Diversity of neural precursor cell types in the prenatal macaque cerebral cortex exists largely within the astroglial cell lineage

Cell lineage analysis with somatic mutations reveals late divergence of neuronal cell types and cortical areas in human cerebral cortex

The mammalian cerebral cortex shows functional specialization into regions with distinct neuronal compositions, most strikingly in the human brain, but little is known in about how cellular lineages shape cortical regional variation and neuronal cell types during development. Here, we use somatic single nucleotide variants (sSNVs) to map lineages of neuronal sub-types and cortical regions. Early-occurring sSNVs rarely respect Brodmann area (BA) borders, while late-occurring sSNVs mark neuron-generating clones with modest regional restriction, though descendants often dispersed into neighboring BAs. Nevertheless, in visual cortex, BA17 contains 30-70% more sSNVs compared to the neighboring BA18, with clones across the BA17/18 border distributed asymmetrically and thus displaying different cortex-wide dispersion patterns. Moreover, we find that excitatory neuron-generating clones with modest regional restriction consistently share low-mosaic sSNVs with some inhibitory neurons, suggesting significant co-generation of excitatory and some inhibitory neurons in the dorsal cortex. Our analysis reveals human-specific cortical cell lineage patterns, with both regional inhomogeneities in progenitor proliferation and late divergence of excitatory/inhibitory lineages.

Translational Utility of the Nonhuman Primate Model

Nonhuman primates are essential for the study of human disease and to explore the safety of new diagnostics and therapies proposed for human use. They share similar genetic, physiologic, immunologic, reproductive, and developmental features with humans and thus have proven crucial for the study of embryonic/fetal development, organ system ontogeny, and the role of the maternal-placental-fetal interface in health and disease. The fetus may be exposed to a variety of inflammatory stimuli including infectious microbes as well as maternal inflammation, which can result from infections, obesity, or environmental exposures. Growing evidence supports that inflammation is a mediator of fetal programming and that the maternal immune system is tightly integrated with fetal-placental immune responses that may set a postnatal path for future health or disease. This review addresses some of the unique features of the nonhuman primate model system, specifically the rhesus monkey (Macaca mulatta), and importance of the species for studies focused on organ system ontogeny and the impact of viral teratogens in relation to development and congenital disorders.

Small molecule therapeutics for neuroinflammation-mediated neurodegenerative disorders

Chronically activated microglia and the resulting cascade of neuroinflammatory mechanisms have been postulated to play a critical role in neurodegenerative disorders. Microglia are the main component of the brain's innate immune system and become activated by infection, injury, misfolded proteins or a multitude of other stimuli. Activated microglia release pro-inflammatory and cytotoxic factors that can damage neurons and transform astrocytes to become toxic to neurons as well. Therapeutic approaches aiming to modulate microglia activation may be beneficial to mitigate the progression of inflammatory-mediated neurodegenerative diseases. In this literature review, we provide an overview of recent progress on key microglia targets and discovery of small molecule compounds advancing in clinical trials to minimize neuroinflammation.

Prenatal ethanol exposure impairs the formation of radial glial fibers and promotes the transformation of GFAPδ‑positive radial glial cells into astrocytes

During embryonic cortical development, radial glial cells (RGCs) are the major source of neurons, and these also serve as a supportive scaffold to guide neuronal migration. Similar to Vimentin, glial fibrillary acidic protein (GFAP) is one of the major intermediate filament proteins present in glial cells. Previous studies confirmed that prenatal ethanol exposure (PEE) significantly affected the levels of GFAP and increased the disassembly of radial glial fibers. GFAPδ is a variant of GFAP that is specifically expressed in RGCs; however, to the best of our knowledge, there are no reports regarding how PEE influences its expression during cortical development. In the present study, the effects of PEE on the expression and distribution of GFAPδ during early cortical development were assessed. It was found that PEE significantly decreased the expression levels of GFAP and GFAPδ. Using double immunostaining, GFAPδ was identified to be specifically expressed in apical and basal RGCs, and was co‑localized with other intermediate filament proteins, such as GFAP, Nestin and Vimentin. Additionally, PEE significantly affected the morphology of radial glial fibers and altered the behavior of RGCs. The loss of GFAPδ accelerated the transformation of RGCs into astrocytes. Using co‑immunostaining with Ki67 or phospho‑histone H3, GFAPδ+ cells were observed to be proliferative or mitotic cells, and ethanol treatment significantly decreased the proliferative or mitotic activities of GFAPδ+ RGCs. Taken together, the results suggested that PEE altered the expression patterns of GFAPδ and impaired the development of radial glial fibers and RGC behavior. The results of the present study provided evidence that GFAPδ may be a promising target to rescue the damage induced by PEE.

