Biomedical subjects

G Fishell

Publications and source records attributed to G Fishell.

34 records · Page 2Linked to original sources

PubMed1997

A short-range signal restricts cell movement between telencephalic proliferative zones.

During telencephalic development, a boundary develops that restricts cell movement between the dorsal cortical and basal striatal proliferative zones. In this study, the appearance of this boundary and the mechanism by which cell movement is restricted were examined through a number of approaches. The general pattern of neuronal dispersion was examined both with an early neuronal marker and through the focal application of DiI to telencephalic explants. Both methods revealed that, although tangential neuronal dispersion is present throughout much of the telencephalon, it is restricted within the boundary region separating dorsal and ventral telencephalic proliferative zones. To examine the cellular mechanism underlying this boundary restriction, dissociated cells from the striatum were placed within both areas of the boundary, where dispersion is limited, and areas within the cortex, where significant cellular dispersion occurs. Cells placed within the boundary region remain round and extend only thin processes, whereas progenitors placed onto the cortical ventricular zone away from this boundary are able to migrate extensively. This suggests that the boundary inhibits directly the migration of cells. To examine whether the signal inhibiting dispersion within the boundary region acts as a long- or short-range cue, we apposed explants of boundary and nonboundary regions in vitro. Within these explants we found that migration was neither inhibited in nonboundary regions nor induced in boundary regions. This suggests that the boundary between dorsal and ventral telencephalon isolates these respective environments through either a contact-dependent or a short-range diffusible mechanism.

Acridine Orange↗

PubMed1997

Regionalization in the mammalian telencephalon.

Regionalization in the telencephalon results in the formation of functionally and anatomically distinct territories. Cell fate analysis and gene expression studies suggest these subdivisions arise relatively late in development compared with the spinal cord or hindbrain. The mechanisms underlying the commitment of telencephalic cells to specific regional identities have been examined through recent transplantation experiments.

Animals↗

PubMed1996

Disruption of the MacMARCKS gene prevents cranial neural tube closure and results in anencephaly.

MacMARCKS is a member of the MARCKS family of protein kinase C (PKC) substrates. Biochemical evidence demonstrates that these proteins integrate calcium and PKC-dependent signals to regulate actin structure at the membrane. We report here that deletion of the MacMARCKS gene prevents cranial neural tube closure in the developing brain, resulting in anencephaly. This suggests a central role for MacMARCKS and the PKC signal transduction pathway in the folding of the anterior neural plate during the early phases of brain formation, and supports the hypothesis that actin-based motility directs cranial neural tube closure.

Anencephaly↗

PubMed1995

Pattern formation in the mammalian forebrain: striatal patch and matrix neurons intermix prior to compartment formation.

The striatum of the mammalian forebrain is divided into two compartments: the patches and the matrix. Neurons of the patch compartment in the rat striatum become postmitotic earlier in neurogenesis than neurons of the matrix compartment. The selective adhesion of patch neurons to one another has been suggested previously to be an important developmental mechanism of striatal compartmentation. We asked if the selective adhesion of patch neurons is expressed before or after the migration of the majority of the matrix neurons into the striatum. Patch neurons were labelled in vivo by a fluorescent retrograde tracer injected into the substantia nigra on embryonic day 19, which almost exclusively labelled patch neurons. Matrix neurons were labelled with a maternal injection of bromodeoxyuridine at embryonic day 18. When animals were killed at embryonic day 20, the majority of the retrogradely labelled patch neurons were intermixed with the bromodeoxyuridine-labelled matrix neurons, although there appeared to be clustering of some of the patch neurons. However, by postnatal day 2 there was a complete segregation of the clusters of the retrogradely labelled patch neurons from the bromodeoxyuridine-labelled matrix neurons in the striatum. This process was modelled in vitro. The patch and matrix compartments were labelled in vivo at embryonic day 13 and 18 respectively, with different birthdate markers ([3H]thymidine or bromodeoxyuridine). At embryonic day 20 the striatal tissue was removed, dissociated and reaggregated in suspension cultures. After 1 day in vitro, labelled patch and matrix neurons were randomly intermixed within the reaggregates. Examination of the cultures at 2.5 and 4 days in vitro revealed clumping of the labelled patch neurons towards the centres of the reaggregates. Over this same period, the labelled matrix neurons did not clump and were dispersed towards the periphery of the reaggregates. The results suggest that patch neuron adhesiveness may appear relatively soon after these neurons become postmitotic, but that this adhesiveness is unable to overcome the initial force produced by the massive migration of matrix neurons into the striatum. We hypothesize that a migratory phase of embryonic striatal development exists, when fated patch and matrix neurons intermix. After this migratory phase, patch neuron adhesiveness can produce the mature segregation of the striatal compartments.

