Glial and neuronal differentiation in the human fetal brain 9-23 weeks of gestation

Systematic development of immunohistochemistry protocol for large cryosections-specific to non-perfused fetal brain

BACKGROUND

Immunohistochemistry (IHC) is an important technique in understanding the expression of neurochemical molecules in the developing human brain. Despite its routine application in the research and clinical setup, the IHC protocol specific for soft fragile fetal brains that are fixed using the non-perfusion method is still limited in studying the whole brain.

NEW METHOD

This study shows that the IHC protocols, using a chromogenic detection system, used in animals and adult humans are not optimal in the fetal brains. We have optimized key steps from Antigen retrieval (AR) to chromogen visualization for formalin-fixed whole-brain cryosections (20 µm) mounted on glass slides.

RESULTS

We show the results from six validated, commonly used antibodies to study the fetal brain. We achieved optimal antigen retrieval with 0.1 M Boric Acid, pH 9.0 at 70°C for 20 minutes. We also present the optimal incubation duration and temperature for protein blocking and the primary antibody that results in specific antigen labeling with minimal tissue damage.

COMPARISON WITH EXISTING METHODS

The IHC protocol commonly used for adult human and animal brains results in significant tissue damage in the fetal brains with little or suboptimal antigen expression. Our new method with important modifications including the temperature, duration, and choice of the alkaline buffer for AR addresses these pitfalls and provides high-quality results.

CONCLUSION

The optimized IHC protocol for the developing human brain (13-22 GW) provides a high-quality, repeatable, and reliable method for studying chemoarchitecture in neurotypical and pathological conditions across different gestational ages.

Histological characterization and development of mesial surface sulci in the human brain at 13-15 gestational weeks through high-resolution histology

Cellular-level anatomical data from early fetal brain are sparse yet critical to the understanding of neurodevelopmental disorders. We characterize the organization of the human cerebral cortex between 13 and 15 gestational weeks using high-resolution whole-brain histological data sets complimented with multimodal imaging. We observed the heretofore underrecognized, reproducible presence of infolds on the mesial surface of the cerebral hemispheres. Of note at this stage, when most of the cerebrum is occupied by lateral ventricles and the corpus callosum is incompletely developed, we postulate that these mesial infolds represent the primordial stage of cingulate, callosal, and calcarine sulci, features of mesial cortical development. Our observations are based on the multimodal approach and further include histological three-dimensional reconstruction that highlights the importance of the plane of sectioning. We describe the laminar organization of the developing cortical mantle, including these infolds from the marginal to ventricular zone, with Nissl, hematoxylin and eosin, and glial fibrillary acidic protein (GFAP) immunohistochemistry. Despite the absence of major sulci on the dorsal surface, the boundaries among the orbital, frontal, parietal, and occipital cortex were very well demarcated, primarily by the cytoarchitecture differences in the organization of the subplate (SP) and intermediate zone (IZ) in these locations. The parietal region has the thickest cortical plate (CP), SP, and IZ, whereas the orbital region shows the thinnest CP and reveals an extra cell-sparse layer above the bilaminar SP. The subcortical structures show intensely GFAP-immunolabeled soma, absent in the cerebral mantle. Our findings establish a normative neurodevelopment baseline at the early stage.

Altered trajectory of neurodevelopment associated with fetal growth restriction

Fetal growth restriction (FGR) is principally caused by suboptimal placental function. Poor placental function causes an under supply of nutrients and oxygen to the developing fetus, restricting development of individual organs and overall growth. Estimated fetal weight below the 10th or 3rd percentile with uteroplacental dysfunction, and knowledge regarding the onset of growth restriction (early or late), provide diagnostic criteria for fetuses at greatest risk for adverse outcome. Brain development and function is altered with FGR, with ongoing clinical and preclinical studies elucidating neuropathological etiology. During the third trimester of pregnancy, from ~28 weeks gestation, neurogenesis is complete and neuronal complexity is expanding, through axonal and dendritic outgrowth, dendritic branching and synaptogenesis, accompanied by myelin production. Fetal compromise over this period, as occurs in FGR, has detrimental effects on these processes. Total brain volume and grey matter volume is reduced in infants with FGR, first evident in utero, with cortical volume particularly vulnerable. Imaging studies show that cerebral morphology is disturbed in FGR, with altered cerebral cortex, volume and organization of brain networks, and reduced connectivity of long- and short-range circuits. Thus, FGR induces a deviation in brain development trajectory affecting both grey and white matter, however grey matter volume is preferentially reduced, contributed by cell loss, and reduced neurite outgrowth of surviving neurons. In turn, cell-to-cell local networks are adversely affected in FGR, and whole brain left and right intrahemispheric connections and interhemispheric connections are altered. Importantly, disruptions to region-specific brain networks are linked to cognitive and behavioral impairments.

Astrocyte Activation in Neurovascular Damage and Repair Following Ischaemic Stroke

Transient or permanent loss of tissue perfusion due to ischaemic stroke can lead to damage to the neurovasculature, and disrupt brain homeostasis, causing long-term motor and cognitive deficits. Despite promising pre-clinical studies, clinically approved neuroprotective therapies are lacking. Most studies have focused on neurons while ignoring the important roles of other cells of the neurovascular unit, such as astrocytes and pericytes. Astrocytes are important for the development and maintenance of the blood-brain barrier, brain homeostasis, structural support, control of cerebral blood flow and secretion of neuroprotective factors. Emerging data suggest that astrocyte activation exerts both beneficial and detrimental effects following ischaemic stroke. Activated astrocytes provide neuroprotection and contribute to neurorestoration, but also secrete inflammatory modulators, leading to aggravation of the ischaemic lesion. Astrocytes are more resistant than other cell types to stroke pathology, and exert a regulative effect in response to ischaemia. These roles of astrocytes following ischaemic stroke remain incompletely understood, though they represent an appealing target for neurovascular protection following stroke. In this review, we summarise the astrocytic contributions to neurovascular damage and repair following ischaemic stroke, and explore mechanisms of neuroprotection that promote revascularisation and neurorestoration, which may be targeted for developing novel therapies for ischaemic stroke.

