Transient populations of presumptive macrophages in the brain of the developing hamster, as indicated by endocytosis of blood-borne horseradish peroxidase

Identification of transient microglial cell colonies in the forebrain white matter of developing rats

Herein, we describe the existence of distinct colonies of transient microglial cells that reside in well-defined zones of the forebrain white matter. Rats, aged at postnatal day (P) 0, P2, P5, P7, P10, P15 or adult, were anaesthetised with halothane gas, and various neural centres were injected unilaterally with the tracer biotinylated Dextran. The neural centres injected were cingulate or sensorimotor cortices, ventral nuclei of the dorsal thalamus, and the pontine reticular formation of the brainstem. Rats were allowed to survive to various stages, from 4 hours to 21 days, after the injection. They were then anaesthetised with sodium pentobarbitone, and their brains were aldehyde-fixed and processed by using standard methods. The following is a description of what is seen after injections at P0, P2, P5, P7, P10; we saw no labelled cells (described below) in the rats injected at P15 or adult. From 2 to 21 days after an injection of dextran into the above-mentioned centres, labelled microglial cell colonies, identified by using double-labelling with anti-OX-6 or Griffonia simplicifolia (Bandeiraea; isolectin B4), were seen in small isolated zones in the forebrain white matter. These colonies were in the corpus callosum, the dorsal and ventral regions of the external capsule, and the internal capsule. A striking feature of these labelled microglial cell colonies was that they were seen on both sides of the brain. Thus, regardless of the location of the injection site in either the cortex, thalamus, or brainstem, the same microglial cell colonies were labelled with dextran in the forebrain white matter. After injections of different coloured fluorescent dextrans into the cortex and into the brainstem of the same animal, many double-labelled cells in each of the colonies were seen. From our short-term survival cases (4 hours to 1 day), a rather strict sequence or progression of labelling of the colonies across the white matter from the injection site was seen; in general, the microglial cell colonies closest to the injection site became labelled well before (about a day) those further away. These results lead us to suggest that the microglial cells in each colony become labelled after a slow diffusion of the tracer through the extracellular space from the injection site.

Labeling of amoeboid microglial cells and intraventricular macrophages in fetal rats following a maternal injection of a fluorescent dye

Amoeboid microglial cells (AMC) in fetal brains were labeled by rhodamine B isothiocyanate (RhIc) when injected intravenously or intraperitoneally into mother rats at late state of pregnancy. The fluorescent cells were immunostained with antibodies OX-42 and OX-18 that recognize complement type 3 (CR3) receptors and major histocompatibility complex class I (MHC-I) surface antigen, respectively. RhIc-labeled AMC were first observed in the cavum septum pellucidum and subependymal cysts associated with the cerebral aqueduct as well as the fourth ventricle, and subsequently at other sites including the corpus callosum and other subcortical white matter. The fluorescence intensity increased with time after RhIc administration so that after 1 day the cells were brightly labeled. The majority of the labeled cells were round, with some elongated ones bearing two or three processes. Besides AMC, macrophages in the ventricular system were also labeled. All fluorescent cells were double labeled with OX-42 and OX-18 antibodies. Present results suggest that when introduced into the maternal circulation, RhIc could readily gain access into the fetal brain through the inefficient placental, blood-brain and blood-cerebrospinal-fluid (blood-CSF) barriers. The avid uptake of RhIc in circulation by brain macrophages indicates an active scavenging role of these cells in fetal brain. The labeling of cells by maternal route offers a rapid method for study of distribution of brain macrophages in fetuses.

