled monocytes were adherent to the luminal wall of blood vessels in the corpus callosum. Numerous carbon-labelled cells appeared to have left the lumen and entered the brain tissue surrounding the vessels. These perivascular carbon-labelled monocytes in the neuropil displayed a large pale nucleus with fine chromatin granules. The phagocytic amoeboid microglia in the corpus callosum were unlabelled at first, although a few cells of a similar nature in the cavum septi pellucidi did show carbon particles in their cytoplasm. Four or five days after the second carbon injection perivascular carbon-labelled monocytes were rare, but carbon particles were now present in the amoeboid microglia. At 8 days amoeboid microglia were virtually absent from the corpus callosum but carbon particles now appeared in cells which closely resembled microglia (flattened nucleus, coarse chromatin, scanty cytoplasm at one pole). The sequential appearance of carbon particles in monocytes, amoeboid microglia, and microglia, suggests that monocytes transform into microglia by way of an amoeboid microglial stage.

Evidence for a haematogenous origin of some of the macrophages appearing in the spinal cord of the rat after dorsal rhizotomy

A single dose of colloidal carbon was given intravascularly to young adult rats in order to label circulating monocytes. Two days after injection dorsal rhizotomies were performed on the fifth to eighth cervical nerves on the right side. The rats were killed 1, 3, 4 and 8 days later. Electron microscopic examination of the spinal cord showed wide-spread tissue degeneration on the operated side in the dorsolateral fasciculus, the dorsal horn and the dorsal neuronal white column, the changes in the last named being the most severe. A variety of non-neuronal elements was found in the dorsolateral fasciculus and dorsal horn. These included astrocytes, oligodendrocytes, microglia-like cells, plasma cells, mast cells, polymorphonuclear leucocytes, monocytes and macrophages. Monocytes and macrophages were most common 3 and 4 days after operation. Some of these cells carried intracytoplasmic carbon particles. Carbon-labelled monocytes were observed in blood vessel lumina, perivascularly and in the neuropil. Monocytes crossing blood vessel walls were also encountered, indicating that the neuropil monocytes were derived from circulating cells. Macrophages were characterized by pleomorphic phagosomes which seemed to be composed largely of myelin remnants. The presence of carbon particles in their cytoplasm, and also their general similarity to monocytes, suggested that they originated from the latter. Local microglial cells were considered to be another source of macrophages. Indeed, there were present some microglia-like cells which were regarded as 'activated microglia' as they showed morphological resemblances to microglia on the one hand and to macrophages on the other. In particular their cytoplasm always included phagosomes. It is concluded that the macrophages which appear in the altered spinal cord following rhizotomy are derived both from circulating monocytes and from indigenous microglia.

Electron microscopic study of macrophages appearing in a stab wound of the brain of rats following intravenous injection of carbon particles

Colloidal carbon was introduced intravenously into young rats to label circulating monocytes before the stabbing of the brain. The rats were sacrificed 1 to 14 days after the stab wound. In the rats sacrificed between 3 to 7 days after the stabbing, numerous phagocytic cells were present in the needle wound. Electron microscope study showed that some of these phagocytic cells carried intracytoplasmic carbon particles. These carbon-labelled cells showed features either of a monocyte, full-blown macrophages, or typical microglia. It is believed that they are all derived from circulating monocytes which have ingested carbon particles in circulation and thereafter invaded the stab wound.

Electron microscopic studies of macrophages in Wallerian degeneration of rat optic nerve after intravenous injection of colloidal carbon

The origin of macrophages in the degenerating optic nerve of rats after eye enucleation was investigated electron microscopically following intravenous labelling of mononuclear leucoytes with colloidal carbon. In the various post-operative periods studied carbon-labelled macrophages were seen at the site of lesion. At 4 and 7 days after enucleation carbon-labelled cells were seen at the site of Wallerian degeneration of the optic nerve over 4 mm distal to the site of the lesion. In the electron microscope these cells showed a flattened nucleus bearing coarse chromatin clumps, their cytoplasm contained a prominent Golgi complex and long isolate profiles of rough endoplasmic reticulum. Clusters of carbon particles in the cytoplasms were membrane-bound. Lysosomal bodies embedded with carbon particles were also observed. In relation to the blood vessels of the optic nerve, endothelial cells and pericytes with ingested carbon were seen. Macrophages in the meninges covering the optic nerve were also labelled. The results suggest that some macrophages in the region of Wallerian degeneration in the optic nerve, as well as those at the actual site of the lesion, were transformed blood leucocytes.

Brain macrophages in rats following intravenous labelling of mononuclear leucocytes with colloidal carbon

Intravenous injections of colloidal carbon were used to label circulating mononuclear leucocytes. In the neonatal rats (3-5 days old), either 24 or 48 hours later, carbon-labelled macrophages were seen in the brain tissue. In the areas examined, notably the corpus callosum and the cerebral cortex, labelled macrophages were distributed randomly. They were either perivascular, perineuronal or lay between the nerve fibres. The labelled cells were mostly spindle-shaped with an eccentric nucleus and the cytoplasm at one pole of the cell was engorged with dark carbon particles. Abundant labelled cells were also seen over the brain surface in the layers of meninges. There was no evidence of leucocytic infiltration into the brain tissue of mature animals. It is concluded from the present work that a proportion (but not all) of the macrophages in the neonatal rat brain are derived from the blood stream.

