Naturally occurring cell death in the cerebral cortex of the rat and removal of dead cells by transitory phagocytes

Microglial chimaerism in human xenografts to the rat brain

Neural tissue from human fetuses is currently used for intracerebral transplantation to treat patients with Parkinson's disease. The development of the human fetal tissue following grafting has been considered mostly, up to now, from the neuronal point of view in xenografts. Very little is known, in contrast, about nonneuronal, glial, or vascular cells in the grafts. Comparison of the data gathered on the development of grafted human neurons with those obtained in comparable studies using rat transplants has demonstrated species-specific features. We have therefore undertaken a series of studies dealing with nonneuronal cells in human-to-rat transplants to reveal other possible species-specificity of the human tissue. This study has, accordingly, been devoted to the immunohistochemical analysis of microglia of host and donor origins in a human to rat xenograft paradigm allowing clear distinction of the origin of the cells. Human neural tissue was transplanted as a cell suspension into the thalamus of adult rats. Amoeboid human microglia were observed in 1-, 2-, and 3-month-old transplants, but their density, already relatively low at the first stage, decreased further over time. Ramified human microglia were only occasional. In sharp contrast, host rat microglia rapidly invaded the transplant in the absence of any sign of necrosis. The rat cells exhibited first an amoeboid morphology but progressed at the later stages toward a more mature, ramified morphology. These results indicate that donor microglia are quite few in number at first and, at least, do not proliferate actively after transplantation.(ABSTRACT TRUNCATED AT 250 WORDS)

Death of neurons in the neonatal rodent and primate globus pallidus occurs by a mechanism of apoptosis

We have examined the developing rat, mouse and marmoset globus pallidus for evidence of cells dying by a process of "naturally occurring" or programmed cell death. We have demonstrated that cells in the developing mammalian globus pallidus die by a process of apoptosis and that by day 7 after birth many of the apoptotic cells possess a neuronal phenotype. Light microscopic and ultrastructural evidence of apoptotic cell death included cell shrinkage, blebbing of the extracellular membrane and condensation of the nuclear chromatin. Additionally we used an in situ nick translation method to assess the integrity of the DNA within the dying cells. This revealed that cells with the morphological characteristics of apoptosis also possessed fragmented DNA typical of cells undergoing Type 1 programmed or apoptotic cell death. The lack of lysosomal enzyme activity within the dying cells and the frequent observations of phagocytosis by neighbouring cells also suggest that the form of programmed cell death is apoptosis and not Type 2 autophagic degeneration. We found no evidence for cells dying by Type 3 non-lysosomal degeneration since all dying cells examined under the electron microscope possessed intact intracellular organelles and cell membranes. We developed a sensitive silver stain which detected balls of condensed chromatin within the apoptotic cells. This enabled identification of apoptotic cells in the developing globus pallidus at low magnification and so allowed us to map the numbers and distribution of dying cells with time. The incidence of apoptotic cells in the neonatal globus pallidus was greatest at birth and then declined such that few cells were detected at one week and none was seen in the adult rat. Although the loss of large numbers of cells in the developing nervous system is a well documented phenomenon, there are only a limited number of reports of the mechanism by which neuronal cells die, and few of these are in the developing mammalian brain. There are at least four different morphological categories of neuronal cell death which are discriminated on morphological and biochemical criteria. Our analysis suggests that apoptotic or Type 1 cell death is the major form of programmed cell death occurring in the mammalian globus pallidus in the first week of life. This report also describes the use of two methods for the ready identification of apoptotic cells at the light microscope level. Because these methods are suitable for use on tissue sections they provide a means to assess the incidence of apoptotic cell death, in parallel with other analyses of the expression of gene products which control cell fate.

Naturally occurring cell death in the developing cerebral cortex of the rat. Evidence of apoptosis-associated internucleosomal DNA fragmentation

Naturally occurring dead cells in the developing rat neocortex, subcortical white matter and hippocampus, which increase in number during the first postnatal week and decrease thereafter to disappear by the end of the first month, were examined by in situ labeling of nuclear DNA fragmentation. These cells showed peripheral chromatin condensation or extremely dark, often fragmented, nuclei. Southern hybridization following agarose gel electrophoresis of DNA extracted from the developing cortex, but not from adult brain, showed a 'ladder' pattern which is typical of internucleosomal DNA fragmentation. Taken together these results show that naturally occurring cell death (programmed cell death) in the developing cerebral cortex has the morphology of apoptosis and is associated with endonuclease activation.

