Dynamic aspects of glial reactions in altered brains

The Role of Oxytocin in Abnormal Brain Development: Effect on Glial Cells and Neuroinflammation

The neonatal period is critical for brain development and determinant for long-term brain trajectory. Yet, this time concurs with a sensitivity and risk for numerous brain injuries following perinatal complications such as preterm birth. Brain injury in premature infants leads to a complex amalgam of primary destructive diseases and secondary maturational and trophic disturbances and, as a consequence, to long-term neurocognitive and behavioral problems. Neuroinflammation is an important common factor in these complications, which contributes to the adverse effects on brain development. Mediating this inflammatory response forms a key therapeutic target in protecting the vulnerable developing brain when complications arise. The neuropeptide oxytocin (OT) plays an important role in the perinatal period, and its importance for lactation and social bonding in early life are well-recognized. Yet, novel functions of OT for the developing brain are increasingly emerging. In particular, OT seems able to modulate glial activity in neuroinflammatory states, but the exact mechanisms underlying this connection are largely unknown. The current review provides an overview of the oxytocinergic system and its early life development across rodent and human. Moreover, we cover the most up-to-date understanding of the role of OT in neonatal brain development and the potential neuroprotective effects it holds when adverse neural events arise in association with neuroinflammation. A detailed assessment of the underlying mechanisms between OT treatment and astrocyte and microglia reactivity is given, as well as a focus on the amygdala, a brain region of crucial importance for socio-emotional behavior, particularly in infants born preterm.

GFAP, S-100 and vimentin proteins in rat after cerebral post-ischemic reperfusion

In the present study astrocytes reactivity during cerebral post-ischemic reperfusion was evaluated immunocytochemically by using antibodies to vimentin, glial fibrillary acidic protein (GFAP) and S-100 protein. At the 7th day of post-ischemic reperfusion few GFAP-positive cells were observed in the hippocampus and cerebellum, the number of GFAP-positive cells increased slightly after 20 days of reperfusion. This poor GFAP-positivity may be due to the inhibition of GFAP polymerization by S-100; in fact, S-100 immuno-reactivity was already evident from the 7th day. Vimentin immuno-staining was evident both at the 7th and 20th day of reperfusion in microglial cells and in oligodendrocytes, suggesting that these cells are involved in the recovery of neurons following brain injury.

Role of endothelin-1 in astrocyte responses after acute brain damage

We examined the possibility of the involvement of endothelin (ET)-1, a potent vasoactive peptide, in the process of astrocyte proliferation after brain injury. Acute brain damage in rats was induced by cold-injury. Astrocytes changed from a differentiated state to an immature, RC-1-positive state immediately after the injury. In the injured site, the level of ET-1-like immunoreactivity in the tissue was significantly increased on the first postoperative day and was sustained at a high level for 5 days. ET(B) receptor mRNA was markedly but transiently down-regulated only on the first day after the injury. Brain extracts (BE) were prepared from the injured tissues, and their effects on the proliferative characteristics of astrocytes were examined in primary culture of astrocytes. The flat morphology, which was observed in association with cell proliferation, and DNA synthesis of astrocytes were enhanced by treatment with each of the BE from 1 (D1-BE), 3 and 5 days after the injury. A monoclonal antibody that recognizes the C-terminus of rat ET-1 and ET-3 inhibited the DNA synthesis of astrocytes induced by D1-BE. These results provide experimental evidence that ET-1 may participate in the initiation of gliosis in the acute phase of brain damage.

Basic fibroblast growth factor (bFGF) injection activates the glial reaction in the injured adult rat brain

Reactive gliosis is a reaction of glial cells to trauma which is characterized by a phenotypic modification of astrocytes, as well as by a proliferation and a migration of some of these cells to form a glial scar. This scar is currently considered as a physical impediment to neuronal regrowth but it may also be involved in wound healing since the astrocytes beside microglia play a phagocytic role in the clearance of post-traumatic debris. Growth factors are released in the area of the injury and at least some of them could be involved in gliosis. In order to test directly this possibility, we have injected one of them, the basic fibroblast growth factor (bFGF), into several brain areas (cortex, striatum, hippocampus or corpus callosum) of adult 2-month-old rats in the absence of lesion. A glial reaction was observed after 3 days and was maximum after 7 days. It was characterized by an increase in astrocyte proliferation and in glial fibrillary acidic protein (GFAP) expression, resulting in a higher number of GFAP-positive cells per surface unit, and by an increase in the size and branching of the astroglial processes. The GFAP mRNA levels were also strongly increased following the bFGF injection. These effects resemble the reactive gliosis observed after lesion and suggest that bFGF is actually involved in the triggering of glial reactions which follow brain injury. In further experiments, bFGF was injected in the site of electrolytic lesions made in the same various parts of the brain. These injections did not increase significantly the normal reactive gliosis induced by the lesion alone, but it accelerated some of the effects. It also resulted in a higher labeling index and GFAP mRNA levels were strongly enhanced after a 3-day-post-operative delay. This last observation strengthens the idea that one of the main factors driving the astrogliosis is the bFGF normally released in and around the site of the lesion.

