Hippocampal Radial Glial Subtypes and Their Neurogenic Potential in Human Fetuses and Healthy and Alzheimer's Disease Adults.

Neuropathological conditions might affect adult granulogenesis in the adult human dentate gyrus. However, radial glial cells (RGCs) have not been well characterized during human development and aging. We have previously described progenitor and neuronal layer establishment in the hippocampal pyramidal layer and dentate gyrus from embryonic life until mid-gestation. Here, we describe RGC subtypes in the hippocampus from 13 gestational weeks (GW) to mid-gestation and characterize their evolution and the dynamics of neurogenesis from mid-gestation to adulthood in normal and Alzheimer's disease (AD) subjects. In the pyramidal ventricular zone (VZ), RGC density declined with neurogenesis from mid-gestation until the perinatal period. In the dentate area, morphologic and antigenic differences among RGCs were observed from early ages of development to adulthood. Density and proliferative capacity of dentate RGCs as well as neurogenesis were strongly reduced during childhood until 5 years, few DCX+ cells are seen in adults. The dentate gyrus of both control and AD individuals showed Nestin+ and/or GFAPδ+ cells displaying different morphologies. In conclusion, pools of morphologically, antigenically, and topographically diverse neural progenitor cells are present in the human hippocampus from early developmental stages until adulthood, including in AD patients, while their neurogenic potential seems negligible in the adult.

Dynamic Expression Patterns of Progenitor and Neuron Layer Markers in the Developing Human Dentate Gyrus and Fimbria.

The molecular mechanisms that orchestrate the development of the human dentate gyrus are not known. In this study, we characterized the formation of human dentate and fimbrial progenitors and postmitotic neurons from 9 gestational weeks (GW9) to GW25. PAX6+ progenitor cells remained proliferative until GW16 in the dentate ventricular zone. By GW11, the secondary dentate matrix had developed in the intermediate zone, surrounding the dentate anlage and streaming toward the subpial layer. This secondary matrix contained proliferating PAX6+ and/or TBR2+ progenitors. In parallel, SOX2+ and PAX6+ fimbrial cells were detected approaching the dentate anlage, representing a possible source of extra-dentate progenitors. By GW16, when the granule cell layer could be delineated, a hilar matrix containing PAX6+ and some TBR2+ progenitors had become identifiable. By GW25, when the 2 limbs of the granule cell layer had formed, the secondary dentate matrix was reduced to a pool of progenitors at the fimbrio-dentate junction. Although human dentate development recapitulates key steps previously described in rodents, differences seemed to emerge in neuron layer markers expression. Further studies are necessary to better elucidate their role in dentate formation and connectivity.

Dynamic Expression Patterns of Progenitor and Pyramidal Neuron Layer Markers in the Developing Human Hippocampus.

The molecular mechanisms underlying the formation of hippocampus are unknown in humans. To improve our knowledge of molecules that potentially regulate pyramidal neurogenesis and layering in various hippocampal fields, we investigated the expression of progenitor markers and cell fate molecules from gestational week (GW) 9 to GW 20. At GW 9, the progenitor cell compartment of the hippocampal formation mainly consisted of PAX6(+) cells in the ventricular zone. Between GW 9 and 11, a second germinal area, the subventricular zone (SVZ), was formed, as shown by TBR2 labeling. Postmitotic markers (TBR1, CTIP2, SATB2, and CUX1) might reflect the inside-out layering of the plate from GW 11 onwards. TBR1(+) neurons appeared in the deep plate, whereas CTIP2(+), SATB2(+), and CUX1(+) neurons occupied the upper layers. From GW 16, differences in layer segregation were observed between the ammonic and subicular plates. Moreover, an ammonic-to-subicular maturation gradient was observed in germinal/postmitotic areas. Taken together, these findings demonstrate for the first time the presence of an SVZ in the hippocampus of human fetuses and laminar differences in transcription factor expression in the pyramidal layer of the human ammonic and subicular plate, and provide new information to further investigate the connectivity of the hippocampal formation.

Activation of microglial N-methyl-D-aspartate receptors triggers inflammation and neuronal cell death in the developing and mature brain.