Effect of a Bone Marrow-Derived Extracellular Matrix on Cell Adhesion and Neural Induction of Dental Pulp Stem Cells

Extracellular matrix (ECM) represents an essential component of the cellular niche. In this conditioned microenvironment, the proliferation rates and differentiation states of stem cells are regulated by several factors. In contrast, in in vitro experimental models, cell growth, or induction procedures toward specific cell lines usually occur in contact with plastic, glass, or biogel supports. In this study, we evaluated the effect of a decellularized ECM, derived from bone marrow stem cells, on the neuronal differentiation of mesenchymal stem cells (MSCs) extracted from dental pulp (Dental Pulp Stem Cells - DPSCs). Since DPSCs derive from neuroectodermal embryonic precursors, they are thought to have a greater propensity toward neuronal differentiation than MSCs isolated from other sources. We hypothesized that the presence of a decellularized ECM scaffold could act positively on neuronal-DPSC differentiation through reproduction of an in vivo-like microenvironment. Results from scanning electron microscopy, immunofluorescence, and gene expression assays showed that ECM is able to positively influence the morphology of cells and their distribution and the expression of specific neuronal markers (i.e., NF-L, NF-M, NF-H, PAX6, MAP2).

Zika Virus Infects Intermediate Progenitor Cells and Post-mitotic Committed Neurons in Human Fetal Brain Tissues

Zika virus (ZIKV) infection is associated with microcephaly in fetuses, but the pathogenesis of ZIKV-related microcephaly is not well understood. Here we show that ZIKV infects the subventricular zone in human fetal brain tissues and that the tissue tropism broadens with the progression of gestation. Our research demonstrates also that intermediate progenitor cells (IPCs) are the main target cells for ZIKV. Post-mitotic committed neurons become susceptible to ZIKV infection as well at later stages of gestation. Furthermore, activation of microglial cells, DNA fragmentation, and apoptosis of infected or uninfected cells could be found in ZIKV-infected brain tissues. Our studies identify IPCs as the main target cells for ZIKV. They also suggest that immune activation after ZIKV infection may play an important role in the pathogenesis of ZIKV-related microcephaly.

Blood-brain barrier dysfunction and recovery after ischemic stroke

The blood-brain barrier (BBB) plays a vital role in regulating the trafficking of fluid, solutes and cells at the blood-brain interface and maintaining the homeostatic microenvironment of the CNS. Under pathological conditions, such as ischemic stroke, the BBB can be disrupted, followed by the extravasation of blood components into the brain and compromise of normal neuronal function. This article reviews recent advances in our knowledge of the mechanisms underlying BBB dysfunction and recovery after ischemic stroke. CNS cells in the neurovascular unit, as well as blood-borne peripheral cells constantly modulate the BBB and influence its breakdown and repair after ischemic stroke. The involvement of stroke risk factors and comorbid conditions further complicate the pathogenesis of neurovascular injury by predisposing the BBB to anatomical and functional changes that can exacerbate BBB dysfunction. Emphasis is also given to the process of long-term structural and functional restoration of the BBB after ischemic injury. With the development of novel research tools, future research on the BBB is likely to reveal promising potential therapeutic targets for protecting the BBB and improving patient outcome after ischemic stroke.

Organization and Histochemical Phenotype of Human Fetal Cerebellar Cells following Transplantation into the Cerebellum of Nude Mice

Previous rodent studies have demonstrated the capacity of cerebellar transplants to organize into trilaminar cell layers typically observed in the normal cerebellum. In Purkinje Cell (PC)-deficient animals, PCs will migrate into the host and form synaptic connections. Recently, fetal cerebellar grafts transplanted into the Purkinje cell degeneration (pcd) mutant mouse were shown to result in an improvement of motor behaviors. These studies indicate the potential therapeutic use of neural transplantation in patients with cerebellar degeneration. In the present study, human fetal cerebellar tissue (8.5 wk postconception) was dissociated and transplanted into the normal cerebellum of nude mice. Six months following transplantation, histological analysis revealed donor cells in recipient mice. Immunostaining for the 28 kDa calcium-binding protein (calbindin) revealed the presence of donor PCs that were organized in discrete cellular layers within the transplant neuropil. In most cases the dendritic processes were oriented in a planar fashion perpendicular to the transplant cell layer. Human neurofilament immunostaining revealed bundles of donor fibers within the core of the transplant and/or at the periphery. These bundles were found to be calbindin positive (PC fibers). Three animals provided evidence of donor PC axon growth ventrally into host white matter, and in one case, this ventral migration reached the deep cerebellar nuclei. Most notable was the development of a pronounced folia-like organization by the implanted cell suspensions. Glial processes within the grafts were aligned perpendicular to the long axis of the transplant folia. These results demonstrate the capacity of human fetal cerebellar cell suspension to reorganize into cell layers typical of the normal cerebellum following transplantation into the rodent cerebellum, and develop an organotypic folia-like organization.

Glial influences on BBB functions and molecular players in immune cell trafficking

The blood-brain barrier (BBB) constitutes an elaborate structure formed by specialized capillary endothelial cells, which together with pericytes and perivascular glial cells regulates the exchanges between the central nervous system (CNS) and the periphery. Intricate interactions between the different cellular constituents of the BBB are crucial in establishing a functional BBB and maintaining the delicate homeostasis of the CNS microenvironment. In this review, we discuss the role of astrocytes and microglia in inducing and maintaining barrier properties under physiological conditions as well as their involvement during neuroinflammatory pathologies. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.

A quantitative framework to evaluate modeling of cortical development by neural stem cells

Neural stem cells have been adopted to model a wide range of neuropsychiatric conditions in vitro. However, how well such models correspond to in vivo brain has not been evaluated in an unbiased, comprehensive manner. We used transcriptomic analyses to compare in vitro systems to developing human fetal brain and observed strong conservation of in vivo gene expression and network architecture in differentiating primary human neural progenitor cells (phNPCs). Conserved modules are enriched in genes associated with ASD, supporting the utility of phNPCs for studying neuropsychiatric disease. We also developed and validated a machine learning approach called CoNTExT that identifies the developmental maturity and regional identity of in vitro models. We observed strong differences between in vitro models, including hiPSC-derived neural progenitors from multiple laboratories. This work provides a systems biology framework for evaluating in vitro systems and supports their value in studying the molecular mechanisms of human neurodevelopmental disease.