Endogenous endothelin-1 initiates astrocytic growth after spinal cord injury

We developed a rat spinal cord injury model and investigated whether endogenous endothelin (ET)-1 plays a role in astrocytic growth after injury. Immunohistochemical study showed that the number of immature astrocytes (ACs) exhibiting strong reactivity to the monoclonal antibody, RC1, markedly increased 24 h after the injury. Injection of a potent nonselective ET receptor antagonist, SB209670, into the lesion sites significantly inhibited the appearance of RC1-positive cells 24 h after the injury. In conjunction with this result, the increase in immunostaining density of 5-bromo-2'-deoxyuridine in the spinal cord 24 h after the injury was inhibited by the injection of SB209670. The tissue content of ET-1-LI was significantly increased 12 and 24 h after the injury. These results suggest that endogenous ET-1 is involved in astrocytic growth after spinal cord injury.

A combined staining method for argyrophilic nucleolar organizer regions and for glial fibrillary acidic protein in astrocytes of human brain

Different protocols are described for the combined staining method by which argyrophilic nucleolar organizer region sites can be evaluated in human astrocytes that are immunoreactive for glial fibrillary acidic protein. Among the four protocols studied, the following method was superior to others in terms of unambiguous visualization of the regions in glial fibrillary acidic protein-positive astrocytes; the first step was immunostaining for the protein with a blue colour reaction of alkaline phosphatase, followed by sequential colloidal silver staining for the regions. By this double staining method, we have demonstrated that the reactive astrocytes found in white matter around the metastatic lesion of carcinoma and the infarction, contain more argyrophilic nucleolar organizer regions in terms of the count as well as the area than glial fibrillary acidic protein-positive astrocytes present in the white matter of the normal brain. In conclusion, the double staining may provide valuable information on the cellular activity of astroglia when performed on routine formalin-fixed paraffin sections of the human brain.

Reactive cell proliferation and microglia following injury to the rat brain

The non-astrocytic cells which proliferate in the rat brain after the induction of an area of necrosis have been characterized and counted by means of combined in vivo bromodeoxyuridine (BrdU) administration and immunohistochemical demonstration of glial fibrillary acid protein (GFAP), vimentin, Ricinus communis agglutinin 120 (RCA-1), Griffonia simplicifolia B4 isolectin (GSI-B4), keratan sulphate (KS), carbonic anhydrase C (CA.C), transferrin (TF) and ferritin. Two days after the injury, 7.5% of the proliferating cells were GFAP-positive reactive astrocytes, 5.7% were RCA-1-positive cells and 17.4% were GSI-B4-positive cells. Lectin-binding cells had the microscopic and ultrastructural aspects of microglia; they proliferated around the needle track and in the corpus callosum. Microglia represented a large fraction of the proliferating cells. Evidence is presented for the origin of at least a proportion of perilesional astrocytes and microglia from the periventricular matrix, and of microglia from blood precursors. Other non-proliferating microglia cells transiently appeared in the normal brain around the wound, in agreement with the existence of two different microglia cell populations reacting with different modalities to an area of necrosis.

Interleukin-1 binding sites on astrocytes

Interleukin-1 has been shown to have regulatory effects on glial cell functions. In this study, we examined the capacity of astroglial cells to specifically bind recombinant iodinated human interleukin-1 alpha. This was performed in mouse brain by both in situ and in vitro autoradiography, on areas of gliosis and on astrocytes and microglia primary and secondary cultures respectively. Specific binding was shown in the brain sections over areas of glial proliferation, and in addition, quantitative autoradiography was performed. Analysis of competition experiments by autoradiography led to EC50 values of 5 x 10(-11) M for human interleukin-1 alpha and approximately 10(-9) M for the interleukin-1 receptor antagonist. In cultures, iodinated human interleukin-1 alpha bound specifically to astrocytes but was unable to bind to microglial cells. Competition binding experiments in astrocyte cultures led to EC50 values of 8 x 10(-11) M and 1 x 10(-10) M for human interleukin-1 alpha and mouse interleukin-1 beta respectively, and an EC50 higher than 10(-9) M for the antagonist. The presence of interleukin-1 receptors on astroglial cells provides biochemical support for the various effects of interleukin-1 in the central nervous system, particularly those concerning the formation of scar tissue, possibly by astroglia proliferation after brain injury.