Downregulation of in vitro neurotoxicity of brain macrophages by prostaglandin E2 and a beta-adrenergic agonist

Brain macrophages (BM), a subpopulation of microglia, have the ability to kill neurons by producing reactive oxygen intermediates. Cocultures of neurons and macrophages derived from the cerebral cortex of rat embryos were used to look for regulation of BM neurotoxicity. Isoproterenol (10(-7) M), a beta-adrenergic agonist, induced a significant inhibition of BM neurotoxicity and this effect was abolished in the presence of propranolol, a beta-adrenergic antagonist. BM neurotoxicity was also reduced in the presence of prostaglandin E2 (10(-8), 10(-6) M), a metabolite derived from arachidonic acid. These results suggest endogenous mechanisms of neuroprotection operating either during development or following lesions.

The SP1 antigen in subplate neurons of the developing cat cortex is an immunoglobulin-like molecule

Monoclonal antibody subplate-1 (mAb SP1) specifically stains somata, dendrites and axons of spiny inverted pyramidal neurons in the subplate zone in the early postnatal kitten neocortex. The SP1 antigen has been previously identified as a cytosolic protein of apparent molecular weight 56 kDa. We have now employed immune-affinity chromatography to further characterize this antigen. An antigen with SP1-like immunoreactivity (ir) is present in various organs, and is particularly enriched in blood plasma. Exsanguination of the organs prior to protein extraction reduces the SP1-ir band dramatically, indicative of a blood-borne molecule. The 56 kDa SP1-ir antigen was purified from plasma by affinity chromatography and subjected to Edman degradation. The first 20 N-terminal amino acids show 80% homology to the N-terminus of immunoglobulin heavy chain of man, the mouse and the dog. If the 56 kDa SP1-ir antigen in plasma is an immunoglobulin, and if an immunoglobulin-like molecule is present in the subplate, then antisera against cat immunoglobulins should stain subplate neurons. A polyclonal antiserum against cat IgG intensely stains the somata and dendrites of subplate neurons. On protein blots, this antiserum recognizes the 56 kDa band, and an additional band of approximately 27 kDa, corresponding in size to immunoglobulin light chains. Preabsorbing mAb SP1 with cat immunoglobulin G abolishes the immunoreactivity in sections of kitten cortex. Further, it dramatically reduces the reactivity on protein blots. The results suggest that the 56 kDa SP1-ir antigen in cortical subplate neurons belongs to the immunoglobulin superfamily.

Stimulation of in vitro myelin synthesis by microglia

Central nervous system myelin is elaborated by oligodendrocytes, which have been studied extensively in cell culture. Dissociated brain cultures allow in vitro analysis of events in myelinogenesis, including cell-cell interactions. Microglia, the primary phagocytic cell of the central nervous system, appear in developing fiber tracts prior to the onset of myelination in vivo. To gain insight into potential oligodendrocyte-microglial interactions during development, these cells were co-cultured and various parameters of myelin synthesis were measured. In co-culture, microglia stimulated the synthesis of sulfatide, a myelin-specific galactolipid, in oligodendrocytes, as well as the expression of the myelin-specific proteins myelin basic protein and proteolipid protein. Activity of the oligodendrocyte cytoplasm-specific enzyme 2',3'-cyclic nucleotide 3'-phosphohydrolase was not elevated, suggesting that the effects of microglia were not due to stimulation of oligodendrocyte proliferation. This was confirmed by the inability of microglia to induce significant DNA synthesis. Conditioned medium from cultured microglia provided a similar stimulatory activity, suggesting that the increase in myelin synthesis does not require contact between oligodendrocytes and microglia. These findings suggest a stimulatory role for microglia during myelinogenesis.