OBJECTIVE: Activated microglia play a central role in the inflammatory and excitotoxic component of various acute and chronic neurological disorders. However, the mechanisms leading to their activation in the latter context are poorly understood, particularly the involvement of N-methyl-D-aspartate receptors (NMDARs), which are critical for excitotoxicity in neurons. We hypothesized that microglia express functional NMDARs and that their activation would trigger neuronal cell death in the brain by modulating inflammation. METHODS AND RESULTS: We demonstrate that microglia express NMDARs in the murine and human central nervous system and that these receptors are functional in vitro. We show that NMDAR stimulation triggers microglia activation in vitro and secretion of factors that induce cell death of cortical neurons. These damaged neurons are further shown to activate microglial NMDARs and trigger a release of neurotoxic factors from microglia in vitro, indicating that microglia can signal back to neurons and possibly induce, aggravate, and/or maintain neurologic disease. Neuronal cell death was significantly reduced through pharmacological inhibition or genetically induced loss of function of the microglial NMDARs. We generated Nr1 LoxP(+/+) LysM Cre(+/-) mice lacking the NMDAR subunit NR1 in cells of the myeloid lineage. In this model, we further demonstrate that a loss of function of the essential NMDAR subunit NR1 protects from excitotoxic neuronal cell death in vivo and from traumatic brain injury. INTERPRETATION: Our findings link inflammation and excitotoxicity in a potential vicious circle and indicate that an activation of the microglial NMDARs plays a pivotal role in neuronal cell death in the perinatal and adult brain.

[Stress during prenatal and early postnatal period when everything begins]. / Les stress pendant les 1 000 premiers jours de la vie quand tout commence.

Early severe stresses are known to affect the biological and psychological development in childhood. Good and adaptable stress during prenatal and early postnatal period can switch to traumatic during these highly susceptible developmental stages. These different stresses modulate genetic/epigenetic processes and the setting up of connectome during these highly plastic and adaptable time periods. The polyvagal processes control the base of the security/well-being perception of the newborn by the onset of synchronized interactions between the mother/parent/nurse and the baby. These positive adjustments in mirror lead to attachment and social links and to implicit learning processes leading to a balanced emotional and cognitive development.

Title: Les stress pendant les 1 000 premiers jours de la vie quand tout commence. Abstract: Les stress présents pendant les 1 000 premiers jours de vie, période de grande vulnérabilité, peuvent avoir un impact sur la biologie de l'enfant et son psychisme. Qu'ils soient bénéfique, adaptable ou toxique, ces stress modulent des régulations génétiques et épigénétiques ainsi que l'installation du connectome du bébé dans la période de grande plasticité et d'adaptation de ces âges précoces. Les régulations des systèmes polyvagaux forment le socle du ressenti de bien-être du bébé, de sa sécurisation dans des synchronies mère, parents, soignants et nouveau-né. Ces régulations positives, en miroir, mènent à l'attachement et aux liens sociaux, aux apprentissages implicites et aux développements émotif, cognitif et comportemental harmonieux.

DYRK1A: a master regulatory protein controlling brain growth.

Copy number variation in a small region of chromosome 21 containing DYRK1A produces morphological and cognitive alterations in human. In mouse models, haploinsufficiency results in microcephaly, and a human DYRK1A gain-of-function model (three alleles) exhibits increased brain volume. To investigate these developmental aspects, we used a murine BAC clone containing the entire gene to construct an overexpression model driven by endogenous regulatory sequences. We compared this new model to two other mouse models with three copies of Dyrk1a, YACtgDyrk1a and Ts65Dn, as well as the loss-of-function model with one copy (Dyrk1a(+/-)). Growth, viability, brain weight, and brain volume depended strongly upon gene copy number. Brain region-specific variations observed in gain-of-function models mirror their counterparts in the loss-of-function model. Some variations, such as increased volume of the superior colliculus and ventricles, were observed in both the BAC transgenic and Ts65Dn mice. Using unbiased stereology we found that, in the cortex, neuron density is inversely related to Dyrk1a copy number but, in thalamic nuclei, neuron density is directly related to copy number. In addition, six genes involved either in cell division (Ccnd1 and pAkt) or in neuronal machinery (Gap43, Map2, Syp, Snap25) were regulated by Dyrk1a throughout development, from birth to adult. These results imply that Dyrk1a expression alters different cellular processes during brain development. Dyrk1a, then, has two roles in the development process: shaping the brain and controlling the structure of neuronal components.

Maternal deprivation induces deficits in temporal memory and cognitive flexibility and exaggerates synaptic plasticity in the rat medial prefrontal cortex.