Fetal Rhesus Monkey First Trimester Zika Virus Infection Impacts Cortical Development in the Second and Third Trimesters

Zika virus is a teratogen similar to other neurotropic viruses, notably cytomegalovirus and rubella. The goal of these studies was to address the direct impact of Zika virus on fetal development by inoculating early gestation fetal rhesus monkeys using an ultrasound-guided approach (intraperitoneal vs. intraventricular). Growth and development were monitored across gestation, maternal samples collected, and fetal tissues obtained in the second trimester or near term. Although normal growth and anatomical development were observed, significant morphologic changes were noted in the cerebral cortex at 3-weeks post-Zika virus inoculation including massive alterations in the distribution, density, number, and morphology of microglial cells in proliferative regions of the fetal cerebral cortex; an altered distribution of Tbr2+ neural precursor cells; increased diameter and volume of blood vessels in the cortical proliferative zones; and a thinner cortical plate. At 3-months postinoculation, alterations in morphology, distribution, and density of microglial cells were also observed with an increase in blood vessel volume; and a thinner cortical plate. Only transient maternal viremia was observed but sustained maternal immune activation was detected. Overall, these studies suggest persistent changes in cortical structure result from early gestation Zika virus exposure with durable effects on microglial cells.

Microglial Activation Modulates Neuroendocrine Secretion During Experimental Sepsis

Sepsis promotes an inflammatory state in the central nervous system (CNS) that may cause autonomic, cognitive, and endocrine changes. Microglia, a resident immune cell of the CNS, is activated in several brain regions during sepsis, suggesting its participation in the central alterations observed in this disease. In this study, we aimed to investigate the role of microglial activation in the neuroendocrine system functions during systemic inflammation. Wistar rats received an intracerebroventricular injection of the microglial activation inhibitor minocycline (100 μg/animal), shortly before sepsis induction by cecal ligation and puncture. At 6 and 24 h after surgery, hormonal parameters, central and peripheral inflammation, and markers of apoptosis and synaptic function in the hypothalamus were analyzed. The administration of minocycline decreased the production of inflammatory mediators and the expression of cell death markers, especially in the late phase of sepsis (24 h). With respect to the endocrine parameters, microglial inhibition caused a decrease in oxytocin and an increase in corticosterone and vasopressin plasma levels in the early phase of sepsis (6 h), while in the late phase, we observed decreased oxytocin and increased ACTH and corticosterone levels compared to septic animals that did not receive minocycline. Prolactin levels were not affected by minocycline administration. The results indicate that microglial activation differentially modulates the secretion of several hormones and that this process is associated with inflammatory mediators produced both centrally and peripherally.

Cortical Interlaminar Astrocytes Are Generated Prenatally, Mature Postnatally, and Express Unique Markers in Human and Nonhuman Primates

Interlaminar astrocytes (ILAs) are a subset of cortical astrocytes that reside in layer I, express GFAP, have a soma contacting the pia, and contain long interlaminar processes that extend through several cortical layers. We studied the prenatal and postnatal development of ILAs in three species of primates (rhesus macaque, chimpanzee, and human). We found that ILAs are generated prenatally likely from radial glial (RG) cells, that ILAs proliferate locally during gestation, and that ILAs extend interlaminar processes during postnatal stages of development. We showed that the density and morphological complexity of ILAs increase with age, and that ILAs express multiple markers that are expressed by RG cells (Pax6, Sox2, and Nestin), specific to inner and outer RG cells (Cryab and Hopx), and astrocyte markers (S100β, Aqp4, and GLAST) in prenatal stages and in adult. Finally, we demonstrated that rudimentary ILAs in mouse also express the RG markers Pax6, Sox2, and Nestin, but do not express S100β, Cryab, or Hopx, and that the density and morphological complexity of ILAs differ between primate species and mouse. Together these findings contribute new information on astrogenesis of this unique class of cells and suggest a lineal relationship between RG cells and ILAs.