NADPH-diaphorase activity in brain macrophages during postnatal development in the rat

NADPH-diaphorase histochemistry, that allows the visualization of cells producing the gaseous intercellular messenger nitric oxide, was used in the study of the forebrain during the first three postnatal weeks in the rat. Subpopulations of NADPH-diaphorase positive neurons were observed at all ages studied. In addition, non-neuronal NADPH-diaphorase-stained cells were detected in the subcortical white matter, and were very numerous in the supraventricular portion of the corpus callosum, and in the internal and external capsules. These cells were present during the first two postnatal weeks, and were especially prominent at the end of the first postnatal week. They were round-shaped and morphologically similar to the brain macrophages, whose phagocytic activity has been shown in previous studies to play a role in naturally occurring cell death and elimination of exhuberant axons. Series of sections adjacent to those stained with NADPH-diaphorase were processed with immunohistochemistry, using two different antibodies (OX-42 and ED-1) that detect macrophagic and microglial markers, and antibodies that recognize the neuronal form of nitric oxide synthase. Furthermore, brain sections from rats at postnatal day 7 were sequentially processed for either OX-42 or nitric oxide synthase immunohistochemistry followed by NADPH-diaphorase histochemistry. The morphological features and distribution of the non-neuronal NADPH-diaphorase-positive cells were superimposable to those obtained with OX-42 and ED-1 immunohistochemistry. In addition, these cells did not display nitric oxide synthase immunoreactivity. Double-labelled NADPH-diaphorase-positive and OX-42-immunoreactive cells were detected at postnatal day 7. The present results show that brain macrophages express NADPH-diaphorase activity during the early stages of the normal postnatal maturation and suggest that nitric oxide produced by brain macrophages could be involved in the development reshaping of the central nervous system.

Developmental profile of non-heme iron distribution in the rat brain during ontogenesis

The entry of iron from blood into the developing rat brain was studied by means of non-heme iron-histochemistry. The content of non-heme iron in the endothelial cells was manifest already from E14, declined from P3 to P5, and was almost absent on P10-P15. The choroid plexus epithelial cells of either ventricle was non-heme iron-containing from E14. Non-heme iron-containing macrophages situated in the stroma of the choroid plexus were also observed from E14. From E19, the macrophage-like cells tended to invade into (a) regions with transitory structures like the intermediate zone of the cerebral hemisphere, (b) developing axonal tracts like corpus callosum and internal capsule, and (c) deep layers of the tectum, a region with an extensive degree of naturally occurring cell death. The amoeboid macrophage-like cells observed in the brain parenchyma gradually acquired prolonged extensions and apparently differentiated into ramified microglia-like cells, which later lost their non-heme iron-content. Thus, at P70, non-heme iron-positive microglia-like cells were hardly seen reflecting the transitory event of non-heme iron in microglia-like cells. At P200, non-heme iron-containing microglia cells and oligodendrocytes appeared in manifestly higher number than at P70, a phenomenon probably related to aging. These results delineate for the first time the appearance of iron in the developing brain. The results are of relevance for understanding the potential of iron-deficiency for harming the developing central nervous system, generally by decreased transport of iron through brain capillaries and choroid plexus, and specifically by an impaired modulation of the developing brain parenchyma by iron-containing macrophages.

Origin of microglia in the quail retina: central-to-peripheral and vitreal-to-scleral migration of microglial precursors during development

The origin, migration, and differentiation of microglial precursors in the avascular quail retina during embryonic and posthatching development were examined in this study. Microglial precursors and developing microglia were immunocytochemically labeled with QH1 antibody in retinal whole mounts and sections. The retina was free of QH1+ macrophages at embryonic day 5 (E5). Ameboid QH1+ macrophages from the pecten entered the retina from E7 on. These macrophages spread from central to peripheral areas in the retina by migrating on the endfeet of the Müller cells and reached the periphery of the retina at E12. While earlier macrophages were migrating along the inner limiting membrane, other macrophages continued to enter the retina from the pecten until hatching (E16). From E9 on, macrophages were seen to colonize progressively more scleral retinal layers as development advanced. Macrophages first appeared in the ganglion cell layer at E9, in the inner plexiform layer at E12, and in the outer plexiform layer at E14. Therefore, it seems that macrophages first migrated tangentially along the inner retinal surface and then migrated from vitreal to scleral levels to gain access to the plexiform layers, where they differentiated into ramified microglia. Macrophages appeared to differentiate shortly after arrival in the plexiform layers, as poorly ramified QH1+ cells were seen as early as E12 in the inner plexiform layer and at E14 in the outer plexiform layer. Radial migration of macrophages toward the outer plexiform layer continued until posthatching day 3, after which retinal microglia showed an adult distribution pattern. We also observed numerous vitreal macrophages intimately adhered to the surface of the pecten during embryonic development, when macrophages migrated into the retina. These vitreal macrophages were not seen from hatching onwards, when no further macrophages entered the retina.