Increase in nucleoside diphosphatase in rat brain striatum lesioned with kainic acid

The activity of ammoniagenesis from guanine nucleotides was found to increase significantly in rat brain after infusion of kainic acid into the striatum. Among the enzymes involved in degrading guanine nucleotides, nucleoside diphosphatase was markedly increased in the lesioned striatum. The enzyme activity began to increase 2 days after the infusion, and reached the maximum on the 13th day, the level being 4 times as high as that of the intact contralateral region. The increased activity was due to Type L enzyme, judging from its substrate specificity. Puromycin and cycloheximide inhibited this increase, indicating that the increased activity resulted from an increase in the net synthesis of the enzyme. These findings suggest that Type L NDPase might play some important roles in gliosis after neuronal lesion.

Reactive astrogliosis after basic fibroblast growth factor (bFGF) injection in injured neonatal rat brain

Reactive gliosis was revealed by immunocytochemistry using antibodies against the glial fibrillary acidic protein (GFAP) after a stab or an electrolytic lesion administered to the cerebral cortex, corpus callosum, striatum, or hippocampus of a 6-day-old rat. The intensity of the gliosis was about the same in the various structures injured and did not change with the delay of 3, 7, or 20 days between the injury and the sacrifice of the animals. When basic fibroblast growth factor (bFGF) was injected in the lesion locus just after the lesion was performed, it resulted (as soon as 3 days after injury) in a strong astrogliosis that was enhanced after a delay of 7 days, the astrocytes in the lesion area exhibiting enlarged cell processes and intense GFAP-positive immunoreactivity. After a delay of 20 days, the astrocytes were not dispersed any more but packed in three or four layers along the borders of the lesion, thus reducing its extension. This suggests a possible role for bFGF in promoting scar formation following brain injury.

Effect of dexamethasone on various stages of experimental brain abscess

Rats were inoculated with staphylococcus aureus to produce cerebral abscesses and treated with either antibiotic or dexamethasone and with antibiotic plus dexamethasone at sequential stages of abscess formation. Antibiotic alone shortened the cerebritis stage, accelerated the encapsulation and affected the bacterial clearance in the abscess centre when it was started early in the course of cerebritis. Dexamethasone impaired the lymphocytic and fibroblastic responses and delayed the collagen deposition as well as suppressed the efficacy of antibiotic. However, it did not halt entirely the encapsulation and did reduce the associated cerebral oedema.

Treatment of experimental brain abscess. 2. Effects of combinations of hyaluronidase with antibiotics and dexamethasone

Brain abscess formation was studied experimentally in rats to determine the most appropriate nonsurgical treatment method by applying different combinations of hyaluronidase, dexamethasone and antibiotic sensitive to the inoculated bacteria in various stages of classical abscess development. The results showed that combined therapy with antibiotic and hyaluronidase started the day before inoculation averted the formation of brain abscess and the same therapy started after encapsulation, effectively eliminated the organisms and resolved the infection leaving a glial scar. But the same therapy, only started at the cerebritis stages, caused an increase of cerebritis. The addition of dexamethasone reduced the oedema but enhanced the cerebritis and delayed encapsulation. Though neurosurgical intervention continues to be the definitive method for eradicating the infection and preventing the pressure-related complications of brain abscess, our concept of management with hyaluronidase and appropriate antibiotic might be a new effective chemotherapeutic method of encapsulated brain abscesses in selected high-risk patients.

Ia-expressing microglial cells in experimental allergic encephalomyelitis in rats

Monoclonal antibodies (MRC OX-6 and OX-17) recognized three types of cells expressing Ia antigen during the course of acute experimental allergic encephalomyelitis (EAE) in rats. In earlier stages of the disease, in animals with or without paralysis, Ia antigens were mostly localized to subarachnoidal and perivascular lymphocytic and histiocytic cell infiltrates, possibly serving as antigen-presenting cells. On the other hand, in convalescent rats, Ia antigens were expressed in a large number of cells with dendritic processes heavily populating the spinal gray matter. The appearance of these Ia-expressing cells in the convalescent stage coincided with the development of degenerating axon terminals in the spinal gray matter. These Ia-expressing cells possessed morphological features characteristic of microglia and were positive for ML-1 lectin but did not express glial fibrillary acidic protein. Immune electron microscopy disclosed the presence of Ia reaction products in the Golgi apparatus, endoplasmic reticulum and plasma membrane of these cells with dendritic processes, indicating active synthesis of Ia molecules in microglia. In addition, Ia antigens were localized to the cells with ultrastructural features of macrophages. Thus, Ia-expressing cells in EAE seems to play dual roles: the induction of immunological reactions during earlier stages and the participation in reparative processes during convalescence.