Microglia: An Intrinsic Component of the Proliferative Zones in the Fetal Rhesus Monkey (Macaca mulatta) Cerebral Cortex

Microglial cells are increasingly recognized as modulators of brain development. We previously showed that microglia colonize the cortical proliferative zones in the prenatal brain and regulate the number of precursor cells through phagocytosis. To better define cellular interactions between microglia and proliferative cells, we performed lentiviral vector-mediated intraventricular gene transfer to induce enhanced green fluorescent protein expression in fetal cerebrocortical cells. Tissues were collected and counterstained with cell-specific markers to label microglial cells and identify other cortical cell types. We found that microglial cells intimately interact with the radial glial scaffold and make extensive contacts with neural precursor cells throughout the proliferative zones, particularly in the rhesus monkey fetus when compared to rodents. We also identify a subtype of microglia, which we term 'periventricular microglia', that interact closely with mitotic precursor cells in the ventricular zone. Our data suggest that microglia are structural modulators that facilitate remodeling of the proliferative zones as precursor cells migrate away from the ventricle and may facilitate the delamination of precursor cells. Taken together, these results indicate that microglial cells are an integral component of cortical proliferative zones and contribute to the interactive milieu in which cortical precursor cells function.

Periventricular microglial cells interact with dividing precursor cells in the nonhuman primate and rodent prenatal cerebral cortex

Cortical proliferative zones have been studied for over 100 years, yet recent data have revealed that microglial cells constitute a sizeable proportion of ventricular zone cells during late stages of cortical neurogenesis. Microglia begin colonizing the forebrain after neural tube closure and during later stages of neurogenesis populate regions of the developing cortex that include the proliferative zones. We previously showed that microglia regulate the production of cortical cells by phagocytosing neural precursor cells (NPCs), but how microglia interact with NPCs remains poorly understood. Here we report on a distinct subset of microglial cells, which we term periventricular microglia, that are located near the lateral ventricle in the prenatal neocortex. Periventricular microglia exhibit a set of similar characteristics in embryonic rat and fetal rhesus monkey cortex. In both species, these cells occupy ~60 μm of the ventricular zone in the tangential axis and make contact with the soma and processes of NPCs dividing at the ventricle for over 50 μm along the radial axis. Periventricular microglia exhibit notable differences across species, including distinct morphological features such as terminal bouton-like structures that contact mitotic NPCs in the fetal rhesus monkey but not in rat. These morphological distinctions suggest differential functions of periventricular microglia in rat and rhesus monkey, yet are consistent with the concept that microglia regulate NPC function in the developing cerebral cortex of mammalian species.

Microglial cell activation and senescence are characteristic of the pathology FXTAS

BACKGROUND

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder associated with premutation alleles of the FMR1 gene. Expansions of more than 200 CGG repeats give rise to fragile X syndrome, the most common inherited form of cognitive impairment. Fragile X-associated tremor/ataxia syndrome is characterized by cerebellar tremor and ataxia, and the presence of ubiquitin-positive inclusions in neurons and astrocytes. It has been previously suggested that fragile X-associated tremor/ataxia syndrome is associated with an inflammatory state based on signs of oxidative stress-mediated damage and iron deposition.

OBJECTIVE

Determine whether the pathology of fragile X-associated tremor/ataxia syndrome involves microglial activation and an inflammatory state.

METHODS

Using ionized calcium binding adaptor molecule 1 and cluster differentiation 68 antibodies to label microglia, we examined the number and state of activation of microglial cells in the putamen of 13 fragile X-associated tremor/ataxia syndrome and 9 control postmortem cases.

RESULTS

Nearly half of fragile X-associated tremor/ataxia syndrome cases (6 of 13) presented with dystrophic senescent microglial cells. In the remaining fragile X-associated tremor/ataxia syndrome cases (7 of 13), the number of microglial cells and their activation state were increased compared to controls.

CONCLUSIONS

The presence of senescent microglial cells in half of fragile X-associated tremor/ataxia syndrome cases suggests that this indicator could be used, together with the presence of intranuclear inclusions and the presence of iron deposits, as a biomarker to aid in the postmortem diagnosis of fragile X-associated tremor/ataxia syndrome. An increased number and activation indicate that microglial cells play a role in the inflammatory state present in the fragile X-associated tremor/ataxia syndrome brain. Anti-inflammatory treatment of patients with fragile X-associated tremor/ataxia syndrome may be indicated to slow neurodegeneration. © 2018 International Parkinson and Movement Disorder Society.