Brain macrophages stimulate neurite growth and regeneration by secreting thrombospondin

The presence of macrophages in the developing or lesioned central nervous system (CNS) led us to study the influence of these cells on neuronal growth. Macrophages were isolated from embryonic rat brain and we observed that factors released in vitro by these cells stimulate neurite growth and regeneration of cultured CNS neurons. This effect was inhibited by antibodies directed against thrombospondin, an extracellular matrix protein that we found to be synthesized and released by brain macrophages. Immunodetection of thrombospondin in the adult rat brain lesioned by kainic acid confirmed the production of this protein by brain macrophages and indicated an early intraparenchymal accumulation of thrombospondin following injury. These results suggest that brain macrophages contribute actively to neurite growth or regeneration during the development or in pathological contexts.

Development of NADPH-diaphorase activity in the rat neocortex

The activity of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d), an enzyme related to the synthesis of nitric oxide (NO), was studied histochemically in rat neocortex from the day of birth (P0) to young adulthood. At birth, NADPH-d containing neurons were already identifiable, sparsely distributed in the deep half of the pallium of the cerebrum. In addition, weakly stained bands of NADPH-d positive neuropil were detectable in layers VI and deep V and the cortical plate (CP). During the first postnatal week, NADPH-d positive neurons increased markedly, especially in CP and the superficial layers. By P7 the cells were mainly in layers VIb and II/III. Differential NADPH-d activity in the neuropil during this period appeared as higher activity gradually moving upwards until the highest intensity localised in layers II and upper III. In the caudal part of the cortex, the higher activity covered the whole of the supragranular layers. By the end of the second postnatal week, both the number and laminar distribution of NADPH-d neurons were adult-like, still mainly in layers VIb and II/III. The staining intensity in the neuropil was generally reduced but the banding pattern seen at P7 was still present. No detectable changes in the patterns of NADPH-d positive cells and neuropil in the neocortex occurred after the second postnatal week. The rostral part of the cortex matured slightly earlier than the caudal part. These results show that the development of NADPH-d activity correlates with the laminar differentiation and suggest that NADPH-d or NO may play an important role in the maturation of cortical neurons including the establishment of functional connections.

Localization of c-fos, c-jun, and hsp70 mRNA expression in brain after neonatal hypoxia-ischemia

The sites of expression of early response mRNAs were determined in the brains of 7-day-old rat pups exposed to unilateral carotid artery ligation followed by 3 h of hypoxia. Pups were sacrificed after recovery periods ranging from 10 min to 24 h. In agreement with our previous northern blot analysis, in situ hybridization of coronal brain sections to probes for c-fos, c-jun, and heat-inducible hsp70 revealed a marked induction and subsequent disappearance of all three mRNAs during this time period. We observed co-localization of the 2 immediate early gene (IEG) mRNAs, c-fos and c-jun, which encode proteins that act in combination to regulate subsequent gene expression. These mRNAs were expressed in all regions known to be vulnerable to permanent injury in this model, such as the cortex, hippocampus, and striatum, as well as in other regions that are spared from permanent damage, such as contralateral cortex and lateral ventricular neuroepithelium. The temporal and regional co-localization of c-fos and c-jun suggests that the transcriptional regulatory activity of their protein products could play a role in plasticity associated with death or recovery from injury in the immature brain. Hsp70 mRNA expression was induced in nearly all of the animals that were positive for IEG mRNAs. Although the most frequent site of expression for all three mRNAs was the ipsilateral cerebral cortex, hsp70 expression was restricted to the ipsilateral hemisphere and absent from a number of structures that were positive for c-fos and c-jun. In addition, the patterns of expression of hsp70 within specific structures frequently differed from those of the IEGs, implying that although both cellular early response systems are activated in this model, their specific functions are carried out within different microenvironments.

Ontogeny of the distribution of tachykinins in rat cerebral cortex: immunocytochemistry and in situ hybridization histochemistry

Tachykinins in the mammalian brain are derived from two genes: preprotachykinin A, encoding substance P and neurokinin A, and preprotachykinin B, encoding neurokinin B. Using immunocytochemistry and in situ hybridization histochemistry, we have investigated the ontogeny and distribution of substance P and neurokinin B in various cortical areas of rat cerebrum at different prenatal and postnatal ages. Preprotachykinin A mRNA-positive and -immunoreactive cells were first detected at birth and were abundant in layer VIb and the adjacent white matter in the cingulate and frontal cortices. By postnatal day 5, the numbers of substance P-expressing cells were diminished dramatically in those layers. However, their number gradually increased and spread out laterally to cover parietal and temporal cortices from P5 to P15 in layer V. At these stages, cells were also observed in layer II, although fewer in number. The number of substance P mRNA-positive neurons and substance P-immunoreactive cells decreased gradually from P10 and P15 onward, respectively. On the other hand, expression of neurokinin B, as detected by in situ hybridization histochemistry or immunocytochemistry, was not evident until P10. Neurons expressing this tachykinin were concentrated in layer II, and to a lesser extent in layers V and VI. This pattern of distribution was retained through P45. The present data show a marked difference between these two tachykinins in onset and trends of development, suggesting functional independence of these two tachykinins in the cerebral cortex.