Early life adverse events can lead to structural and functional impairments in the prefrontal cortex (PFC). Here, we investigated whether maternal deprivation (MD) alters PFC-dependent executive functions, neurons and astrocytes number and synaptic plasticity in adult male Long-Evans rats. The deprivation protocol consisted of a daily separation of newborn Long-Evans pups from their mothers and littermates 3h/day postnatal day 1-14. Cognitive performances were assessed in adulthood using the temporal order memory task (TMT) and the attentional set-shifting task (ASST) that principally implicates the PFC and the Morris water maze task (WMT) that does not essentially rely on the PFC. The neurons and astrocytes of the prelimbic (PrL) area of the medial PFC (mPFC) were immunolabelled respectively with anti-NeuN and anti-GFAP antibodies and quantified by stereology. The field potentials evoked by electrical stimulation of ventral hippocampus (ventral HPC) were recorded in vivo in the PrL area. In adulthood, MD produced cognitive deficits in two PFC-dependent tasks, the TMT and ASST, but not in the WMT. In parallel, MD induced in the prelimbic area of the medial PFC an upregulation of long-term potentiation (LTP), without any change in the number of neurons and astrocytes. We provide evidence that MD leads in adults to an alteration of the cognitive abilities dependent on the PFC, and to an exaggerated synaptic plasticity in this region. We suggest that this latter phenomenon may contribute to the impairments in the cognitive tasks.

Cerebellar abnormalities following hypoxia alone compared to hypoxic-ischemic forebrain injury in the developing rat brain.

Two-day-old (P2) rat pups were subjected to either a global hypoxia or to electrocoagulation of the right carotid artery followed by 2.5 h hypoxia. Cellular and regional injury in the cerebellum (CB) was studied at 1, 2 and 19 days using immunohistology. Following hypoxia and hypoxia-ischemia, all neuronal populations of the CB were damaged in a subset of Purkinje cells. The decrease in the number of interneurons, as well as the thickness of molecular and granular layers was significant following hypoxia. Diffuse white matter damage, with loss of preoligodendrocytes was more severe following hypoxia than hypoxia-ischemia. Global hypoxia in the rat at P2 produces extensive damage to many cell types in different areas of the CB. The addition of unilateral forebrain ischemia does not increase the severity of these changes. Our data provide insight into the mechanisms of the changes observed in the CB of premature newborns.

[Anatomy and physiology of traumatic stress]. / Anatomie et physiologie du stress traumatique.

Facing a more or less intrusive stress, some individuals can cope as they are more resilient, while others get traumatized and further develop a Post Traumatic Stress Disorder (PTSD). Individuals are not equal facing traumatic stress for genetic/epigenetic or personal reasons. This review analyzes from animal models to human, the neurobiological changes detected when the stress switch from adaptable in everyday life to pathological leading to PTSD. Fear memories lead to the disruption of the anatomy/morphology of emotional-memory networks centered on the amygaloïd complex and hippocampal hub associated with the homeostatic unbalance of the body-brain exchanges of molecules such as hormones, neuromodulators or peptides. Persistent fear memories are hardly handled by the frontal ability for decision making towards action. But these fear memories can be revisited by different therapies recruiting cerebral plasticity and resilience. Current understanding of PTSD allowed to develop a series of efficient treatments associating precise medicine to diverse body-mind therapies.

TITLE: Anatomie et physiologie du stress traumatique. ABSTRACT: Le stress prend des formes très variées, allant de bénéfique, bénigne à traumatique. Chaque individu avec son patrimoine génétique et épigénétique et ses mémoires émotionnelles singulières réagit différemment face au stress. L'effet du stress aigu ou chronique est objectivé par l'élévation d'hormones, comme le cortisol, et d'autres molécules circulantes, évoluant au cours du temps. Après avoir décrit les comportements face au danger, nous exposons dans cette Synthèse, les différentes régulations anatomiques et physiologiques susceptibles de varier lors du passage d'un stress adaptable à un stress traumatique (et de ses mémoires), pouvant entraîner l'installation de troubles de stress post-traumatique (TSPT). Des traitements médicamenteux et des thérapies novatrices permettent d'initier l'extinction des mémoires associées à la peur et d'améliorer la prise en charge des troubles de stress post-traumatiques.

Early microglial colonization of the human forebrain and possible involvement in periventricular white-matter injury of preterm infants.