Glial influence on the blood brain barrier

The Blood Brain Barrier (BBB) is a specialized vascular structure tightly regulating central nervous system (CNS) homeostasis. Endothelial cells are the central component of the BBB and control of their barrier phenotype resides on astrocytes and pericytes. Interactions between these cells and the endothelium promote and maintain many of the physiological and metabolic characteristics that are unique to the BBB. In this review we describe recent findings related to the involvement of astroglial cells, including radial glial cells, in the induction of barrier properties during embryogenesis and adulthood. In addition, we describe changes that occur in astrocytes and endothelial cells during injury and inflammation with a particular emphasis on alterations of the BBB phenotype. GLIA 2013;61:1939–1958

Specification of functional neurons and glia from human pluripotent stem cells

Human pluripotent stem cells (PSCs) such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) hold great promise in regenerative medicine as they are an important source of functional cells for potential cell replacement. These human PSCs, similar to their counterparts of mouse, have the full potential to give rise to any type of cells in the body. However, for the promise to be fulfilled, it is necessary to convert these PSCs into functional specialized cells. Using the developmental principles of neural lineage specification, human ESCs and iPSCs have been effectively differentiated to regional and functional specific neurons and glia, such as striatal gama-aminobutyric acid (GABA)-ergic neurons, spinal motor neurons and myelin sheath forming oligodendrocytes. The human PSCs, in general differentiate after the similar developmental program as that of the mouse: they use the same set of cell signaling to tune the cell fate and they share a conserved transcriptional program that directs the cell fate transition. However, the human PSCs, unlike their counterparts of mouse, tend to respond divergently to the same set of extracellular signals at certain stages of differentiation, which will be a critical consideration to translate the animal model based studies to clinical application.

Directed differentiation of functional astroglial subtypes from human pluripotent stem cells

Regionally and functionally diverse types of astrocytes exist throughout the central nervous system and participate in nearly every aspect of normal and abnormal neural function. Therefore, human astrocyte subtypes are useful tools for understanding brain function, modulating disease processes and promoting neural regeneration. Here we describe a protocol for directed differentiation and maintenance of functional astroglia from human pluripotent stem cells in a chemically defined system. Human stem cells are first differentiated into neuroepithelial cells with or without exogenous patterning molecules (days 0-21). Regular dissociation of the neuroepithelial clusters in suspension, and in the presence of mitogens, permits generation of astroglial subtypes over a long-term expansion (days 21-90). Finally, the astroglial progenitors are either amplified for an extended time or differentiated into functional astrocytes on removal of mitogens and the addition of ciliary neurotrophic factor (days >90). This method generates robust populations of functionally diversified astrocytes with high efficiency.

Emerging cerebral connectivity in the human fetal brain: an MR tractography study

Cerebral axonal connections begin to develop before birth during radial migration in each brain area. A number of theories are still actively debated regarding the link between neuronal migration, developing connectivity, and gyrification. Here, we used high angular resolution diffusion tractography on postmortem fetal human brains (postconception week (W) 17-40) to document the regression of radial and tangential organization likely to represent migration pathways and the emergence of corticocortical organization and gyrification. The dominant radial organization at W17 gradually diminished first in dorsal parieto-occipital and later in ventral frontotemporal regions with regional variation: radial organization persisted longer in the crests of gyri than at the depths of sulci. The dominant tangential organization of the ganglionic eminence at W17 also gradually disappeared by term, together with the disappearance of the ganglionic eminence. A few immature long-range association pathways were visible at W17, gradually became evident by term. Short-range corticocortical tracts emerged prior to gyrification in regions where sulci later developed. Our results suggest that the regional regression of radial organization and regional emergence of fetal brain connectivity proceeds in general from posterodorsal to anteroventral with local variations.

Specification of transplantable astroglial subtypes from human pluripotent stem cells

Human pluripotent stem cells (hPSCs) have been differentiated efficiently to neuronal cell types. However, directed differentiation of hPSCs to astrocytes and astroglial subtypes remains elusive. In this study, hPSCs were directed to nearly uniform populations of immature astrocytes (>90% S100β(+) and GFAP(+)) in large quantities. The immature human astrocytes exhibit similar gene expression patterns as primary astrocytes, display functional properties such as glutamate uptake and promotion of synaptogenesis, and become mature astrocytes by forming connections with blood vessels after transplantation into the mouse brain. Furthermore, hPSC-derived neuroepithelia, patterned to rostral-caudal and dorsal-ventral identities with the same morphogens used for neuronal subtype specification, generate immature astrocytes that express distinct homeodomain transcription factors and display phenotypic differences of different astroglial subtypes. These human astroglial progenitors and immature astrocytes will be useful for studying astrocytes in brain development and function, understanding the roles of astrocytes in disease processes and developing novel treatments for neurological disorders.

Cell proliferation in human ganglionic eminence and suppression after prematurity-associated haemorrhage

In premature infants, germinal matrix haemorrhage in the brain is a common occurrence. However, cell proliferation and fate determination in the normal human germinal matrix is poorly understood. Human ganglionic eminence samples were collected prospectively from autopsies of premature and term infants with no evidence of pathological process (n=78; dying at post-menstrual age 14-88 weeks). The ganglionic eminence was thickest at 20-26 weeks and involuted by 34-36 weeks. Proliferating cells, detected by Ki67 immunoreactivity, were abundant throughout the ganglionic eminence prior to 18 weeks, after which a sharp boundary between the dorsal and ventral ganglionic eminence appeared with reduced cell proliferation in the dorsal region. Ki67 immunoreactivity persisted in the majority of ventral cells until ∼28 weeks, after which time the proportion of proliferating cells dropped quickly. The expression of cell lineage markers (such as Olig2, SOX2, platelet-derived growth factor receptor alpha) showed partitioning at the microscopic level. The hypothesis that germinal matrix haemorrhage suppresses cell proliferation was then addressed. In comparison to controls, germinal matrix haemorrhage (n=47; born at post-menstrual age 18-34 weeks followed by survival of 0 h to 98 days) was associated with significantly decreased cell proliferation if survival was >12 h. The cell cycle arrest transcription factor p53 was transiently increased and the oligodendroglial lineage markers Olig2 and platelet-derived growth factor receptor alpha were decreased. Cell death was negligible. A low level of microglial activation was detected. Haemorrhage-associated suppression of cell proliferation in premature human infants could partially explain the reduced brain size and clinical effects in children who suffer germinal matrix haemorrhage after premature birth.