Quantitative studies on proliferative changes of reactive astrocytes in mouse cerebral cortex

Cell number and proliferation of reactive astrocytes were studied quantitatively in the stabbed cerebral cortex of adult mice, using immunohistochemistry for glial fibrillary acidic protein (GFAP) and [3H]thymidine autoradiography. GFAP-positive astrocytes increased in cell number gradually from 24 to 96 h after stabbing, and their immunoreactivity became intense. The maximum number of GFAP-positive cells was about 4.5 times normal in the layers II-VI of the cortex, whereas it was only 1.5 times normal in the layer I (molecular layer). In contrast to the gradual increase in cell number, no GFAP-positive astrocytes were labeled with [3H]thymidine prior to 48 h after stabbing, in either the layer I or the layers II-VI. Then 3-5% of them were labeled at 72 and 96 h, but very few again after 6 days. By injecting [3H]thymidine successively for 6 days after stabbing, only 17% of GFAP-positive astrocytes of the layer I or the layers II-VI were labeled. These results reveal that, in the cortical layers II-VI, many GFAP-negative source cells initially express much more GFAP-antigen without proliferation and change into GFAP-positive reactive astrocytes. Proliferation of reactive astrocytes is not the major factor for the marked increase in number of them. The cortical layer I would have few GFAP-negative source cells for reactive astrocytes. These source cells may be protoplasmic astrocytes.

Immunohistochemical studies on the proliferation of reactive astrocytes and the expression of cytoskeletal proteins following brain injury in rats

The appearance of reactive astrocytes following brain injury was investigated in 4-week-old rats with special reference to their proliferation and chronological changes in the cytoskeletal proteins. Two days after the injury, glial fibrillary acidic protein (GFAP)-positive cells had increased in number around the lesion and spread to the entire ipsilateral cortex by 3 days after the injury. To investigate the distribution of mitotic cells and its chronological change, immunohistochemical staining with monoclonal antibody to bromodeoxyuridine (BrdU) was performed. BrdU-positive cells began to appear around the lesion and spread to the entire ipsilateral cortex by 3 days and their distribution was the same as that of GFAP-positive cells. To investigate the association of GFAP-positive cells with cell division, double labeling experiments using [3H]thymidine autoradiography and immunohistochemical staining with antiserum to GFAP were performed. Cells doubly labeled with GFAP and [3H]thymidine were localized in the area adjacent to the lesion, in the molecular layer of the cortex and in the white matter. By contrast, none of the cells were doubly labeled in the IInd to VIth layers of the cortex. Furthermore, only astrocytes in the former areas expressed vimentin transiently from 2 to 10 days after the injury. In the rats administered vincristine, cells arrested during mitosis were found in the regions which express vimentin. From these results, it was suggested that astrocytes in the molecular layer of the cortex and the white matter adjacent to the lesion proliferated in response to the injury and expressed vimentin transiently, then acquired GFAP, and that astrocytes in the IInd to VIth layers of the cortex became reactive astrocytes without mitosis.

Interleukin-2-like activity in injured rat brain

The lymphokine interleukin-2 (IL-2) promotes division and maturation of oligodendrocytes in culture. We now report that a IL-2-like activity was present in injured rat brain. The ion-exchange properties of this activity were similar to those of splenocyte IL-2 but its apparent molecular weight was higher. Brain IL-2-like activity was highest in the tissue immediately adjacent to the injury, reaching a maximal activity of about 8000 U/g tissue after 10 days postlesion. The mitogenic activity of injured-brain extracts on astrocytes and CTLL thymocytes was partially inhibited by monoclonal antibodies to murine IL-2 receptor. However, pure human IL-2 did not have mitogenic activity for cultured rat astrocytes. Purified astrocytes, alone or stimulated in a variety or ways, did not produce IL-2-like activity.

Microglia in the hypendyma of the rat subcommissural organ following brain lesion with serotonin neurotoxin

The population of microglial cells in the subependymal layer of the subcommissural organ is sparse in normal adult rats. The number of microglial cells was substantially increased in this area following intraventricular injection of the serotonin neurotoxin 5,6-dihydroxytryptamine (5,6-DHT). In sections of plastic embedded material, 1 micron thick, the majority of phagocytic cells scattered in the subependymal layer had an appearance similar to that described in classical studies of microglial cells. At the electron microscopic level microglial cells exhibited the characteristic elongate nucleus with peripheral chromatin condensation. The perikaryon was scanty, containing strands of rough endoplasmic reticulum. The abundant organelles in the processes included Golgi complexes, mitochondria, rough and smooth endoplasmic reticulum as well as dense and multivesicular bodies. In addition, the processes contained phagocytosed axon terminals originating from the dense serotoninergic input to the subcommissural organ, which had degenerated on accumulating the serotonin neurotoxin. A fraction of the phagocytosed material was contained in subependymal subcommissural organ cells, astrocytes and oligodendrocytes. At the light microscopic level the phagocytosed terminals were visualized histochemically with Schmorl's reaction, which resulted in Prussian Blue precipitates. This allowed screening of microglial cells in complete series of sections through the well-defined subependymal layer of the subcommissural organ.