Amoeboid microglial subpopulations visualized by antibodies against ionized calcium-binding adapter molecule 1, CD68, and CD45 enter the forebrain starting at 4.5 postovulatory or gestational weeks (gw). They penetrate the telencephalon and diencephalon via the meninges, choroid plexus, and ventricular zone. Early colonization by amoeboid microglia-macrophages is first restricted to the white matter, where these cells migrate and accumulate in patches at the junctions of white-matter pathways, such as the three junctions that the internal capsule makes with the thalamocortical projection, external capsule and cerebral peduncle, respectively. In the cerebral cortex anlage, migration is mainly radial and tangential towards the immature white matter, subplate layer, and cortical plate, whereas pial cells populate the prospective layer I. A second wave of microglial cells penetrates the brain via the vascular route at about 12-13 gw and remains confined to the white matter. Two main findings deserve emphasis. First, microglia accumulate at 10-12 gw at the cortical plate-subplate junction, where the first synapses are detected. Second, microglia accumulate in restricted laminar bands, most notably around 19-30 gw, at the axonal crossroads in the white matter (semiovale centre) rostrally, extending caudally in the immature white matter to the visual radiations. This accumulation of proliferating microglia is located at the site of white-matter injury in premature neonates. The spatiotemporal organization of microglia in the immature white and grey matter suggests that these cells may play active roles in developmental processes such as axonal guidance, synaptogenesis, and neurodevelopmental apoptosis as well as in injuries to the developing brain, in particular in the periventricular white-matter injury of preterm infants.

Inflammation processes in perinatal brain damage.

Once viewed as an isolated, immune-privileged organ, the central nervous system has undergone a conceptual change. Neuroinflammation has moved into the focus of research work regarding pathomechanisms underlying perinatal brain damage. In this review, we provide an overview of current concepts regarding perinatal brain damage and the role of inflammation in the disease pathomechanism.

GABA receptor subunits in human auditory cortex in normal and stroke cases.

GABA receptors are ubiquitous in the cerebral cortex and play a major role in shaping responses of cortical neurons. GABA(A) and GABA(B) receptor subunit expression was visualized by immunohistochemistry in human auditory areas from both hemispheres in 9 normal subjects (aged 43-85 years; time between death and fixation 6-24 hours) and in 4 stroke patients (aged 59-87 years; time between death and fixation 7-24 hours) and analyzed qualitatively for GABA(A) and semiquantitatively for GABA(B) receptor subunits. In normal brains, the primary auditory area (TC) and the surrounding areas TB and TA displayed distinct GABA(A) receptor subunit labeling with differences among cortical layers and areas. In postacute and chronic stroke we found a layer-selective downregulation of the alpha-2 subunit in the anatomically intact cerebral cortex of the intact and of the lesioned hemisphere, whereas the alpha-1, alpha-3 and beta-2/3 subunits maintained normal levels of expression. The GABA(B) receptors had a distinct laminar pattern in auditory areas and minor differences among areas. Unlike in other pathologies, there is no modulation of the GABA(B) receptor expression in subacute or chronic stroke.

Analysis of neuronal, glial, endothelial, axonal and apoptotic markers following moderate therapeutic hypothermia and anesthesia in the developing piglet brain.

Hypothermia (HT) by whole body (WBC) or selective head cooling (SHC) reduces hypoxic-ischemic (HI) brain injury; however, whether prolonged hypothermia and/or anesthesia disrupts immature brain development, eg, increases apoptosis, is unknown. Anesthesia increases apoptosis in immature animals. We investigated whether neuroprotective hypothermia and anesthesia disrupts normal brain development. Thirty-eight pigs <24 h old were randomized between five groups and were killed after 72 h: eighteen received a global hypoxic-ischemic insult under anesthesia, eight subsequently cooled by SHC with WBC to T(rectal) 34.5 degrees C for 24 h, followed by 48 h normothermia (NT) at T(rectal) 39.0 degrees C, while 10 remained normothermic. Sixteen underwent anesthetized sham hypoxic-ischemic, six then following normothermia and 10 following hypothermia protocols. There were four normothermic controls. The hypothermia groups demonstrated significant brain hypothermia. In the hypoxic-ischemic groups this conferred approximately 60% neuroprotection reducing histological injury scores in all brain areas. Immunohistochemical/histochemical analyses of neuronal, glial, endothelial, axonal, transcriptional apoptotic markers in areas devoid of histological lesions revealed no hypothermia/normothermia group and differences whether exposed to hypoxic-ischemic or not. Neither 36-h anesthesia nor 24-h hypothermia produced adverse effects at 4-day survival on a panel of brain maturation/neural death markers in newborn pigs. Longer survival studies are necessary to verify the safety of hypothermia in the developing brain.