A Radial Glia Fascicle Leads Principal Neurons from the Pallial-Subpallial Boundary into the Developing Human Insula

The human insular lobe, in the depth of the Sylvian fissure, displays three main cytoarchitectonic divisions defined by the differentiation of granular layers II and IV. These comprise a rostro-ventral agranular area, an intermediate dysgranular area, and a dorso-caudal granular area. Immunohistochemistry in human embryos and fetuses using antibodies against PCNA, Vimentin, Nestin, Tbr1, and Tb2 reveals that the insular cortex is unique in that it develops far away from the ventricular zone (VZ), with most of its principal neurons deriving from the subventricular zone (SVZ) of the pallial-subpallial boundary (PSB). In human embryos (Carnegie stage 16/17), the rostro-ventral insula is the first cortical region to develop; its Tbr1+ neurons migrate from the PSB along the lateral cortical stream. From 10 gestational weeks (GW) onward, lateral ventricle, ganglionic eminences, and PSB grow forming a C-shaped curvature. The SVZ of the PSB gives rise to a distinct radial glia fiber fascicle (RGF), which courses lateral to the putamen in the external capsule. In the RGF, four components can be established: PF, descending from the prefrontal PSB to the anterior insula; FP, descending from the fronto-parietal PSB toward the intermediate insula; PT, coursing from the PSB near the parieto-temporal junction to the posterior insula, and T, ascending from the temporal PSB and merging with components FP and PT. The RGF fans out at different dorso-ventral and rostro-caudal levels of the insula, with descending fibers predominating over ascending ones. The RGF guides migrating principal neurons toward the future agranular, dysgranular, and granular insular areas, which show an adult-like definition at 32 GW. Despite the narrow subplate, and the absence of an intermediate zone except in the caudal insula, most insular subdivisions develop into a 6-layered isocortex, possibly due to the well developed outer SVZ at the PSB, which is particularly prominent at the level of the dorso-caudal insula. The small size of the initial PSB sector may, however, determine the limited surface expansion of the insula, which is in contrast to the exuberant growth of the opercula deriving from the adjacent frontal-parietal and temporal VZ/SVZ.

Folding, But Not Surface Area Expansion, Is Associated with Cellular Morphological Maturation in the Fetal Cerebral Cortex

Altered macroscopic anatomical characteristics of the cerebral cortex have been identified in individuals affected by various neurodevelopmental disorders. However, the cellular developmental mechanisms that give rise to these abnormalities are not understood. Previously, advances in image reconstruction of diffusion magnetic resonance imaging (MRI) have made possible high-resolution in utero measurements of water diffusion anisotropy in the fetal brain. Here, diffusion anisotropy within the developing fetal cerebral cortex is longitudinally characterized in the rhesus macaque, focusing on gestation day (G85) through G135 of the 165 d term. Additionally, for subsets of animals characterized at G90 and G135, immunohistochemical staining was performed, and 3D structure tensor analyses were used to identify the cellular processes that most closely parallel changes in water diffusion anisotropy with cerebral cortical maturation. Strong correlations were found between maturation of dendritic arbors on the cellular level and the loss of diffusion anisotropy with cortical development. In turn, diffusion anisotropy changes were strongly associated both regionally and temporally with cortical folding. Notably, the regional and temporal dependence of diffusion anisotropy and folding were distinct from the patterns observed for cerebral cortical surface area expansion. These findings strengthen the link proposed in previous studies between cellular-level changes in dendrite morphology and noninvasive diffusion MRI measurements of the developing cerebral cortex and support the possibility that, in gyroencephalic species, structural differentiation within the cortex is coupled to the formation of gyri and sulci.SIGNIFICANCE STATEMENT Abnormal brain morphology has been found in populations with neurodevelopmental disorders. However, the mechanisms linking cellular level and macroscopic maturation are poorly understood, even in normal brains. This study contributes new understanding to this subject using serial in utero MRI measurements of rhesus macaque fetuses, from which macroscopic and cellular information can be derived. We found that morphological differentiation of dendrites was strongly associated both regionally and temporally with folding of the cerebral cortex. Interestingly, parallel associations were not observed with cortical surface area expansion. These findings support the possibility that perturbed morphological differentiation of cells within the cortex may underlie abnormal macroscopic characteristics of individuals affected by neurodevelopmental disorders.