Aging↗

PubMed1995

Optical microscopy. 3. Tracking fluorescently labeled neurons in developing brain.

For decades, time-lapse microscopy has been used to track dynamic events associated with biological phenomena. Time-lapse studies of the developing nervous system have been restricted to analysis of dissociated cell cultures or of a series of static images from living organisms. The advent of new fluorescent dyes and video imaging technology has produced novel views of the behavior of neurons in the context of the developing nervous tissue, such as migrations within and away from proliferative zones and navigation of axonal processes to synaptic targets. After fixation of the tissue preparation, time-lapse monitoring can be followed by other analytical techniques and forms of microscopy, e.g., immunocytochemistry or electron microscopy, producing information on the interactions of individual cells whose behavioral histories are known. The power of video time-lapse microscopy of living brain tissue lies in the firsthand documentation of developmental patterning, which in turn can serve as an experimental assay.

Animals↗

PubMed1995

Striatal precursors adopt cortical identities in response to local cues.

One of the early steps in the regionalization of the CNS is the subdivision of the forebrain into dorsal and basal telencephalic ventricular zones. These ventricular zones give rise to the cortex and striatum respectively, in the mature brain. Previous work suggests that while neural precursors are able to move within both the dorsal cortical and basal striatal ventricular zones, they are unable to cross the boundary area between them. To determine if the regional identities of the cells in these ventricular zones are restricted, cells from the basal striatal ventricular zone were either transplanted back into their original environment or into the dorsally adjacent cortical ventricular zone. Use of in vitro explants of mouse telencephalon demonstrated that striatal precursors are able to integrate heterotopically within 12 hours of being placed onto the surface of cortical ventricular zone. To examine whether heterotopically placed neural precursors have phenotypes appropriate to their host or donor environment, in vivo transplants in rats were performed. Striatal ventricular zone cells transplanted to a striatal environment adopt morphologies and axonal projections characteristic of striatal cells. In contrast, striatal ventricular zone cells transplanted in vivo to a cortical environment acquired morphologies and axonal projections specific to cortex. These findings suggest that within forebrain, position-specific cues play an instructive role in determining critical aspects of regional phenotype.

Animals↗

PubMed1993

Dispersion of neural progenitors within the germinal zones of the forebrain.

One of the early events in the establishment of regional diversity in brain is the subdivision of the forebrain into the cerebral cortex and underlying basal ganglia. This subdivision is of special interest, owing to the striking difference in cellular patterning in these two regions. Whereas the dorsal aspect of the telencephalon gives rise to the laminar, cortical regions of brain, the basal aspect gives rise to nuclear, subcortical regions. To examine early events in the regionalization of the forebrain, we visualized cell movement within the ventricular zones of the dorsal and basal regions of the E15 murine telencephalon. Over an 8-24-hour observation period, labelled cells moved extensively in the plane of the cortical ventricular zone. Cell dispersion was restricted, however, at the border between the cortical ventricular zone and the lateral ganglionic eminence, the basal telencephalic ventricular zone. We suggest that this restriction of cell movements establishes a regional pattern of neurogenesis in the developing brain.

Animals↗

PubMed1993

Spatially localized neuronal cell lineages in the developing mammalian forebrain.