Intraventricular hemorrhage in premature infants: mechanism of disease

Intraventricular hemorrhage (IVH) is a major complication of prematurity. IVH typically initiates in the germinal matrix, which is a richly vascularized collection of neuronal-glial precursor cells in the developing brain. The etiology of IVH is multifactorial and is primarily attributed to the intrinsic fragility of the germinal matrix vasculature and the disturbance in the cerebral blood flow (CBF). Although this review broadly describes the pathogenesis of IVH, the main focus is on the recent development in molecular mechanisms that elucidates the fragility of the germinal matrix vasculature. The microvasculature of the germinal matrix is frail because of an abundance of angiogenic blood vessels that exhibit paucity of pericytes, immaturity of basal lamina, and deficiency of glial fibrillary acidic protein (GFAP) in the ensheathing astrocytes endfeet. High VEGF and angiopoietin-2 levels activate a rapid angiogenesis in the germinal matrix. The elevation of these growth factors may be ascribed to a relative hypoxia of the germinal matrix perhaps resulting from high metabolic activity and oxygen consumption of the neural progenitor cells. Hence, the rapid stabilization of the angiogenic vessels and the restoration of normal CBF on the first day of life are potential strategies to prevent IVH in premature infants.

Region-specific maturation of cerebral cortex in human fetal brain: diffusion tensor imaging and histology

INTRODUCTION

In this study, diffusion tensor imaging (DTI) and glial fibrillary acidic protein (GFAP) immunohistochemical analysis in different cortical regions in fetal brains at different gestational age (GA) were performed.

METHODS

DTI was performed on 50 freshly aborted fetal brains with GA ranging from 12 to 42 weeks to compare age-related fractional anisotropy (FA) changes in different cerebral cortical regions that include frontal, parietal, occipital, and temporal lobes at the level of thalami. GFAP immunostaining was performed and the percentage of GFAP-positive areas was quantified.

RESULTS

The cortical FA values in the frontal lobe peaked at around 26 weeks of GA, occipital and temporal lobes at around 20 weeks, and parietal lobe at around 23 weeks. A significant, but modest, positive correlation (r = 0.31, p = 0.02) was observed between cortical FA values and percentage area of GFAP expression in cortical region around the time period during which the migrational events are at its peak, i.e., GA < or = 28 weeks for frontal cortical region and GA < or = 22 weeks for rest of the lobes.

CONCLUSIONS

The DTI-derived FA quantification with its GFAP immunohistologic correlation in cortical regions of the various lobes of the cerebral hemispheres supports region-specific migrational and maturational events in human fetal brain.

Carbonic anhydrase II in the developing and adult human brain

Carbonic anhydrase II (CA II) is one of 14 isozymes of carbonic anhydrases, zinc metalloenzymes that catalyze the reversible hydration of carbon dioxide to bicarbonate. Mutations in CA II in humans lead to osteopetrosis with renal tubular acidosis and cerebral calcifications, a disorder often associated with mental retardation. Recently, new avenues in CA II research have opened as a result of discoveries that the enzyme increases bicarbonate and proton fluxes and may play an important role in brain tissue. In the human brain, CA II was localized to oligodendrocytes, myelin, and choroid plexus epithelium. Because this conclusion was based on a few fragmentary reports, we analyzed in more detail the expression of the enzyme in human telencephalon. By immunoblotting, we found a gradual increase in CA II levels from 17 weeks' gestation to childhood and adolescence. By immunohistochemistry, CA II was found to be present not only in oligodendrocytes and choroid plexus epithelium (declining with aging in both these locations), but also in a subset of neurons mostly with GABAergic phenotype, in a few astrocytes, and transiently during brain development in the endothelial cells of microvessels. The enzyme also occurred in oligodendrocyte processes in contact with myelinating axons, myelin sheaths, and axolemma, but was either absent or appeared in minute amounts in compact myelin. These findings suggest the possible involvement of CA II in a wide spectrum of biologic processes in the developing and adult human brain and may contribute to better understanding of the pathogenesis of cerebral calcifications and mental retardation caused by CA II deficiency.

Astrocyte end-feet in germinal matrix, cerebral cortex, and white matter in developing infants

Astrocyte end-feet ensheathe blood vessels in the brain and are believed to provide structural integrity to the cerebral vasculature. We sought to determine in developing infants whether the coverage of blood vessels by astrocyte end-feet is decreased in germinal matrix (GM) compared with cerebral cortex and white matter (WM), which may cause fragility of the GM vasculature. Therefore, we evaluated the perivascular coverage by astrocyte end-feet in these areas. We double-labeled the brain sections with astroglial markers [glial fibrillary acidic protein (GFAP), aquaporin-4 (AQP4), and S-100beta] and a vascular marker, laminin. Perivascular coverage by GFAP+ astrocyte end-feet increased consistently as a function of gestational age (GA) in cortex and WM from 19 to 40 wk. Compared with GFAP, AQP4+ astrocyte end-feet developed at an earlier GA, ensheathing about 63% of blood vessels for 23-40 wk in cortex, WM, and GM. Coverage by GFAP+ perivascular end-feet was decreased in GM compared with cortex and WM from 23 to 34 wk. There was no difference in the coverage by AQP4+ end-feet among the three areas in these infants. The expression of AQP4, a water channel molecule, in the astrocyte end-feet was not significantly different between premature and mature infants, suggesting similar risk of brain edema in preterm and term infants in pathologic conditions. More importantly, the lesser degree of GFAP expression in astrocyte end-feet of GM compared with cortex and WM may reflect a cytoskeletal structural difference that contributes to the fragility of GM vasculature and propensity to hemorrhage.

Specific characteristic of radial glia in the human fetal telencephalon

Phenotypic characteristics of cells in the developing human telencephalic wall were analyzed using electron microscopy and immunocytochemistry with various glial and neuronal cell markers. The results suggest that multiple defined cell types emerge in the neocortical proliferative zones and are differentially regulated during embryonic development. At 5-6 weeks gestation, three major cell types are observed. Most proliferating ventricular zone (VZ) cells are labeled with radial glial (RG) markers such as vimentin, glial fibrillary acidic protein (GFAP), and glutamate astrocyte-specific transporter (GLAST) antibodies. A subpopulation of these RG cells also express the neuronal markers beta III-tubulin, MAP-2, and phosphorylated neurofilament SMI-31, in addition to the stem cell marker nestin, indicating their multipotential capacity. In addition, the presence of VZ cells that immunoreact only with neuronal markers indicates the emergence of restricted neuronal progenitors. The number of multipotential progenitors in the VZ gradually decreases, whereas the number of more restricted progenitors increases systematically during the 3-month course of human corticogenesis. These results suggest that multipotential progenitors coexist with restricted neuronal progenitors and RG cells during initial corticogenesis in the human telencephalon. Since the multipotential VZ cells disappear during the major wave of neocortical neurogenesis, the RG and restricted neuronal progenitors appear to serve as the main sources of cortical neurons. Thus, the diversification of cells in human VZ and overlying subventricular zone (SVZ) begins earlier and is more pronounced than in rodents.