Neuropeptides shaping the central nervous system development: Spatiotemporal actions of VIP and PACAP through complementary signaling pathways

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) are neuropeptides with wide, complementary, and overlapping distributions in the central and peripheral nervous systems, where they exert important regulatory roles in many physiological processes. VIP and PACAP display a large range of biological cellular targets and functions in the adult nervous system including regulation of neurotransmission and neuroendocrine secretion and neuroprotective and neuroimmune responses. As the main focus of the present review, VIP and PACAP also have been long implicated in nervous system development and maturation through their interaction with the seven transmembrane domain G protein-coupled receptors, PAC1, VPAC1, and VPAC2, initiating multiple signaling pathways. Compared with PAC1, which solely binds PACAP with very high affinity, VPACs exhibit high affinities for both VIP and PACAP but differ from each other because of their pharmacological profile for both natural accessory peptides and synthetic or chimeric molecules, with agonistic and antagonistic properties. Complementary to initial pharmacological studies, transgenic animals lacking these neuropeptides or their receptors have been used to further characterize the neuroanatomical, electrophysiological, and behavioral roles of PACAP and VIP in the developing central nervous system. In this review, we recapitulate the critical steps and processes guiding/driving neurodevelopment in vertebrates and superimposing the potential contribution of PACAP and VIP receptors on the given timeline. We also describe how alterations in VIP/PACAP signaling may contribute to both (neuro)developmental and adult pathologies and suggest that tuning of VIP/PACAP signaling in a spatiotemporal manner may represent a novel avenue for preventive therapies of neurological and psychiatric disorders. © 2016 Wiley Periodicals, Inc.

From stem cells to comparative corticogenesis: a bridge too far?

It has been hypothesized that the higher number of neurons in human cortex compared to the chimpanzee and other primate species is key to high cognitive function. Are human cortical precursors endowed with specific properties that drive greater neuronal expansion than in other non-human primates? Otani et al. 2016 addressed this issue taking advantage of comparative in vitro corticogenesis models based on human, chimpanzee and macaque pluripotent stem cells. Clonal analysis revealed a heterochrony of early developmental events possibly leading to a relatively higher expansion of human cortical precursor population. In absence of evidence going beyond putative correlation, the claim that stem cell models of cortical development indicate mechanism of cortical size regulation needs to be further examined notably with respect to in vivo observations of cortical precursor lineages.

Evolutionary origin of Tbr2-expressing precursor cells and the subventricular zone in the developing cortex

The subventricular zone (SVZ) is greatly expanded in primates with gyrencephalic cortices and is thought to be absent from vertebrates with three-layered, lissencephalic cortices, such as the turtle. Recent work in rodents has shown that Tbr2-expressing neural precursor cells in the SVZ produce excitatory neurons for each cortical layer in the neocortex. Many excitatory neurons are generated through a two-step process in which Pax6-expressing radial glial cells divide in the VZ to produce Tbr2-expressing intermediate progenitor cells, which divide in the SVZ to produce cortical neurons. We investigated the evolutionary origin of SVZ neural precursor cells in the prenatal cerebral cortex by testing for the presence and distribution of Tbr2-expressing cells in the prenatal cortex of reptilian and avian species. We found that mitotic Tbr2(+) cells are present in the prenatal cortex of lizard, turtle, chicken, and dove. Furthermore, Tbr2(+) cells are organized into a distinct SVZ in the dorsal ventricular ridge (DVR) of turtle forebrain and in the cortices of chicken and dove. Our results are consistent with the concept that Tbr2(+) neural precursor cells were present in the common ancestor of mammals and reptiles. Our data also suggest that the organizing principle guiding the assembly of Tbr2(+) cells into an anatomically distinct SVZ, both developmentally and evolutionarily, may be shared across vertebrates. Finally, our results indicate that Tbr2 expression can be used to test for the presence of a distinct SVZ and to define the boundaries of the SVZ in developing cortices.

Characterization of Laminar Zones in the Mid-Gestation Primate Brain with Magnetic Resonance Imaging and Histological Methods

Distinct populations of progenitor and postmitotic neural and glial cells are stratified in the fetal primate brain across developmentally transient tissue zones between the ventricular and pial surfaces. These zones were originally identified by light microscopy. However, it has subsequently been shown that various forms of magnetic resonance image (MRI) contrast can be used to distinguish layers of developing neural tissue in ex vivo, as well as in vivo (including in utero) conditions. Here we compare mid-gestation rhesus macaque tissue zones identified using histological techniques to ex vivo as well as in utero MRI performed on the same brains. These data are compared to mid-gestation fetal human brain MRI results, obtained in utero. We observe strong similarity between MRI contrast in vivo and post mortem, which facilitates interpretation of in utero images based on the histological characterization performed here. Additionally, we observe differential correspondence between the various forms of ex vivo MRI contrast and microscopy data, with maps of the water apparent diffusion coefficient providing the closest match to histologically-identified lamina of the nonhuman primate brain. Examination of histology and post mortem MRI helps to provide a better understanding of cytoarchitectrual characteristics that give rise to in utero MRI contrast.