Human Motor Thalamus Reconstructed in 3D from Continuous Sagittal Sections with Identified Subcortical Afferent Territories.

Classification and delineation of the motor-related nuclei in the human thalamus have been the focus of numerous discussions for a long time. Difficulties in finding consensus have for the most part been caused by paucity of direct experimental data on connections of individual nuclear entities. Kultas-Ilinsky et al. (2011) showed that distribution of glutamic acid decarboxylase isoform 65 (GAD65), the enzyme that synthesizes inhibitory neurotransmitter γ-aminobutyric acid, is a reliable marker that allows to delineate connectionally distinct nuclei in the human motor thalamus, namely the territories innervated by nigral, pallidal, and cerebellar afferents. We compared those immunocytochemical staining patterns with underlying cytoarchitecture and used the latter to outline the three afferent territories in a continuous series of sagittal Nissl-stained sections of the human thalamus. The 3D volume reconstructed from the outlines was placed in the Talairach stereotactic coordinate system relative to the intercommissural line and sectioned in three stereotactic planes to produce color-coded nuclear maps. This 3D coordinate-based atlas was coregistered to the Montreal Neurological Institute (MNI-152) space. The current report proposes a simplified nomenclature of the motor-related thalamic nuclei, presents images of selected histological sections and stereotactic maps illustrating topographic relationships of these nuclei as well as their relationship with adjacent somatosensory afferent region. The data are useful in different applications such as functional MRI and diffusion tractography. The 3D dataset is publicly available under an open license and can also be applicable in clinical interventions in the thalamus.

Entry and distribution of microglial cells in human embryonic and fetal cerebral cortex.

Microglial cells penetrate into and scatter throughout the human cortical grey and white matter according to a specific spatiotemporal pattern during the first 2 trimesters of gestation. Routes of entry were quantitatively and qualitatively different from those identified in the diencephalon. Starting at 4.5 gestational weeks, amoeboid microglial cells, characterized by different antibodies as Iba1, CD68, CD45, and MHC-II, entered the cerebral wall from the ventricular lumen and the leptomeninges. Migration was mainly radial and tangential toward the immature white matter, subplate layer, and cortical plate, whereas pial cells populated the prospective layer I. The intraparenchymal vascular route of entry was detectable only from 12 gestational weeks. Interestingly, microglial cells accumulated in restricted laminar bands particularly at 19 to 24 gestational weeks among the corona radiata fibers rostrally, extending caudally in the immature white matter to reach the visual radiations. This accumulation of proliferating MIB1-positive microglia (as shown by MIB1-Iba1 double immunolabeling) was located at the site of white matter injury in premature neonates. The spatiotemporal organization of microglia in the immature white and grey matter suggests that these cells may play active roles in developmental processes and in injury to the developing brain.

Apoptosis-inducing factor deficiency induces early mitochondrial degeneration in brain followed by progressive multifocal neuropathology.

Apoptosis-inducing factor (AIF) deficiency compromises oxidative phosphorylation. Harlequin mice, in which AIF is downregulated, develop a severe mitochondrial complex I (CI) deficiency, suggesting that Harlequin mice may represent a natural model of the most common oxidative phosphorylation disorders. However, the brain phenotype specifically involves the cerebellum, whereas human CI deficiencies often manifest as complex multifocal neuropathologies. To evaluate whether this model can be used as to study CI-deficient disorders, the whole brain of Harlequin mice was investigated during the course of the disease. Neurodegeneration was not restricted to the cerebellum but progressively affected thalamic, striatal, and cortical regions as well. Strong astroglial and microglial activation with extensive vascular proliferation was observed by 4 months of age in thalamic, striatal, and cerebellar nuclei associated with somatosensory-motor pathways. At 2 months of age, degenerating mitochondria were observed in most cells in these structures, even in nondegenerating neurons, a finding that indicates mitochondrial injury is a cause rather than an effect of neuronal cell death. Thus, apoptosis-inducing factor deficiency induces early mitochondrial degeneration, followed by progressive multifocal neuropathology (a phenotype broader than previously described), and resembles some histopathologic features of devastating human neurodegenerative mitochondriopathies associated with CI deficiency.