Sexual differences in the numerical density of synaptic profiles of developing rat visual cortex

Axo-dendritic synaptic profiles were quantified along the whole depth of the visual cortex of 10-day-old male and female rats. In both sexes the numerical density of synaptic profiles on spine-like structures was greater than the numerical density of synapses on dendritic shafts. Females had a significantly greater numerical density of synaptic profiles on spine-like structures, than did males at a distance of 200-400 and 500-600 microns from the pia surface, which corresponds to layers II-III and IV of the cortex, respectively. A small percentage (2%-4%) of spine-like structures received two presynaptic terminals. This type of double synapses was three times more abundant in females. No sex differences were found in the numerical density of synapses on dendritic shafts in any cortical layer.

Cellular forms and functions of brain microglia

Consistent with the recent characterization of microglial cells as macrophages, an overall picture for the unique function of these cells in CNS tissue has developed. The microglia are derived from blood monocytes that migrate into the tissue during fetal development and subsequently remain after complete formation of the blood-brain barrier. These monocytes give rise to the ramified microglia of adult tissue through the developmental intermediate of amoeboid microglia. Ramified microglia appear uniquely adapted in contrast to other tissue macrophages based on their stability or lack of turnover and mitotic capability. The ramified cells, while usually downregulated, can convert into active macrophages termed reactive microglia; this conversion appears to occur nonspecifically in response to any injury. Further, reactive microglial cells can fuse to form giant multinucleated cells during viral infections. Each microglia cell form possesses a characteristic morphology and differing functional state with regard to macrophage activity. In their role as tissue macrophages, microglia are involved in immune responses, tissue transplantation, and AIDS dementia complex, as well as many other neurological mechanisms and diseases.

Microglial invasion and activation in response to naturally occurring neuronal degeneration in the ganglion cell layer of the postnatal cat retina

Retinae of kittens between postnatal (P) days 2 and 10 were examined for the presence of degenerating neuronal profiles, normal nucleoli and microglia. Comparison of the numbers of degenerating profiles with numbers of axons lost from the optic nerve suggest that the majority of these profiles result from the degeneration of retinal ganglion cells. Analysis of local densities of the different profiles revealed different rates of cell loss, occurring at different times in central and peripheral retina. The period of rapid cell loss occurred between P2 and P3 in central retina compared to between P8 and P10 in peripheral retina. At both locations, these periods of rapid cell loss were accompanied by a decrease in the ratio of microglia to dying cells even though the absolute densities of microglia increased. However, calculation of the clearance times of cellular debris indicate that the speed of removal of degeneration products is greater during rapid cell loss, which suggests that cellular degeneration serves to activate the phagocytic process.

Nuclear patterns of apoptotic and developing neurons of superior cervical ganglion of newborn rat

Superior cervical ganglion (SCG) neurons of female rats aged 3, 5 and 7 days revealed conspicuous nuclear changes in neurons undergoing postnatal cell death. Several qualitative and quantitative data such as nuclear size and shape, the presence of atypical chromocenters and chromatin textural features discriminated well neurons candidate to degeneration and those advancing in the direction of adult maturation. At least on morphological grounds, postnatal death of SCG neurons appears to be of apoptotic type. The sequence of nuclear events observed enables the recognition of the early stages of involution which prelude neuron degeneration.