Diversity of cortical interneurons in primates: the role of the dorsal proliferative niche

Evolutionary elaboration of tissues starts with changes in the genome and location of the stem cells. For example, GABAergic interneurons of the mammalian neocortex are generated in the ventral telencephalon and migrate tangentially to the neocortex, in contrast to the projection neurons originating in the ventricular/subventricular zone (VZ/SVZ) of the dorsal telencephalon. In human and nonhuman primates, evidence suggests that an additional subset of neocortical GABAergic interneurons is generated in the cortical VZ and a proliferative niche, the outer SVZ. The origin, magnitude, and significance of this species-specific difference are not known. We use a battery of assays applicable to the human, monkey, and mouse organotypic cultures and supravital tissue to identify neuronal progenitors in the cortical VZ/SVZ niche that produce a subset of GABAergic interneurons. Our findings suggest that these progenitors constitute an evolutionary novelty contributing to the elaboration of higher cognitive functions in primates.

Radial glia in the proliferative ventricular zone of the embryonic and adult turtle, Trachemys scripta elegans

To better understand the role of radial glial (RG) cells in the evolution of the mammalian cerebral cortex, we investigated the role of RG cells in the dorsal cortex and dorsal ventricular ridge of the turtle, Trachemys scripta elegans. Unlike mammals, the glial architecture of adult reptile consists mainly of ependymoradial glia, which share features with mammalian RG cells, and which may contribute to neurogenesis that continues throughout the lifespan of the turtle. To evaluate the morphology and proliferative capacity of ependymoradial glia (here referred to as RG cells) in the dorsal cortex of embryonic and adult turtle, we adapted the cortical electroporation technique, commonly used in rodents, to the turtle telencephalon. Here, we demonstrate the morphological and functional characteristics of RG cells in the developing turtle dorsal cortex. We show that cell division occurs both at the ventricle and away from the ventricle, that RG cells undergo division at the ventricle during neurogenic stages of development, and that mitotic Tbr2+ precursor cells, a hallmark of the mammalian SVZ, are present in the turtle cortex. In the adult turtle, we show that RG cells encompass a morphologically heterogeneous population, particularly in the subpallium where proliferation is most prevalent. One RG subtype is similar to RG cells in the developing mammalian cortex, while 2 other RG subtypes appear to be distinct from those seen in mammal. We propose that the different subtypes of RG cells in the adult turtle perform distinct functions.

The interaction between microglia and neural stem/precursor cells

In the adult mammalian brain, neurogenesis from neural stem/precursor cell occurs within two regions, the subgranular zone (SGZ) in the dentate gyrus (DG) and the subventricular zone (SVZ) lining the lateral ventricles. The function of neural stem cell is enhanced by external stimuli, such as injury and inflammation. Microglia, as the main immune modulating cells, play important roles in the central nervous system (CNS). Recently, select discoveries reported that microglia might influence the proliferation, differentiation and survival of neural precursor cells (NPCs). Other studies revealed that NPCs might reversibly regulate the function of microglia. Accordingly, in this review we focus on the interaction between microglia and NPCs.

Cortical and Clonal Contribution of Tbr2 Expressing Progenitors in the Developing Mouse Brain

The individual contribution of different progenitor subtypes towards the mature rodent cerebral cortex is not fully understood. Intermediate progenitor cells (IPCs) are key to understanding the regulation of neuronal number during cortical development and evolution, yet their exact contribution is much debated. Intermediate progenitors in the cortical subventricular zone are defined by expression of T-box brain-2 (Tbr2). In this study we demonstrate by using the Tbr2(Cre) mouse line and state-of-the-art cell lineage labeling techniques, that IPC derived cells contribute substantial proportions 67.5% of glutamatergic but not GABAergic or astrocytic cells to all cortical layers including the earliest generated subplate zone. We also describe the laminar dispersion of clonally derived cells from IPCs using a recently described clonal analysis tool (CLoNe) and show that pair-generated cells in different layers cluster closer (142.1 ± 76.8 μm) than unrelated cells (294.9 ± 105.4 μm). The clonal dispersion from individual Tbr2 positive intermediate progenitors contributes to increasing the cortical surface. Our study also describes extracortical contributions from Tbr2+ progenitors to the lateral olfactory tract and ventromedial hypothalamic nucleus.