Interleukin-1-like activity in rat brain: sources, targets, and effect of injury

Extracts of injured rat brain contained molecules that shared biological and physiochemical properties with interleukin-1 (IL-1). Brain IL-1-like activity increased following brain injury in parallel with the increase in astrocyte and fibroblast mitogenic activity. After 3 days postlesion, it reached about 20 times the basal (noninjured) level. Monoclonal antibodies to human IL-1 inhibited this brain IL-1-like activity. One of the cellular sources of brain IL-1-like activity seems to be astroglial cells. Primary cultures of purified rat brain astrocytes secreted into the culture medium more IL-1 activity than comparable numbers of peripheral blood monocytes. Brain IL-1, as well as authentic monocyte IL-1, appear to act on brain glial cells, promoting thymidine incorporation into purified astrocytes in culture.

The spatio-temporal pattern of Wallerian degeneration in the rhesus monkey optic nerve

Three patterns of degeneration were distinguished in the Rhesus monkey optic nerve following eye enucleation. A traumatic zone, extending for 1 to 2 mm from the site of transection and characterized by a rapid infiltration of haematogenous macrophages, the rapid degeneration and removal of all neural elements and the formation of astrocytic scar tissue within 35 days. A conical zone, closely associated with the central retinal blood vessels, in which the pattern of degeneration was similar to the traumatic zone except that the onset was delayed and the removal of debris was slower. There was extensive vesiculation of myelin sheaths in these two zones which was indicative of haematogenous cell infiltration. The remainder of the nerve underwent classical Wallerian degeneration in which endogenous cells slowly phagocytosed the degenerating nerve fibres. These observations are considered relevant to the controversy concerning the identification of the macrophage involved in Wallerian degeneration; many studies have described the macrophage within the area of trauma rather than the macrophage involved at some distance from the site of trauma.

Macrophages in the central nervous system of the rat

In an immunohistochemical study using monoclonal antibodies, which exclusively recognize cells of the monocyte-macrophage lineage, and monoclonal antibodies against the Ia-antigen, we describe the occurrence of macrophages in the developing and adult central nervous system (CNS). In normal adult brain, no macrophages could be detected in the CNS parenchyma; only in the meninges and the choroid plexes were a few macrophages found. During ontogeny, numerous phagocytic cells infiltrated the CNS parenchyma; these cells which did not express Ia are blood-borne. About three weeks after birth, all macrophages had disappeared from the CNS. As microglia in adult and developing brain do not stain with the anti-macrophage antibodies, we suggest that microglial cells are not related to the mononuclear phagocyte system and do not have a hematogenous origin.

Histochemical studies of the differentiation of microglial cells in the cerebral hemispheres of chick embryos and chicks

Using histochemical procedures to reveal the presence of nucleoside diphosphatase (NDPase), thiamine pyrophosphatase (TPPase) and acid phosphatase (AcPase), we investigated the appearance, distribution and ultrastructure of amoeboid and microglial cells in the cerebral hemispheres of chick embryos and young chicks, in order to elucidate the relationship between these two cell populations. On day 6 of incubation, a few round cells exhibiting NDPase, TPPase and AcPase activity were first detected in the thin mantle layer of the cerebral hemisphere. In the corpus striatum, these round cells increased rapidly in abundance until day 13 of incubation, after which their numbers gradually decreased, so that, on day 19 of incubation, they had entirely disappeared. Between day 10 and day 17 or 18 of incubation, round cells were located mainly in the zone of the mantle layer closest to the lumen. On day 10 of incubation, NDPase-, TPPase- and AcPase-positive cells that had a few short cytoplasmic processes (poorly ramified cells) were detected in the intermediate and basal zones of mantle layer. They increased in abundance until day 17 or 18 of incubation and thereafter rapidly decreased in number. Round and poorly ramified cells exhibited NDPase activity on their plasma membranes and in their cytoplasmic vacuoles, with TPPase and AcPase activity being localized within their vacuoles. On day 19 of incubation, NDPase- and TPPase-positive cells with long, well-ramified cytoplasmic processes (well-ramified cells) were observed in the corpus striatum, these being mainly localized in the basal zone. After hatching, these cells increased rapidly in abundance and were distributed throughout the corpus striatum.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of microglia in plaque formation in senile dementia of the Alzheimer type. An immunohistochemical study