Macrophages are required for cell death and tissue remodeling in the developing mouse eye

To identify and characterize tissue remodeling processes mediated by macrophages, we have generated transgenic mice in which diphtheria toxin is expressed from a macrophage-specific transgene. Expression of the transgene disrupts subsets of mature macrophages in both the eye and the peritoneal cavity and results in the persistence of two normally transient ocular tissues, the hyaloid vasculature and the pupillary membrane. Furthermore, the cells comprising the pupillary membrane appear alive up to 14 days after the structure is normally remodeled, suggesting that the macrophage actively elicits target cell death. Thus, these transgenic mice provide direct evidence for the active involvement of macrophages in developmentally programmed tissue remodeling and identify the hyaloid vessels and the pupillary membrane in the eye as targets of macrophage-mediated remodeling.

First appearance, distribution, and origin of macrophages in the early development of the avian central nervous system

A phagocytic cell system of hemopoietic origin exists in the early avian embryo (Cuadros, Coltey, Nieto, and Martin: Development 115:157-168, '92). In this study we investigated the presence of cells belonging to this system in the central nervous system (CNS) of chick and quail embryos by using both histochemical staining for acid phosphatase and immunolabelling with antibodies recognizing cells of quail hemangioblastic lineage. The origin of these cells was traced in interspecific chick-quail yolk sac chimeras. Hemopoietic cells were detected within the CNS from developmental stage HH15 on, and steadily increased in number at subsequent stages. Analysis of yolk sac chimeras revealed that most of these cells were of yolk sac origin, although some hemopoietic cells of intramebryonic origin were also found in the CNS. Immunocytochemical, histochemical, and ultrastructural characterization allowed us to identify hemopoietic cells in the CNS as macrophages. These cells were consistently found in the brain vesicles and spinal cord, appearing (1) between undifferentiated neuroepithelial cells at dorsal levels of the CNS; (2) in areas of cell death; (3) in the marginal layer in close relationship with developing axons; (4) in large extracellular spaces in the subventricular layer; (5) on vascular buds growing through the marginal and subventricular layers; and (6) in the ventricular lumen. Macrophages in different locations varied in morphology and ultrastructure, suggesting that in addition to their involvement in phagocytosis, they play a role in other processes in the developing CNS, such as axonal growth and vascular development. The first macrophages migrate to the CNS independently of its vascularization, apparently traversing the pial basal lamina to reach the nervous parenchyma. Other macrophages may enter the CNS together with vascular buds at subsequent stages during CNS vascularization.

Developmental regulation of cytokine expression in the mouse brain

Expression of four cytokine genes, transforming growth factor (TGF) beta 2, tumor necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), and macrophage colony-stimulating factor (CSF-1 also known as M-CSF) was examined to determine whether these genes are developmentally regulated in the brain. Northern blots were performed on RNA isolated from the mouse brain from embryonic day 15 (E15) through postnatal day 9. TGF beta 2 gene expression was relatively high in the earliest embryos studied and decreased after E16-E17, and the three transcripts were developmentally regulated. TNF-alpha and IL-6 were detected in total RNA on all days studied. CSF-1 was detected only in polyadenylated RNA. The data suggest that expression of these cytokines is related to specific developmental events that share cellular functions with regenerative or inflammatory processes.

The origin and nature of ramified and amoeboid microglia: a historical review and current concepts

The origin of ramified microglia has been a longstanding controversial issue, with 4 major schools of thought, which state that they are derived (1) from invasion of mesodermal pial elements, (2) from neuroectodermal matrix cells together with the macroglia, (3) from pericytes, and (4) from invasion of monocytes in early development. This paper is in support of the last-mentioned hypothesis. It is known that ramified microglial cells do not divide under normal circumstances, and since our studies in the corpus callosum have shown that these cells do not appear until the fifth postnatal day, it is reasoned that they must be derived from some preexisting mitotically active cells. The putative precursor is the preponderant amoeboid microglia in the same region. Our experimental studies with the carbon labelling technique have demonstrated for the first time that blood monocytes invade into the early postnatal brain to become amoeboid microglia, which then differentiate into ramified microglia. Just like other tissue macrophages, the monocyte-derived amoeboid microglia exhibit features indicative of phagocytic activities. These include the content of hydrolytic enzymes, uptake of carbon, and a characteristic surface morphology, as seen by scanning electron microscopy. The transformation of amoeboid microglia into ramified microglia, which occurs between the second and third postnatal week, is considered to be a regressive phenomenon, as shown by the diminution of their content of hydrolytic enzymes and the downregulation of membrane antigen. Apart from their primary role as active phagocytes, their involvement in Alzheimer's disease (AD) is evidenced recently by the fact that the cells are specifically marked by antibodies present in the cerebrospinal fluid of AD patients. In conclusion, ramified microglial cells are derived from monocytes, but through an intermediate amoeboid microglia stage as active macrophages in the perinatal period.