Structural proteins during brain development in the preterm and near-term ovine fetus and the effect of intermittent umbilical cord occlusion

OBJECTIVE

The purpose of this study was to determine the immunoreactivity of selected structural proteins in the preterm and near-term ovine fetal brain and the response to intermittent umbilical cord occlusion as a measure of altered cellular growth. The intermediate filament proteins nestin, vimentin, and glial fibrillary acidic protein was used as markers for astroglial maturation and astrogliosis, and myelin basic protein as a marker for oligodendrocytes and myelin formation.

STUDY DESIGN

Fetal sheep (control and experimental groups at 0.75 and 0.90 of gestation) were studied over 4 days; umbilical cord occlusion was performed in the experimental group by complete inflation of an occluder cuff for 90 seconds every 30 minutes for 3 to 5 hours each day. Animals were then killed, and the fetal brain was perfusion fixed and processed for immunohistologic examination of the gray and white matter. Immunoreactivity was quantified with an image analysis system and expressed as the fractional area positive stain for each protein.

RESULTS

In both preterm and near-term animal groups, umbilical cord occlusion caused a large decline in arterial Po(2) (to approximately 7 mm Hg), a modest decline in pH (to approximately 7.30), and a modest rise in Pco(2) (to approximately 61 mm Hg; all P <.01), with a return to control values after the occluder release and no cumulative acidosis over each day of study. Vimentin and glial fibrillary acidic protein immunoreactivity showed reciprocal changes, with vimentin decreased and glial fibrillary acidic protein increased in both the gray and white matter of the control group from 0.75 to 0.90 of gestation, which can be attributed to the transition of radial glia into mature astrocytes. Myelin basic protein immunoreactivity increased approximately 3-fold in the white matter of the control group with advancing gestation, which likely reflected active oligodendrocyte differentiation and increased myelination at this time of development. Intermittent umbilical cord occlusion over 4 days resulted in an approximately 60% decrease in nestin, vimentin, and glial fibrillary acidic protein immunoreactivity, which was qualitatively similar for both the gray and white matter and likely indicative of altered protein synthesis and/or degradation, but only in the preterm group and with no change in myelin basic protein immunoreactivity.

CONCLUSION

There is considerable change in the immunoreactivity of structural proteins within the ovine fetal brain over the latter part of gestation and consistent with a high rate of protein turnover, as previously reported. Intermittent umbilical cord occlusion as studied with minimal evidence for necrotic cell injury appears capable of altering selected protein synthesis/degradation, more so in younger animals when protein turnover is higher, which might then impact on the brain's development.

Cortical radial glial cells in human fetuses: depth-correlated transformation into astrocytes

In the human brain, the transformation of radial glial cells (RGC) into astrocytes has been studied only rarely. In this work, we were interested in studying the morphologic aspects underlying this transformation during the fetal/perinatal period, particularly emphasizing the region-specific glial fiber anatomy in the medial cortex. We have used carbocyanine dyes (DiI/DiA) to identify the RGC transitional forms and glial fiber morphology. Immunocytochemical markers such as vimentin and glial fibrillary acidic protein (GFAP) were also employed to label the radial cells of glial lineage and to reveal the early pattern of astrocyte distribution. Neuronal markers such as neuronal-specific nuclear protein (NeuN) and microtubule-associated protein (MAP-2) were employed to discern whether or not these radial cells could, in fact, be neurons or neuronal precursors. The main findings concern the beginning of RGC transformation showing loss of the ventricular fixation in most cases, followed by transitional figures and the appearance of mature astrocytes. In addition, diverse fiber morphology related to depth within the cortical mantle was clearly demonstrated. We concluded that during the fetal/perinatal period the cerebral cortex is undergoing the final stages of radial neuronal migration, followed by involution of RGC ventricular processes and transformation into astrocytes. None of the transitional or other radial glia were positive for neuronal markers. Furthermore, the differential morphology of RGC fibers according to depth suggests that factors may act locally in the subplate and could have a role in the process of cortical RGC transformation and astrocyte localization. The early pattern of astrocyte distribution is bilaminar, sparing the cortical plate. Few astrocytes (GFAP+) in the upper band could be found with radial processes at anytime. This suggests that astrocytes in the marginal zone could be derived from different precursors than those that differentiate from RGCs during this period.

Expression of the alpha4 isoform of the nicotinic acetylcholine receptor in the fetal human cerebral cortex

Nicotinic acetylcholine receptors are likely to play an important role in neuronal migration during development. Furthermore, the alpha4 receptor subunit gene is related to a hereditary juvenile form of epilepsy. Only little information is available, however, on the expression of cerebrocortical nicotinic acetylcholine receptors during human fetal development. Using non-isotopic in situ hybridization and immunohistochemistry, we have studied the distribution of the alpha4 subunit of the nicotinic acetylcholine receptor mRNA and protein in the human frontal cortex at middle (17-24 weeks of gestation) and late (34-42 weeks of gestation) fetal stages. Both, alpha4 receptor mRNA and alpha4 receptor protein were observed beginning during week 17-18 of gestation. At this time of development, a few weakly labeled mRNA-containing cells were present mainly in the ventricular zone, the subplate and the cortical plate. A similar distribution pattern was found for the receptor protein. Around week 38 of gestation, the distribution in the cerebral cortex of alpha4 subunit-containing cells was similar to that of adult human cortices with the highest densities of labeled neurons found in layers II/III, followed by layers V and VI. Nicotinic acetylcholine receptor-containing neurons appear rather early in human fetal development. Given functional maturity, they may interact during cortical development with acetylcholine released from corticopetal fibers or other yet unknown sources subserving the process of neuronal migration and pathfinding.