Early ultrastructural changes in blood-brain barrier vessels of the rat embryo

The blood-brain barrier (BBB) in fetal rat brain has been shown by others to be more permeable to a variety of blood-borne solutes than the BBB in adults. We used ultrastructural morphometric methods to measured the density of putative vascular pores between the ages of embryonic day (E) 11 and birth to determine the structural basis for this relatively high permeability. We found that fenestrations, that are frequent at E11, declined rapidly and were last seen at E13 in intraparenchymal vessels and at E17 in pial vessels. Interendothelial junctions in fetal brain contained expanded clefts suggestive of paracellular channels at all ages examined, although they disappear after birth. Both of these features likely contribute to high fetal BBB permeability, but endothelial vesicles probably do not. The central nervous system is vascularized by ingrowth of capillary sprouts from the perineurial vascular plexus. Invading capillaries express BBB features in response to inductive signals from the surrounding neural tissue. We compared early ultrastructural changes in perineurial vessels, which are separated from neural tissue by a sizeable perivascular space, with those in intraneural vessels, which are totally enveloped by neural tissue, to determine whether the inductive interaction requires close cellular contact. For the most part, the perineurial and intraneural vessels matured in parallel. Furthermore, cerebellar vessels developed in parallel with cerebral vessels, even though they did not invade neural tissue until a comparatively late stage. These results suggest that intimate contact between neural tissue and vessel walls is not a requirement for BBB expression.

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.

The ontogenesis of the forebrain commissures and the determination of brain asymmetries

We have reviewed the organization and development of the interhemispheric projections through the forebrain commissures, especially those of the CC, in connection with the development of brain asymmetries. Analyzing the available data, we conclude that the developing CC plays an important role in the ontogenesis of brain asymmetries. We have extended a previous hypothesis that the rodent CC may exert a stabilizing effect over the unstable populational asymmetries of cortical size and shape, and that it participates in the developmental stabilization of lateralized motor behaviors.

Amoeboid and ramified microglia: their interrelationship and response to brain injury

Rio-Hortega's hypothesis that transiently appearing amoeboid microglia might become ramified microglia in the adult and that the latter could differentiate into brain macrophages in the event of brain damage could not be proved because of inherent limitations in existing techniques. The present investigation used a novel method of labelling the rat supraventricular amoeboid microglia with an enduring fluorescent marker, rhodamine B isothiocyanate, introduced intraperitoneally. Observation of their subsequent development showed that they became transformed into the ramified microglia. Both the amoeboid and ramified microglia were OX-42 positive, indicating their macrophage/monocyte lineage. Other microglia in the cerebral neocortex, which were also OX-42 positive, were not derived from any of the rhodamine-labelled cells. Rhodamine-labelled microglia did not migrate toward the site of a superficial cerebral injury. Following a deep lesion reaching the corpus callosum, greatly increased numbers of labelled amoeboid microglia were frequently observed at or near the lesion site. Large rhodamine-labelled cells, which were OX-42 positive, appeared at all lesioned sites, and such were most likely blood derived monocytes. The antigenicity of the ramified microglia became elevated when rhodamine B isothiocyanate was present intracellularly and even more so with the presence of a nearby intracerebral stab wound.

Macrophages, microglial cells, and HLA-DR antigens in fetal and infant brain

Immunohistochemical reactions for macrophages, microglia, and HLA-DR antigens were tested on frozen sections of necropsy brain tissue from 20 fetuses and infants ranging in age from 18 weeks' gestation to 8 months post term. No primary central nervous system disease was present but there were four cases of sudden infant death syndrome (SIDS). Macrophages were detected in all the samples studied and were located in the germinal matrix zone, in perivascular spaces throughout the brain, and in the leptomeninges and subependymal layer. Well differentiated microglia were present in all cases examined after 35 weeks' gestation and less well ramified forms were seen at earlier stages of gestation. HLA-DR antigens were detected on a small number of macrophages, chiefly in a perivascular location, in all but three cases. The fewest reactive cells and the weakest reactions occurred in the youngest fetuses. One case of SIDS showed increased foci of microglia in perivascular white matter: this case and one other case of SIDS were the only cases with well ramified microglia that expressed HLA-DR antigens. These findings may be relevant to an understanding of local immune responses in fetal brain infections, including human immunodeficiency virus infection.