[The importance of controls in immunohistochemistry: how to validate an antibody, from research to diagnosis]. / Les contrôles nécessaires en immunohistochimie: de la recherche au diagnostic.

The specificity of an immunohistochemical reaction is guaranteed by two sets of controls. Positive controls verify the specificity of the primary antibody and demonstrate that it binds only to the protein which was used as an immunogen. Negative controls ensure that the labelling technique is specific and that the primary antibody is responsible for generation of the immunostaining. In fact, the production of a labelling may also be related to cross reactivity or to non-specific physical or chemical interactions. This paper reviews the characteristics of various epitopes and antibodies, describes different strategies which prove the specificity of the immunohistochemical reaction in research or diagnostic pathology and point towards the essential information which should be reported in a paper.

GluNs Detection and Functions in Microglial Cells.

Proving endogenous GluN presence and functions in microglia require complementary steps to demonstrate (1) that GluN genes are transcripted and translated, (2) their cellular localization, (3) that the GluN are functional, and (4) the role of the functional GluN. The complete demonstration is performed by using mRNA detection technics, western blots, immunofluorescence, electrophysiology, calcium imaging, morphology studies, multiplex immunoassay together with conditional microglial Knock-Out mice and brain lesion models.

Immunohistochemical expression of prion protein (PrPC) in the human forebrain during development.

The cellular prion protein (PrPC) is a ubiquitous protein whose expression in the adult brain occurs mainly in synapses. We used monoclonal antibodies to study fetal and perinatal PrPC expression in the human forebrain. Double immunofluorescence and confocal microscopy with GFAP, Iba1, MAP2, doublecortin, synaptophysin, and GAP-43 were used to localize PrPC. PrPC immunoreactivity was observed in axonal tracts and fascicles from the 11th week to the end of gestation. Synapses expressed PrPC at increasing levels throughout synaptogenesis. At midgestation, a few PrPC-labeled neurons were detected in the cortical anlage and numerous ameboid and intermediate microglial cells were PrPC-positive. In contrast, at the end of gestation, microglial PrPC expression decreased to almost nothing, whereas neuronal PrPC expression increased, most notably in ischemic areas. In adults, PrPC immunoreactivity was restricted to the synaptic neuropil of the gray matter. At all ages, choroid plexus, ependymal, and endothelial cells were labeled, whereas astrocytes were only occasionally immunoreactive. In conclusion, the early expression of PrPC in the axonal field may suggest a specific role for this molecule in axonal growth during development. Moreover, PrPC may play a role in early microglial cell development.

Distribution and differentiation of microglia in the human encephalon during the first two trimesters of gestation.

We describe the topographical distribution of microglial subpopulations during development of the human diencephalon and telencephalon. Brains from embryos and fetuses age 5-23.5 gestational weeks (gw) were subjected to single- and double-immunolabeling for lectin RCA-1 (Ricinus Communis Agglutinin 1), Iba1 (a microglial marker), CD68 (specific of macrophages), CD45 (marker for mononucleate cells of hematopoietic lineage), CD34 (expressed on endothelial cells), and MIB1 and Ki67 (markers for cell proliferation). At 5.5 gw the first intracerebral microglial cells were seen close to the meninges and choroid plexus near the di-telencephalic fissure. They were amoeboid and positive for Iba1, CD45, and RCA-1, whereas cells in the deep parenchyma expressed Iba1/CD68/RCA-1 and constituted clusters. In the developing diencephalon, microglial clusters were located in junctional regions of the white matter anlagen, most notably at the junctions of the internal capsule with the thalamic projections, the external capsule, and the cerebral peduncle. In the cortical anlagen, Iba1+/RCA-1/CD68+/CD45+ cells accumulated at 10-12 gw, constituting a tangential band at the junction between the cortical plate and the subplate. Between 10 and 16 gw microglial clusters increased markedly in size and cellular density. Contact between Iba1+ microglia and CD34+ blood vessels was clearly visible from 10-12 gw onward, first in microglial clusters of the white matter anlagen and subsequently throughout the parenchyma. From the middle of the second trimester onward microglial cells colonized the entire cerebral parenchyma, developed a ramified morphology, and downregulated their surface antigens, but remained more numerous in the white matter.