Development of GABA-immunoreactivity in the neocortex of the mouse

The prenatal and postnatal development of GABAergic elements in the neocortex of the mouse was analyzed by GABA-immunocytochemistry. Radial distribution of cells and laminar numerical densities were calculated at each developmental stage to substantiate qualitative observations. The first immunoreactive neurons were observed in the cortical anlage at embryonic day 12-embryonic day 13 (E12-E13) in the primitive plexiform layer. At following prenatal stages (E14-E19), most GABA-positive neurons were present in the marginal zone, subplate, and subventricular zone. GABA-immunoreactivity in the cortical plate appeared early (E14), although the complete maturation of its derivatives was achieved postnatally. At prenatal stages we noted a well-developed system of immunopositive fibers in the subplate. As indicated by the direction of growth cones, most of these fibers had an extracortical origin and invaded the cortex laterally through the internal capsule and striatum. In rostral and middle telencephalic levels, fibers originating in the septal region contributed to the cingulate bundle. Presumably corticofugal fibers and callosal axons were also noticed. At postnatal stages the maturation of GABA-immunoreactivity appeared to be a complex, long-lasting process, in which the adult pattern was produced at the same time as the appearance of certain regressive phenomena. Thus, between postnatal day 0 and postnatal day 8 (P0-P8), GABA-positive populations disappeared from the subventricular zone, marginal zone and to a lesser extent from the subplate. At the same ages we noticed the presence of morphologically abnormal, GABA-immunoreactive neurons in the subventricular zone and subplate which are interpreted as correlates of neuronal degeneration. Most GABA-positive subplate fibers also disappeared whereas GABA-immunoreactive axons were seen in the cingulate bundle until the adult stage. In the derivatives of the cortical plate, the maturation of GABA-immunoreactive elements progressed according to the "inside-out" gradient of cortical development, with the important exception of layer IV, which was the last layer to exhibit an adult-like appearance. Within each layer deriving from the cortical plate (layers VIa to II-III), GABA-immunoreactivity showed a protracted maturation in which the first GABA-positive cells were detected a few days after cell birth but substantial numbers of neurons began to express GABA considerably later. The later phase occurred concurrently with the maturation of GABA-positive axonal plexuses. These results suggest that different GABA-positive populations show different developmental regulation of GABA expression during cortical ontogenesis.

Cell death and the creation of regional differences in neuronal numbers

Regional variations in cell death are ubiquitous in the nervous system. In the retina, cell death in retinal ganglion cells is elevated in the retinal periphery and may be important in setting up the initial conditions that produce central retinal specializations such as an area centralis or visual streak. In central visual system structures, pronounced spatial and spatiotemporal inhomogeneities in cell death are seen both in layers and regions of the lateral geniculate nucleus and superior colliculus; similar indications of inhomogeneities are seen in those nonvisual structures that have been examined. Cell death in the cortex is highly nonuniform, by layer and by cortical area. A variety of possible functions for these regional losses are proposed, in the context of a uniform mechanism for cell death that allows it to assume multiple functions.

Perinatal hypoxic ischemic encephalopathy affects the proportion of GABA-immunoreactive neurons in the cerebral cortex of the rat

The hypothesis was tested whether perinatal hypoxic ischemia leads to a preferential degeneration of the GABAergic inhibitory neurons in the cerebral cortex which, in turn, could account for the reported higher risk of developing epilepsy later in life. To that end rat pups, aged 12-13 days, were made hypoxic by employing a combination of unilateral ligation of one of the carotid arteries and a 90-min exposure to 8% O2. After recovery periods of 3, 7, 35 and 150 days, the animals were sacrificed by perfusion fixation and the brains embedded in Epon. Transverse semi-thin sections were alternately stained with an antibody against GABA and with Toluidine blue. By using an unbiased morphometric method (the disector) the number of GABA-immunoreactive (GABA-IR) neurons and the total number of nerve cells per unit volume of tissue were estimated in corresponding neocortical areas in the ipsilateral (damaged) and contralateral ('control') hemisphere. For all animals with post-ischemic survival times of 3 and 7 days GABA-IR cells constituted a lower proportion of the total number of nerve cells in the damaged than in the 'control' cortical areas. This finding was consistent with the outcome of an earlier in vitro study. By contrast, in all animals with a survival time of 35 and 150 days, the proportion of GABA-IR neurons was higher on the damaged than on the 'control' side. This switch in the direction of the left/right differences, apparently depending on the length of the post-ischemic survival time, was statistically significant. No lateralization in the proportion of GABA-IR cells was detected in the cerebral cortex of the control rats. These observations, therefore, do not support the hypothesis that perinatal hypoxic ischemia ultimately leads to a preferential loss of GABAergic neurons in the cerebral cortex.