Lipid-containing cells in the brain in sudden infant death syndrome

The authors report a semi-quantitative autopsy study on the content of lipid-containing cells (LCC) in corpus callosum and periventricular frontal white-matter from the brains of 96 infants. These were 55 cases of sudden infant death syndrome (SIDS), 28 with conditions expected to have caused hypoxia (hypoxic control), and 13 with conditions not expected to have caused more than brief, terminal hypoxia (non-hypoxic control). Appreciable numbers of LCC were found in the non-hypoxic control cases, but significantly more LCC were found in the hypoxic controls. LCC in SIDS cases were intermediate between the non-hypoxic and hypoxic controls. The findings are discussed in the light of an experimental primate study. The authors conclude that the slight excess of LCC in SIDS cases is more likely to be an exaggeration of normal developmental LCC accumulation than evidence of pre-terminal episodes of hypoxia.

Neurogenesis and development of callosal and intracortical connections in the hamster

The developmental time-course of callosal and ipsilateral corticocortical projections was studied in embryonic and postnatal hamsters, from the time of neurogenesis until the appearance of adult patterns. Callosal neurogenesis was determined by combining the incorporation of [3H]thymidine injected on specific embryonic days with retrograde labelling of callosal neurons in the adult animal. The development of both callosal and corticocortical projections was studied by the transport of wheat germ agglutinin conjugated to horseradish peroxidase. Despite a significant radial disperson of postmigratory neurons born on the same day, it was found that the birthdates of callosally-projecting neurons in the frontal cortex were not restricted to a short period of time, but extended between embryonic days 13 and 15. This period covers the neurogenesis of cells in cortical layers III-V. Elongation of callosal axons (and possibly also of corticocortical fibres) started a couple of days before birth in the frontal cortex, and continued through the first postnatal days. After a "waiting period" of a few days, axons from both sets of projections were seen innervating restricted target sectors of the cortex. The zones of origin of these projections were initially exuberant, but were subsequently trimmed to overlap completely with the corresponding terminal fields. It is concluded that callosal and ipsilateral corticocortical projections undergo similar sequences of ontogenetic stages, suggesting that the development of neocortical connectivity as a whole may be governed by one and the same set of rules.

Heterogeneity in the distribution and morphology of microglia in the normal adult mouse brain

We have examined the distribution of microglia in the normal adult mouse brain using immunocytochemical detection of the macrophage specific plasma membrane glycoprotein F4/80. We were interested to learn whether the distribution of microglia in the adult brain is related to regional variation in the magnitude of cell death during development and resulting monocyte recruitment, or whether the adult distribution is influenced by other local microenvironmental cues. We further investigated the possibility that microglia are sensitive to their microenvironment by studying their morphology in different brain regions. Microglia are present in large numbers in all major divisions of the brain but are not uniformly distributed. There is a more than five-fold variation in the density of immunostained microglial processes between different regions. More microglia are found in gray matter than white. Particularly, densely populated areas include the hippocampus, olfactory telencephalon, basal ganglia and substantia nigra. In comparison, the less densely populated areas include fibre tracts, cerebellum and much of the brainstem. The cerebral cortex, thalamus and hypothalamus have average cell densities. There was no simple relationship between the amount of developmental cell death and the adult distribution of microglia. An estimate of the total number of microglia in the adult mouse brain, 3.5 x 10(6), is comparable to that found in the liver on a weight for weight basis. However, microglia possess up to twice the surface area of membrane of Kupffer cells, the large resident macrophages of the liver. The proportion of cells that were microglia varied from 5% in the cortex and corpus callosum, to 12% in the substantia nigra. Microglia vary in morphology depending on their location. They were broadly classified into three categories. Compact cells are rounded cells, sometimes with one or two short thick limbs, bearing short processes ("bristles"). They resemble Kupffer cells of the liver and are found exclusively in sites lacking a blood-brain barrier. Longitudinally branched cells are found in fibre tracts and possess several long processes which are usually aligned parallel to, or more occasionally perpendicular to, the longitudinal axis of the nerve fibres. Radially branched cells are found throughout the neuropil. They can be extremely elaborate and there is wide variation in the length and complexity of branching of the processes. There was no evidence of monocyte-like cells in the adult CNS. The systematic variation in microglial morphology provides further evidence that these cells are sensitive to their microenvironment.