Cell proliferation and death: morphological evidence during corticogenesis in the developing human brain

Cell proliferation and death account for the refinement of the cell number during corticogenesis. These processes have been investigated in the human developing telencephalon (12th-24th week of gestation) and cerebellum (16th-24th week). Only foetal brains, which had normal neuropathological examination, were utilised. Cell proliferation was analysed by classical histology and PCNA immunohistochemistry; cell death was investigated by the TUNEL method, which makes evident the different stages of apoptosis. High figures of mitotic nuclei were seen in the ventricular zone at the 12th-15th week of gestation, before sharply declining. The decrease of the proliferating cells occurs synchronously in both frontal and occipital germinal zones. Conversely, a slow increase of the number of the mitotic cells was observed in the more dorsal regions, probably due to the presence of proliferating glial elements. The amount of apoptotic nuclei was always remarkably low in the transient compartments of the wall of the telencephalon. The moderate number of apoptotic cells suggests that cellular mechanisms other than apoptosis are involved in the dissolution of the ventricular zone. Neither proliferating nor apoptotic cells were seen in the cortical plate. The topography of cell proliferation and death in the developing cerebellum did not account for a mutual relationship between the two events. The prolonged duration of the cell-cycle in the human developing CNS may explain its increased vulnerability to various DNA-damaging conditions, which can lead to either destructive lesions or malformations.

Colonisation of the developing human brain and spinal cord by microglia: a review

Microglia are the immune effector cells of the nervous system. The prevailing view is that microglia are derived from circulating precursors in the blood, which originate from the bone-marrow. Colonisation of the central nervous system (CNS) by microglia is an orchestrated response during human fetal development related to the maturation of the nervous system. It coincides with vascularisation, formation of radial glia, neuronal migration and myelination primarily in the 4th-5th months and beyond. Microglial influx generally conforms to a route following white matter tracts to gray areas. We have observed that colonisation of the spinal cord begins around 9 weeks, with the major influx and distribution of microglia commencing around 16 weeks. In the cerebrum, colonisation is in progress during the second trimester, and ramified microglial forms are widely distributed within the intermediate zone by the first half of intra-uterine life (20-22 weeks). A distinct pattern of migration occurs along radial glia, white matter tracts and vasculature. The distribution of these cells is likely to be co-ordinated by spatially and temporally regulated, anatomical expression of chemokines including RANTES and MCP-1 in the cortex; by ICAM-2 and PECAM on radiating cerebral vessels and on capillaries within the germinal layer, and apoptotic cell death overlying this region. The phenotype and functional characteristics of fetal microglia are also outlined in this review. The need for specific cellular interactions and targeting is greater within the central nervous system than in other tissues. In this respect, microglia may additionally contribute towards CNS histogenesis.

Early generation of glia in the intermediate zone of the developing cerebral cortex

Radial glia are present at the earliest stage of cerebral cortical development, and later they transform into astrocytes. Other glial cells including astrocytes and oligodendrocytes are thought to appear only after neuron generation is complete and the cortical layers are formed. Little is known of when and where microglia enter the central nervous system and proliferate. We addressed the question of the origin of these three glial cell types in the developing ferret cerebral cortex. We assessed the temporal pattern of glial cell division by administering [3H]thymidine to label cells in S phase, and by using survival periods of 1-2 h to label dividing cells in situ. Labeled cells were identified in the developing intermediate zone of the ferret cerebral wall. These cells were present at E28, and reached a maximum number at P1. Double labeling experiments identified these cells as astrocytes, oligodendrocytes or microglia. None of the dividing cells expressed neuronal markers. These data show that all three types of glia are generated in the developing subcortical white matter, and that glial progenitors are present in the intermediate zone as soon as it becomes a recognizable structure. These data also show that the period of glial generation overlaps extensively with the period of neuron generation, since neuron generation is not complete until the end of the second postnatal week in the ferret.

Maturation of fetal human neural xenografts in the adult rat brain

Transplantation of human fetal neural cells has been used for several years as a treatment for Parkinson's disease. These therapeutic trials were based on a large number of rat allografts studies, and the species to species extrapolation appeared valid in many respects. One major difference between neurons of various species, however, is their rate of maturation; indeed, human neurons have been proven to grow much more slowly than rat neurons. This has been studied mostly, up to now, at the light microscope level. In an attempt to determine the fine structural correlates of this protracted development and to detail the schedule of morphogenesis and synaptogenesis, human fetal brain stem tissue (at 8 weeks of gestation) was transplanted into a previously lesioned brain area of immunosuppressed adult rats. Transplants, which were allowed to develop for 15 days to 3 months, were analyzed using the electron microscope. At 15 days, small cells containing a large nucleus were surrounded by wide extracellular spaces. At 1 month, grafted neurons displayed a thin rim of cytoplasm and few thin processes. At 2 months, extracellular spaces tended to diminish. Thin processes formed bundles and large processes extended from enlarged neurons. Major changes were observed at 3 months survival as the neuropile filled up with cells and processes and synaptogenesis began. Comparison with a similar ultrastructural study of thalamic rat allografts shows that human cells develop following a pattern similar to that in rat cells but that the duration of each maturation step is largely extended.

Human fetal hippocampal development: II. The neuronal cytoskeleton

In this study of the developing human hippocampus, we monitor the timing of onset and the sequential patterns of expression of 11 developmentally regulated proteins that are important components of the neuronal cytoskeleton. Immunohistochemistry using well-characterized antibodies was conducted with fixed paraffin-embedded sections from hippocampi at various stages of fetal and postnatal development. At 9 weeks gestational age, immunoreactivity was evident for the microtubule-associated proteins (MAPs), MAP2 and MAP5, low molecular weight (Mr) neurofilament (NF) protein (NF-L), poorly phosphorylated mid-Mr NF protein (NF-M/P-), vimentin, and alpha-and beta-tubulins within the somatodendritic domain of neurons of the hippocampal plate. Weak immunoreactivity for moderately phosphorylated, high Mr NF protein (NF-H/P + + +), tau, and nestin was observed. Highly phosphorylated mid-Mr NF protein (NF-M/P + + +) and alpha-internexin were first detected at 15 weeks and highly phosphorylated, high Mr NF protein (NF-H/P+3) at 20 weeks. At 15 weeks, MAP2, MAP5, and tubulins were expressed in an "inside-out" gradient and in a gradient between hippocampal subfields with subiculum > ammonic subfields > dentate gyrus. These gradients paralleled the maturational gradients seen in cytoarchitectural and neuronal morphologic studies. The adult pattern of neuronal cytoskeletal protein expression in the hippocampus was attained by the second postnatal year for all proteins. Our findings demonstrate an elaborate orchestration of cytoskeletal protein expression within the hippocampus that is qualitatively similar to what is seen in other brain regions and in nonhuman species but which also has some important differences in timing and pattern. The differences in the developmentally regulated expression of neuronal cytoskeletal proteins in separate regions of the central nervous system (CNS) suggest that there are region-specific differences in composition and function of the neuronal cytoskeleton. These observations have implications for understanding the role of the neuronal cytoskeleton in the developing, mature, and diseased CNS.