Using immunohistochemical and enzyme histochemical methods, we have investigated the presence of mononuclear phagocytic cells around senile plaques in six brains from patients with senile dementia of the Alzheimer type (SDAT). It is generally supposed that reactive microglial cells are involved in amyloid formation "as representatives of the reticuloendothelial system in the brain." We used different monoclonal antibodies directed against cells of the mononuclear phagocyte lineage, antibodies against the macrophage markers alpha 1-antichymotrypsin and lysozyme, and the lectin WGA, in addition to enzyme histochemical staining for nonspecific esterase and acid phosphatase. It was concluded that no macrophages of the mononuclear phagocyte lineage are involved in plaque formation. The role of glial cells in amyloid formation is discussed.

Genesis of resting microglia in the gray matter of mouse hippocampus

The genesis of resting microglia in the gray matter of mouse hippocampus was studied by 3H-thymidine autoradiography in combination with electron microscopy. Newborn mice were injected with 3H-thymidine singly or repeatedly at different postnatal stages, and killed shortly after the injection or after various intervals. Tissue specimens of the hippocampus at CA1 and CA2 were processed for light and electron microscopic autoradiography. The results showed that at least 91% of glial cells in the stratum radiatum of the hippocampus are produced after birth. About three-fourths of astroglia in this area are produced before the sixth postnatal day, and a larger part of resting microglia are formed after the ninth postnatal day. Morphological transition can be traced from either proliferating cells in the stratum radiatum at late postnatal days to resting microglia, or from those in early postnatal days to astroglia. A continuous morphological transition was observed between the proliferating cells at the late postnatal days (microglial production period) and those at the early postnatal days (astroglial production period). The latter retain some fine structural characteristics similar to small glioblasts in the subependymal layer. These findings strongly suggest that resting microglia, as well as astroglia, are derived from glioblasts, and are of neurectodermal origin.

Immunohistochemical studies of blood monocytes infiltrating into the neonatal rat brain

Brains of normal rats ranging in age from newborn to adult were observed with immunofluorescence technique using anti- granulomonocyte antiserum. For the first 10 days after birth, many cells with positive fluorescence were found in the white matter, the subependyma , the extra-parenchymal spaces, and the leptomeninx , but very few in the gray matter. They were mononuclear, rich in cytoplasm, and globular or irregular in shape. After about day 10 p.n., the positive cells decreased in number and became slender. However, there was no change in the distribution pattern. After about 3 weeks of age, no positive cells were detected in the brain parenchyma, except for very rare necrobiotic ones. It was suggested that blood monocytes infiltrate into the brain parenchyma of normal neonatal rat, but only for a while in the limited areas (white matter and subependyma ). They have the morphology and distribution of the "ameboid microglia" of neonatal brain. These monocytes disappear from the brain finally by the end of month 1 p.n.

A qualitative and quantitative ultrastructural study of glial cells in the developing visual cortex of the rat