Light and electron microscopic demonstration of some lysosomal enzymes in the amoeboid microglia in neonatal rat brain

A cytochemical study of the amoeboid microglial cells in the brain of the neonatal rat has shown that these vacuolated cells exhibit strong acid phosphatase, aryl sulphatase and adenosine triphosphatase (ATPase) activities. Endogenous peroxidase, however, was not present. With the electron microscope the reaction product of acid phosphatase was found to be localized in some of the Golgi cisternae, in the majority of the electron-dense secretory granules, and in an occasional long tubular profile. The secretory granules were not uniformly stained for this enzyme, some showing only a focal reaction or none at all. The distribution of the activity of aryl sulphatase corresponded to that of acid phosphatase except that all the granules appeared to contain the former enzyme. With the light microscope the amoeboid microglial cells were intensely stained for ATPase. From these observations it was concluded that amoeboid microglia are active phagocytes and their enzyme-rich secretory granules are lysosomes.

Some aspects of amoeboid microglia in the corpus callosum and neighbouring regions of neonatal rats

The distribution and form of amoeboid microglia in the brain of neonatal rats have been studied with the light and electron microscope. With the silver carbonate method of Rio-Hortega, two major 'colonies' of amoeboid microglia are identified: (1) in the supraventricular corpus callosum in which the nerve fibres are widely spaced, and (2) at the medial angle of the lateral ventricle inferior to the corpus callosum. Scattered amoeboid cells are also seen in the cavum septum pellucidum and in the lumen of the lateral ventricle. Associated with the subependyma forming the roof of the lateral ventricle there are also numerous amoeboid cells. Ultrastructural studies show that the subependyma includes cellular elements with features intermediate between those of immature subependymal cells and full-blown amoeboid microglia. It is suggested that the latter are derived from the subependymal cells and that, once they are formed, they leave the subependyma and migrate into the corpus callosum and elsewhere. With the metallic stain, the amoeboid microglia present a wide diversity of appearances, some of which bear a close resemblance to typical microglia. It is therefore suggested that amoeboid microglia change into typical microglia. The present study clearly demonstrates that amoeboid microglia are active phagocytes. Their cytoplasm is heavily loaded with secretory granules (lysosomes) and give a positive reaction with PAS and acid phosphatase.

Electron-microscopic identification of amoeboid microglia in the spinal cord of newborn rats

Electron-microscopic studies in the spinal cord of neonatal rats show, though rarely, the existence of amoeboid microglial cells. The cells are distributed singly and are found only in the spinal grey matter. They are characterized by having a round, prominent nucleus with peripheral chromatin clumps. The cytoplasm is endowed with a well-developed Golgi complex, and isolated profiles of rough endoplasmic reticulum. One striking feature of the cytoplasm is the presence of abundant lysosome-like granules. The cytoplasm, which is of moderate density, may be vacuolated. The surface of the cells often sends out pseudopodium-like processes. These, together with the presence of occasional phagosomes, indicate that the amoeboid cells are phagocytic. Thus, it is possible to generalize from the present and earlier findings that amoeboid microglial cells are normal cellular constituents in the maturing central nervous system and their temporary existence in the neonatal stage indicates the necessity of these cells for subsequent maturation of the nervous tissue.

The antecubital organ of the primate slow loris (Nycticebus coucang coucang)

A study of the 'antecubital organ' of the slow loris (Nycticebus coucang coucang), was undertaken in light and electron microscopes. As distinct from other prosimian primates there is a complete absence of interstitial cells in the gland suggesting its different functional role. The acinar cells in the 'antecubital organ', of slow loris contain large number of smooth ER and electron-dense secretory granules. The granules are seen both in the apical region of the cells as well as in their basal cytoplasmic processes. Some of these processes appear to terminate close to a blood capillary. The structural features of the 'antecubital organs' of slow loris suggest that it is a mixed gland of both exocrine and endocrine nature.

Study in the changes of the proportions and numbers of the various glial cell types in the spinal cord of neonatal and young adult rats

Glioblasts, astrocytes, microglia and the three classes of oligodendroyctes were enumerated in the grey and white matter of the spinal cord of rats of various ages. Both regions showed fairly similar results. Glioblasts, which made up the major glial population in the newborn rats, declined steadily with age and their number became negligible by 22 days post natum. They were absent in the young adult rats (aged 70 days). Contrary to this, the major glial types increased rapidly with age, the increase being most drastic in the oligodendrocytic population. The growth continued through about 22 days after birth and became more or less stabilized thereafter. Of the three classes of oligodendrocytes, the light cells appeared to develop first, followed by the medium dense cells and subsequently the dark ones. While there was a gradual disappearance of the light and medium dense cells with age, there was an accumulation of the dark cells, so that they were predominant in the spinal cord of the young adult rats.