Development of human fetal ventral mesencephalic grafts in rats with 6-OHDA lesions of the nigrostriatal pathway

Neuronal transplantation is an approach that can be exploited to study the development of the human central nervous system as well as being used in attempts to restore neurological function. In the present study, we have examined cellular events that appear to precede the development of dopamine nerve fiber extension by neurons from the human fetal ventral mesencephalon. These cellular events were examined using neuronal cell suspensions from human fetal ventral mesencephalic tissue (gestational ages 7-10 weeks) transplanted into the striatum of unilaterally lesioned 6-hydroxydopamine (6-OHDA) rats. Animals were sacrificed for immunohistochemistry 9-10 weeks after the transplantation prior to the manifestation of behavioral recovery. Histological analysis revealed tyrosine hydroxylase (TH) immunoreactive neurons in the grafts. The majority of these neurons had very short TH positive processes (60-70 microns), indicating that the maturation of grafted dopaminergic neurons was still incomplete. Immunostaining for the human specific intermediate neurofilament (hNF, clone: BF-10) showed dense neuronal fibers in the grafts. These fibers extended deeper into the host brain than the TH positive neuronal processes. The whole striatum, particularly the medial part of the striatum, exhibited long NF positive processes. Glial fibrillary acidic protein (GFAP) immunohistochemistry revealed fine astrocytic processes inside the grafts, which were clearly different from host reactive glial cells surrounding the grafts. These graft-derived glial processes tended to extend into the host brain deeper than the TH positive neuronal processes from the grafts. These early histological findings of the grafted human fetal ventral mesencephalon suggest that the graft-derived NF positive neuronal processes, as well as the glial processes, radiate from the grafted tissue and extend into the host brain prior to the extension of TH positive processes. These results further suggest that human-to-rat xenografts can be used to study the neural development of human fetal brain tissue.

Immunohistochemical localization of glial and neuronal cell markers in the developing bovine brain

In the present immunohistochemical study, the distribution and differentiation of glial and neuronal cells in bovine fetal brains (age range: between 1-2 and 7-8 months) was examined using antibodies against nervous system-specific proteins, i.e., glial fibrillary acidic protein (GFAP), vimentin, neuron specific enolase (NSE) and a neurofilament protein subunit (NF200kD).

Postnatal development of vimentin-positive cells in the rabbit superior colliculus

The present study examined the postnatal development of the radial glia in the rabbit superior colliculus during the first 40 postnatal days. An antivimentin monoclonal antibody and the carbocyanine fluorescent tracer DiI were used in order to investigate the development of laminar connectivity in the superior colliculus. We focused our study on the superficial gray layer, the intermediate layer, and the deep layers of the superior colliculus, the periaqueductal gray matter (PAGM), and the medial intercollicular region. Vimentin-positive structures of glial lineage consisted of 1) the main radial system, which in the newborn rabbit was made up of wavy fibers that ran from the aqueduct to the pial surface, where they terminated in end-feet. At postnatal day 15, these fibers diminished to 100-200 microns long wavy tracts, which emanated from the aqueduct, and to a few straight or arched fragments in the superficial gray layer; 2) the median ventricular formation, which extends from mesencephalic aqueduct to the intercollicular sulcus, was characterized by a series of ascending, vimentin-positive fibers, some of large caliber, which persisted until postnatal day 40; 3) the tangential fiber system, which was made up of fibers that diverged from the median ventricular formation and of a number of short tracts running perpendicular to the periaqueductal radial fibers; these structures may provide support for migrating subpopulations of neurons; 4) immature and mature-like protoplasmic and fibrous astrocytes, which appeared during the second postnatal week. Thereafter, the number of vimentin-positive astrocytes decreased sharply. Our findings generally support earlier descriptions of the radial glia, except for the persistence, in superficial layers of the superior colliculus, of straight and curved fragments of fibers, which may participate in the organization of visual afferents at this level.

Expression of phosphorylated high molecular weight neurofilament protein (NF-H) and vimentin in human developing dorsal root ganglia and spinal cord

The expression of vimentin and the phosphorylated variant of high molecular weight neurofilament protein (NF-H) was studied in developing human fetal dorsal root ganglia and spinal cord. The technique used for examination of cryosections was double-label fluorescence with monoclonal antibodies. Both proteins were present in the nerve fibres inside the ganglia of 6- and 8-week-old embryos. During further development the expression of vimentin continued to increase in the satellite cells, but was found to be decreasing in the ganglion cells. Phosphorylated NF-H was found in the processes of ganglion cells, as well as in the perikarya at all developmental stages. In the spinal cord of 6- and 8-week-old embryos, phosphorylated NF-H protein was found in the longitudinal fibres of the marginal layer and in processes of the mantle zone; some of the fibres also contained vimentin. Later the co-expression of the two proteins ceased and vimentin was found only in glial and mesenchymal derivatives. Phosphorylated NF-H was located, at all developmental stages, in the axons of both white and grey matter, but not in the neuronal perikarya. The results indicate that phosphorylation of the NF-H in human dorsal root ganglia starts in the perikarya of the ganglion cells while in the ganglion cells of the spinal cord it takes place in the axons.