(i) This paper provides new information on the time course and fine structural features of glial cell differentiation, on the relative frequencies of glioblasts, astroblasts, astrocytes, oligodendrocytes and microglial cells, and on neuron: glia ratios in visual cortex of the rat between birth and maturity. The analyses were done on montages of electron micrographs of 75 pm wide strips extending the full depth of the cortex from animals 12 h and 4, 6, 8, 10, 12, 14, 20, 24, 90 and 180 days old (six montages from two or three animals at each age). (ii) At birth, and up to 4 days, most non-neuronal cells are poorly differentiated, irregularly shaped cells with dark nuclei (glioblasts). A few at this stage and progressively larger numbers over the next few days, can be recognized asastroblastsby the presence of a distinctive form of granular reticulum (distended cisterns with a moderately electron dense content), and some also by their position in contact with the subpial or perivascular basal laminae. Astroblasts enlarge, develop processes and transform into immature astrocytes: their nuclei become paler, the granular reticulum is no longer distended, and glial filaments begin to accumulate.Mature astrocyteswith pale nuclei, filaments and a low concentration of perikaryal organelles in a pale cytoplasmic matrix predominate at 24 days, and at 3-6 months 51 % of all glial cells are astrocytes. (iii) Concentrations of glioblasts (at 0 and 4 days) and subsequently of cells of the astrocytic lineage are apparent in the most superficial and in the deepest cortical layers, and an additional small peak is seen at the level of layer IV in the adult animals. The superficial concentration is probably associated with the subpial glia limitans and the layer IV concentration with the high density of synapses in this region; several probable explanations are considered for the concentration in layer VI. (iv) Processes ofradial glial cellsare apparent from birth to day 8 but not thereafter. No evidence was found for transformation of radial glia into astrocytes. A peak in phagocytic activity by immature microglial cells at days 6-8 suggests the possibility of loss of radial glial processes by degeneration rather than transformation. (v)Oligodendroblasts, intermediate in morphology between glioblasts and light oligodendrocytes, appear suddenly in the deep cortex and subcortical white matter at day 6 and are rapidly replaced bylight oligodendrocytes. These are large, organelle-rich cells with characteristically distended Golgi saccules, and are the only oligodendrocytes present during early myelination, which begins at day 10. Early in the 3rd postnatal week some light oligodendrocytes are replaced bymedium oligodendrocytes, which are smaller and darker, with abundant orderly stacks of granular reticulum.Dark oligodendrocytesare first apparent at the end of the 3rd week, account for about one-third of all oligodendrocytes at day 24, predominate at day 40 and constitute 90 % of all oligodendrocytes at 3 and 6 months, at which time oligodendrocytes comprise 39% of all cortical glial cells. We suggest that the progression from light to medium oligodendrocytes does not simply represent a diminution in the overall level of synthetic activity but that different components of the myelin sheath are being synthesized at the two stages. (vi)Microgliaare present from birth but are seen in significant numbers at days 6—10 and thereafter. Some are relatively mature in appearance, even in the youngest animals, and almost all are similar to the resting microglia of adult brain by day 16. At 3-6 months, 8 % of all cortical glial cells are identified as microglia and these cells are fairly evenly distributed throughout the cortical depth but are surprisingly and consistently poorly represented in layer VI. From day 6 to the end of the 2nd postnatal week, cells with poorly differentiated cytoplasm (many free polyribosomes), but containing phagocytosed products of cell degeneration, are identified asimmature microglia. However, it is possible that such cells do not mature into classical resting microglia but that they represent a different cell type. (vii) Theneuron: glia ratiois 4.54 at birth, rises to 5.09 at 4 days, and falls to approximately 2.5 at days 12-24. At 3-6 months the ratio is 2.13.

Trauma-induced glial proliferation: possible involvement of the immune system

Rat neurologlial cells proliferate following trauma to the frontal cortex. Since previous reports have indicated that activated T-lymphocytes secrete a factor which can induce the proliferation of glial cells in vitro, we investigated the effects of immunosuppression on the trauma-induced proliferation of glial cells in the rat. Animals were treated with either methotrexate (2.5 and 10 mg/kg/day), hydrocortisone (100 mg/kg/day), or saline for five days prior to lesioning of the cortex. Immunocompetence was estimated by measuring sheep red blood cell hemagglutination titers at the time of killing. On the last day of drug treatment, rats were mechanically lesioned on the frontal cortex, and the incorporation of intraventricularly injected 3H-thymidine (3H-TdR) into cortical DNA, a measure of glial cell proliferation, was determined two days after lesioning. Intraperitoneal treatment with methotrexate before the lesioning depressed 3H-TdR incorporation into brain DNA, and reduced hemagglutination titers and thymus and spleen weight. However, intramuscular hydrocortisone pretreatment, which had immunosuppressive actions similar to methotrexate, had a much smaller effect on 3H-TdR incorporation into brain DNA following trauma. Methotrexate given acutely after lesioning did not depress thymidine utilization in the lesioned rat cortex, apparently because of poor penetration of the folate analog into the brain. Thus, we conclude that inhibition of glial cell proliferation resulting from methotrexate pretreatment had been produced indirectly, probably by its severe immunosuppressive effects.

Glial DNA synthesis and cell proliferation in the lesioned frontal cortex of the rat

Proliferation of rat neurological cells was quantified following a lesion of the frontal cortex, with the rate of incorporation of intraventricularly administered [3H]thymidine ([3H]TdR) into cortical DNA serving as an index of glial proliferation. Incorporation of [3H]uridine into the corresponding RNA fractions did not serve this purpose. The intraventricular route of administration of thymidine greatly reduced the amount of [3H]TdR needed to label DNA relative to systemic injection. The rate of incorporation of [3H]TdR into DNA was linear for 75 min post-injection. Significantly more [3H]TdR was incorporated into DNA of the lesioned frontal cortex than that of the contralateral control cortex, during the first 4 days post-trauma. The majority of the acid-insoluble radioactivity (from [3H]TdR) was localized in the nuclear subcellular fraction of the cortex. Experiments indicated that the enhanced incorporation of [3H]TdR was not the result of altered metabolism or pool sizes of TdR in the lesioned cortex. Histological analysis indicated that there was a significant increase in the number of glial cells in the lesioned cortex by day 4 post-lesion, which corresponded to the increase in DNA synthetic activity. It was concluded that mechanical trauma to the frontal cortex of the rat results in an increase in the number of glial cells at and near the lesion which is accompanied by an increase in incorporation of [3H]TdR into cortical DNA. This method of measuring posttraumatic DNA synthesis has several advantages over autoradiography.