Lipid-containing cells in brains of normal and hypoxic infant monkeys: a quantitative and ultrastructural study

The significance of lipid-containing cells found at autopsy in the white matter of infant brains is controversial, particularly with respect to their postulated role as markers of the "sudden infant death syndrome." To determine whether such cells are indicative of prior nonlethal hypoxic insult, we quantitated them in the brains of control infant monkeys and in two groups of infant monkeys that were subjected to 30 minutes of hypoxic insult. One group consisting of monkeys that died less than 48 hours after the hypoxia, and the other of those that survived 7 to 13 days following the insult. The quantification of lipid-containing cells was undertaken in frozen brain sections stained with Oil red O; sections of brains from 4 perfusion-fixed animals were evaluated by electron microscopy. Lipid-containing cells were found in the corpus callosum, in the septum, and in periventricular white matter in both posthypoxic and control animals. There was a relationship between numbers of lipid-containing cells and the age of the animal; animals with large numbers were less than 28 days old. Decreases in numbers of lipid-containing cells correlated with advancing myelination as well as with age. Electron microscopic evaluation revealed lipid droplets in the cytoplasm of cells with irregularly shaped nuclei, densely clumped chromatin, occasional microtubules, and narrow cytoplasmic processes. We suggest that lipid-containing cells in the white matter of the brains of infants are related to age and to maturational factors and, in the absence of other pathologic signs, are not related to prior hypoxic injury.

Brain macrophages: questions of origin and interrelationship

Brain tissue appears to contain several distinct types of macrophages. An effort is made here to present a description of the complete cohort of macrophages and sources of phagocytic activity in this tissue. Initially, the criteria and methods used for the identification of tissue macrophages in general are considered. These include some morphological and ultrastructural features, assessment of phagocytic activity, and histochemistry for intracellular and surface components. Each of these methods or criteria has certain advantages but also associated problems and limitations; all have been applied in various instances to brain tissue. In a final analysis, the most reliable means of identification of tissue macrophages involves a combination of all of these approaches. The identification and characterization of macrophages have been rendered extremely confusing in the brain because of so many different sources of these cells, both intrinsic and blood-derived. The classes of macrophages or phagocytic cells in brain tissue are microglia, supraependymal cells, epiplexus cells, meningeal macrophages, pericytes, and direct blood-derived macrophages. The morphology, location, and functional properties of each of these classes is described. In an overall view, brain tissue is very well protected by intrinsic macrophages, and the locations and distribution of these cells are consistent with other tissues. Finally, in a consideration of origin and interrelationship, the idea is presented that the most likely source for all or most brain macrophages is monocytic blood cells. The latter cells appear to migrate into the tissue from several sites during embryogenesis and may continue to enter, at least from blood vessels, in the adult state.

Shared recognition molecules in the brain and lymphoid tissues: the polypeptide mediator network of psychoneuroimmunology

Nervous and immune systems share specific recognition molecules for signals that originate in both systems. The information substances are polypeptides and their receptors. They comprise the multi-directional information exchange network whereby brain and nervous system function including mood and emotion can be integrated with immune and endocrine system activity throughout the body. This serves as a biochemical rationale for multiple interactions between the systems. It permits us at the tissue, cellular and molecular level to start to understand the "psychoneuroimmunology" of the whole person. Initiated changes in our view of the nervous and immune systems will undoubtedly lead to new strategies for the prophylaxis, diagnosis and treatment of human pathology.

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.