The effect of cycloheximide on natural and X-ray-induced cell death in the developing cerebral cortex

Naturally occurring cell death in the cerebral cortex and subcortical white matter is increased after X-irradiation, and this process is curbed with cycloheximide, an inhibitor of protein synthesis. However, cycloheximide alone increases cell death during development, and this effect is dose-dependent. This suggests that, in both normal and experimentally-induced cortical cell death during development, different proteins are activated or inhibited, depending on the agent, the time of its application, and the previous metabolic or functional state of the cell.

Sex differences in neuron number in the binocular area of the rat visual cortex

We have previously shown that the thickness of the binocular area of the primary visual cortex is sexually dimorphic in rats. In the present study, sex differences in the number of neurons in this cortical area were examined in nine littermate pairs of 90-day-old Long-Evans hooded rats. Cytoarchitectonic characteristics were used to define the binocular visual cortex, and its volume was estimated through three-dimensional reconstruction of serial coronal sections for each hemisphere. Neuronal and glial density as well as neuronal soma size were estimated from semithin sections through a stereological technique, the disector, in the same animals that were used to estimate volume. The volume of the binocular area was 19% greater in males than in females. While there were no sex differences in soma size or in neuronal density, the differences in the volume of the binocular area resulted in significant sex differences (male greater than female) in the number of neurons overall and in every layer, except layer IV. Glial density was not different between the sexes, but the total number of glial cells was higher in males than in females. These results demonstrate that the binocular visual cortex of the rat is sexually dimorphic in its volume and much of the difference is due to sex differences in the number of neurons and glial cells.

Brain macrophages: evaluation of microglia and their functions

There is now evidence approaching, if not having already surpassed, overwhelming in support of microglial cells as macrophages. Consistent with this cellular identity, they appear to arise from monocytes in developing brain where amoeboid microglia function in removing cell death-associated debris and in regulating gliogenesis. In normal adult tissue, ramified microglial cells with down-regulated macrophage functional properties may serve a constitutive role in cleansing the extracellular fluid. Under all conditions of brain injury, microglia appear to activate and convert into active macrophages. Activated and reactive microglia participate in inflammation, removal of cellular debris and wound-healing, the latter through regulation of gliosis in scar formation and a potential contribution to neural regeneration and neovascularization. In the activated state, microglia also express MHC's and, thus, may function in antigen presentation and lymphocyte activation for CNS immune responses. As uniquely adapted tissue resident macrophages within the CNS, microglia serve a variety of functional roles over the lifespan of this tissue. These cells may therefore be involved in or contribute to some disease states; such has been indicated in multiple sclerosis and AIDS dementia complex.

A transitory population of substance P-like immunoreactive neurones in the developing cerebral cortex of the mouse

Immunocytochemical methods were used to investigate the developmental expression of substance P (SP) in mouse cerebral cortex. SP-like-immunoreactive cells were first detected at postnatal day 0 (P0), their numbers being notably increased by P2. Immunopositive cells were especially abundant in layer VIb and in the subjacent future white matter, although they were also present in layer V. Between P5 and P8 the number of SP-like-immunoreactive cells gradually decreased, being almost completely absent by P12. At these stages cells were only observed in the deepest cortical layers. From P16 onwards, the adult pattern of SP-like immunoreactivity emerged with a few immunopositive cells scattered throughout the cortical layers. The present data show a transitory population of SP-like-immunoreactive cells present in the mouse cerebral cortex during the first postnatal week. On the basis of close correlations of SP-like expression with the distribution or transitory populations and the timing of cell death in rodents, we propose that most of the SP-like-immunoreactive cells reported here would probably disappear by cell death.