Morphological studies on neuroglia. VI. Postnatal development of microglial cells

The postnatal development of microglial cells was investigated in the neonatal rat brain by use of light- and electron microscopy, including enzyme-histochemical techniques. Microglial cells were selectively stained by demonstration of their nucleoside diphosphatase (NDPase) activity and classified into three types: 1) In the early postnatal period "primitive microglial cells" showing scantily ramified processes were found in the cerebral cortex, the hippocampal formation, and the hypothalamus. During the course of the first postnatal week the processes of this cell type developed gradually and the cells were transformed into typical ramified microglial cells, called "resting microglial cells". 2) "Amoeboid microglial cells "showing typical features of macrophages were characteristic of the cerebral white matter. 3) "Round microglial cells" possessing a round soma and few pseudopodia but no characteristic processes occurred in large numbers in the subventricular zone of the lateral ventricle and as single elements in the vicinity of blood vessels. Histochemically, thiamine pyrophosphatase (TPPase) was demonstrated only in the fully developed, ramified microglial cells ("resting microglial cells"), which could be readily observed in the central nervous tissue from the age of 14 day. "Round and amoeboid microglial cells" did not show TPPase activity and disappeared after 14 days of postnatal life. By use of electron microscopy, in neonatal rats NDPase activity was apparent in the plasma membrane of the three types of microglial cells ("primitive, round, and amoeboid" types). They showed basically similar submicroscopic characteristics, i.e., well-developed Golgi apparatus, long strands of rough-surfaced endoplasmic reticulum, single dense bodies and vacuoles, and numerous ribosomes. "Amoeboid microglial cells" were characterized by their well-developed cytoplasmic vacuoles and phagocytic inclusion bodies. The present study strongly suggests a mesodermal origin for these microglial elements.

Morphological studies on neuroglia. V. Microglial cells in the cerebral cortex of the rat, with special reference to their possible involvement in synaptic function

Electron-microscopic survey of selectively stained microglial cells in the cerebral cortex of the rat reveals that the processes of this cell type often encircle axo-dendritic synapses. Enzyme-histochemical methods for thiamine pyrophosphatase (TPPase) or nucleoside diphosphatase (NDPase) were used for the selective marking of the microglial cells; TPPase and NDPase activities were observed in the plasma membrane of microglial cells. The synapses encircled by microglial processes displayed presynaptic structures containing round clear vesicles (50 nm in diameter) and a prominent thickening of the postsynaptic membrane. In vitro, the above-mentioned enzymatic activities were completely suppressed by neuroactive agents such as catecholamines and phenothiazine derivatives. Examination using enzyme-histochemical techniques suggests that a single enzyme may be responsible for both microglial cells in the normal central nervous tissue is discussed.

Are resting and/or reactive microglia macrophages?

According to recent submicroscopic, cytokinetics, and functional (particularly cytoimmunologic) investigations, no relationship exists between "resting" microglia (the small argyrophilic cells appearing in undamaged brain tissue, first described by Rio Hortega) and "reactive" microglia (the argyrophilic cells appearing under pathologic conditions). While "resting" microglia are apparently cells of neuro-ectodermal origin, all observations tend to indicate that "reactive" microglia are derived from extravasated blood monocytes and should be called brain macrophages. In the intact brain parenchyma, no macrophages are demonstrable. Free subarachnoidal cells in the cerebrospinal fluid (CSF), perivascular cells, and epiplexus and/or supraependymal cells in the CSF-containing spaces of the normal central nervous system are cells of the mononuclear phagocyte system and must be considered as CSF macrophages. According to rough estimates, the normal adult central nervous system contains a maximum of 280,000 CSF macrophages.

Immunofluorescence studies of the monocytes in the injured rat brain

Rat brain was obtained on the 4th day after its damage by a stab wound. The injured and the normal control brain tissues were stained by the immunofluorescence technique using anti-granulomonocytic rabbit serum. After the fluorescence observation the same tissues were further stained by hematoxylin and eosin (HE) and studied comparatively by light microscopy. The following results were obtained: (1) The normal adult rat brain lacks the cells which react with the antiserum, thus the resting microglia occurring in the normal adult brain are antigenically different from the cells of the monocyte-macrophage system. (2) In the injured brain tissues monocytes extravasate, enter-brain parenchyma, and take ameboid forms or become macrophages. (3) Among the reactive cells in the injured brain, all of the brain macrophages and most of the ameboid cells were reactive with the antiserum thereby indicating their monocytic origin.