An immunohistochemical study of the fetal sheep neocortex and cerebellum with antibodies against nervous system-specific proteins

The topographical distribution of glial fibrillary acidic protein (GFAP), vimentin, neuron-specific enolase (NSE) and neurofilament (NF) proteins in the developing neocortex and cerebellum of sheep fetuses of different gestational ages (60-149 days) was described. For comparison, brain tissues from a lamb and two adult sheep were included in this study. In the walls of the developing cerebral hemispheres GFAP- and vimentin-immunoreactive radial glial fibres were demonstrated. From 80 days of gestation onwards a continuous decrease of radial fibres occurred which was accompanied by an increase of GFAP-positive mature astrocytes. In Bergmann glial fibres of the cerebellum, which are the equivalent of radial fibres in the telencephalon, both GFAP and vimentin were detectable in fetuses and adult sheep. With polyclonal antibodies against NSE and NF proteins (NF-M, NF-H) prominent staining of neuronal fibre tracts was seen in fetuses of all gestational ages studied. In the neocortex, staining for NF-L did not occur before day 80 of gestation. With monoclonal antibodies against phosphorylated NF-H (clone SMI 31), however, reaction of neocortical fibre tracts was first seen at 85 days of gestation, and cytoplasmic staining of single neocortical neurons was first found in a 149-day-old fetus. Several fixatives and proteolytic pretreatment were examined for their effects on preservation and re-establishment of marker protein expression, respectively. GFAP and vimentin in radial glial fibres were not demonstrable without pretrypsinization of tissue sections. The most intensive staining of NF proteins with polyclonal antisera was seen in brains fixed in Bouin's fluid.

Expression of neuronal and glial polypeptides during histogenesis of the human cerebellar cortex including observations on the dentate nucleus

In order to gain a more complete understanding of the sequential pattern of gene expression during neurogenesis and gliogenesis in humans, we followed the expression of well-characterized, developmentally regulated polypeptides in the cerebellar cortex and dentate nucleus by immunohistochemistry using monoclonal antibodies of highly defined specificity. At 8-10 weeks gestational age (GA), progenitor cells and their immediate progeny in the rhombencephalic ventricular zone expressed vimentin and nestin and, to a lesser extent, microtubule-associated protein 5 (MAP5) and glial fibrillary acidic protein (GFAP), but not the low affinity nerve growth factor receptor (NGFR). In contrast, postmitotic, migrating immature neurons in the intermediate zone gave strong reactions for MAP2, tau, and a nonphosphorylated form of middle molecular weight neurofilament (NF) protein (NF-M) and weak reactivity for NGFR. At 15 weeks GA, proliferating cells of the superficial part of the cerebellar external granular layer stained only for NGFR, while more deeply situated cells of the external granular layer stained positively for NGFR, MAP2, MAP5, tau, and chromogranin A, which correlates with the early outgrowth of parallel fibers. All phosphoisoforms of NF-M as well as the low (NF-L) and high (NF-H) molecular weight NF proteins and alpha-internexin were expressed in the somatodendritic domain of Purkinje cells and dentate nucleus neurons from about 20 weeks GA with a gradual compartmentalization of highly phosphorylated forms of NF-M and NF-H into axons by the end of gestation. Alpha-internexin was also expressed strongly in axons of the deep white matter from 20 weeks GA to adulthood. MAP2, synaptophysin, and NGFR showed early, transient expression in the somatodendritic domain of Purkinje cells followed by the appearance of a 220 kDa nestin-like peptide that continued to be expressed in adult Purkinje cells. Notably, developing dentate nucleus neurons expressed many of these proteins in a similar temporal sequence. Early in the developing cerebellar cortex, the expression of NF protein and synaptophysin occurred in discrete patches or columns similar to those described for other antigens (i.e., zebrins). Finally, radial glia were positive for vimentin, GFAP, and nestin from 8 weeks GA to 8 months postnatal. This study describes the distinct molecular programs of lineage commitment in cerebellar progenitor cells and in differentiating neurons and astrocytes of the human cerebellum. The acquisition of a mature molecular neuronal phenotype correlates with the establishment of structural polarity in cerebellar neurons.

Immunohistochemical characterization of primitive neuroectodermal tumors and their possible relationship to the stepwise ontogenetic development of the central nervous system. 1. Ontogenetic studies

Aim of the present study was to establish different immunohistochemical staining patterns for a subsequent comparison with those of primitive neuroectodermal (PNET) subsets, i.e. PNET-NOS (not otherwise specified) or PNET with focal neuronal, astrocytic or ependymal differentiation, to relate neoplastic to embryonal development. Tissue of the developing central nervous system, with special emphasis on the stepwise development of the rhombencephalon, the cerebellar and the retinal anlage, from 20 different human embryos and fetuses ranging from 3 to 30 weeks of gestational age (GA) was examined. Six neuronal markers, synaptophysin, chromogranin A, neuron-specific enolase (NSE), neurofilament protein (NFP; 160 kDa, 200 kDa, 70 and 200 kDa) and six other markers, glial fibrillary acidic protein (GFAP), S-100 protein, vimentin, myoglobin, desmin, cytokeratin, were assessed immunohistochemically. GFAP and S-100 protein appeared at the 6th week of GA in primitive glial cells of the cerebellar anlage, brain stem, rhombencephalon, and developing spinal cord, together with--as first neuronal marker--chromogranin A, then NFP (70 and 200 kDa, and 160 kDa) from the 8th week onward. NSE started in the 11th week and synaptophysin not earlier than the 16th week of GA. Interestingly, the differentiation of the retinal anlage started rather late with NSE positivity beginning from the 16th week and positive reactions to synaptophysin and NFPs only from the 25th and chromogranin A from the 28th week of GA onward.

Glial cell differentiation in neuron-free and neuron-rich regions. II. Early appearance of S-100 protein positive astrocytes in human fetal hippocampus

The development of the human fetal hippocampus and dentate gyrus has been studied immunocytochemically. The first glial cells to appear are vimentin-positive radial glial cells. A gradual transition from vimentin to glial fibrillary acidic protein (GFAP) reactivity in the radial glial cells occurs at week 8. The GFAP-positive radial glial cells transform into astrocytes from week 14. A population of small S-100-positive somata which morphologically and spatially are distinct from GFAP-positive radial glial cells and their transformed progeny, are found as early as week 9.5 in the hippocampus during the period of peak neurogenesis. The well-defined immunoreactivity of the morphologically homogenous cell subpopulation for S-100 protein, which has been used as an astrocytic marker in the adult hippocampus, indicates that astrocytes may differentiate at very early gestational ages in human fetuses. The S-100-positive astrocytes are thought to be derived from ventricular zone cells, which at the time of their appearance do not express any of the applied astrocytic markers (S-100, GFAP, vimentin). It is suggested that the S-100-positive astrocytic cell population interacts with the first incoming projection fibers, so modulating the pattern of connectivity.