Cell death in the developing trigeminal nuclear complex of the rat

The time course and distribution of cell death in the trigeminal nuclear complex of the rat has been examined with the aid of sections stained for Nissl substance and succinic dehydrogenase activity. Pyknotic figure counts in the principal trigeminal nucleus and in each of the three spinal trigeminal subnuclei revealed that cell death commences at E19, in the region of the junction between the principal nucleus and the subnucleus oralis, close to the site of entry of trigeminal afferents into the brainstem. Cell death subsequently spreads rostrally and caudally into the rest of the principal and spinal trigeminal nuclei. Cell death ceases simultaneously, at about P10, in all parts of the trigeminal nuclear complex examined. Neurons could not be reliably distinguished from glial cells in prenatal animals, but data for neuronal numbers postnatally indicate that much of this cell death is indeed due to loss of neurons. The data suggest that, in the trigeminal nuclear complex, only half the number of neurons produced survive to maturity. These findings are of significance for those investigators using this system in studies of plasticity.

Naturally occurring, postnatal cell death in the cerebral cortex of the micrencephalic rat induced by prenatal X-irradiation

Naturally occurring postnatal cell death in the somatosensory cortex and medial cortical areas was examined in micrencephalic rats produced after exposure to 100 cGy X-rays at embryonic day 15. The absolute numbers of dead cells per tissue section were not affected in the future subcortical white matter, but were severely decreased in the cerebral cortex when compared with age-matched controls. Furthermore, the proportion of cortical dead cells per 1000 live neurons in the cerebral cortex and the cumulative cell death across time were significantly reduced in irradiated animals. These results suggest that naturally occurring postnatal cell death in the cerebral cortex is reduced in micrencephaly.

Cytotoxic Effect of Brain Macrophages on Developing Neurons

Brain macrophages are transiently present in different regions of the central nervous system during development or in the course of tissue remodelling following various types of injuries. To investigate the influence of these phagocytes on neuronal growth and survival, brain macrophages stemming from the cerebral cortex of rat embryos were added to neuronal primary cultures. A neurotoxic effect of brain macrophages was demonstrated by the reduction of the number of neurons bearing neurites within two days of contact between the two cell types. Neuronal death and phagocytosis were also directly observed in video recordings of living cultures. This toxicity involved the production by brain macrophages of reactive oxygen intermediates, as shown by the protective effect of catalase, a scavenger of H2O2. In addition, the respiratory bursts of brain macrophages were stimulated in the presence of neurons. These results suggest that brain macrophages could favour the appearance of neuroregressive events which occur either during neurogenesis or in neurodegenerative diseases, implying intracerebral recruitment of mononuclear phagocytes.

Fibronectin and laminin regulate the in vitro differentiation of microglial cells

During development, the differentiation of ameboid microglia (brain macrophages) into ramified microglia is marked by a loss of macrophage-like properties and the extension of thin cytoplasmic projections. We have studied the influence of two extracellular matrix proteins, laminin and fibronectin, on microglia differentiation, using cell cultures. Brain macrophages were isolated from primary glial layers derived from embryonic rat brain and further cultured in serum-free medium. The addition of fibronectin induced the transformation of round or spindle-shaped brain macrophages into cells displaying a reduced cell body and extending thin and long processes. This morphological transformation was associated with a reorganization of the vimentin network, including a condensation of dispersed filaments into thick bundles and a modification of the phosphorylation state of vimentin monomers. In addition, compared to brain macrophages, the process-bearing microglia lost the ability to engulf zymosan particles, and showed reduction in non-specific esterase activity and superoxide anion generation. In contrast, laminin reduced the spontaneous transformation of brain macrophages into process-bearing cells. Moreover, laminin and serum induced a reverse transformation of process-bearing cells when added to cultures pretreated with fibronectin. Altogether these results demonstrate antagonist effects of fibronectin and laminin on the in vitro differentiation of brain macrophages towards a "resting" phenotype, which shares several properties with the ramified microglia present in the adult brain. We suggest that fibronectin and laminin regulate the differentiation of microglial cells, which takes place during development or following various types of lesions in the adult brain.