The role of cell lineage in the organization of the cerebral cortex and striatum of the developing rat forebrain was analysed using retroviral-mediated gene transfer to mark the progeny of individual progenitors. Injections around the onset of neurogenesis (embryonic day 14) produced neuronal- and glial-specific clones in the striatum and cortex. The majority of the neuronal clones were restricted to either the deep or superficial layers of the cortex and to either the striatal patch or matrix compartments of the striatum. Moreover, modeling the distributions of the neuronal clones in various ways revealed that grouping the clones into deep vs superficial cortical compartments and patch vs matrix striatal compartments best accounted for the clone distributions. These results suggest that at the onset of neurogenesis there is a heterogeneity of neuronal progenitors within the proliferative ventricular zone.

Animals↗

PubMed1992

Embryonic lesions of the substantia nigra prevent the patchy expression of opiate receptors, but not the segregation of patch and matrix compartment neurons, in the developing rat striatum.

Unilateral lesions of the substantia nigra on embryonic day 19 prevent the development of the normal patchy distribution of opiate receptors in the ipsilateral rat striatum. Independent, early and permanent labelling of patch compartment neurons in the same brains on embryonic day 14 with [3H]thymidine revealed that the substantia nigra lesions did not prevent the aggregation of early born neurons into patches, but rather blocked the normal expression of one phenotype (dense opiate receptor binding) of these patches. Thus, early nigrostriatal connections may not be critical for the fundamental patch/matrix compartmentation of the striatum, but may be important in the maturation of phenotypic markers of these compartments.

Animals↗

PubMed1991

Pattern formation in the striatum: neurons with early projections to the substantia nigra survive the cell death period.

During the early postnatal period the striatum undergoes significant cell death. The specificity and regulation of this regressive event may be particularly interesting in the light of recent findings demonstrating that a developmentally organized compartmental architecture exists in the striatum. The striatum can be divided into two complementary and phenotypically distinct compartments, the patches and the matrix. In the adult, these two striatal compartments can be distinguished on the basis of their anatomy and a series of compartment-specific biochemical and hodological markers. We have previously demonstrated that the neurons within the patch and matrix compartments become postmitotic and make connections with the substantia nigra at distinct and sequential developmental times. The majority of patch neurons become postmitotic between embryonic days 12 and 15 and make a striatonigral connection prenatally. In contrast, a majority of matrix neurons become postmitotic between embryonic days 17 and 20 and do not form an efferent connection to the substantia nigra until the first postnatal week. Here we investigated whether either neuronal birthdate or time of making an efferent projection correlates with a neuron's probability of surviving the cell death period. We found that both the patch and matrix compartments undergo their entire cell death period by the end of the first postnatal week. During this period approximately 30% of striatal neurons are subject to cell death, regardless of striatal compartment. Neuronal counts within the striatal patch compartment suggest that both early born neurons (embryonic day 13) and early projecting neurons (to the substantia nigra) are preferentially spared. However, their considerable overlap (i.e., most early born neurons also have a nigral projection) prevents assessment of which feature is critical for survival. In contrast, there are small, but mostly separate, populations of early born and early projecting neurons within the matrix compartment. Quantitative analysis of these two distinct populations suggests that while early projection neurons within the matrix are spared, the early born matrix neurons lacking an early nigral projection undergo significant cell death. This proposal is further supported by the observation that the percentage of early born neurons in both the patch and matrix compartments that also have an early nigral projection increases from postnatal day 2 to 17. This finding suggests that among the early born striatal neurons in both compartments, those that do not project to the nigra selectively die during the cell death period. Together these results support the hypothesis that completion of an early projection to the substantia nigra gives neurons an advantage for surviving the cell death period.

Animals↗

PubMed1991

Astrotactin provides a receptor system for CNS neuronal migration.

CNS neuronal migration is a specialized form of cell motility that sets forth the laminar structure of cortical regions of brain. To define the neuronal receptor systems in glial-guided neuronal migration, an in vitro assay was developed for mouse cerebellar granule neurons, which provides simultaneous tracking of hundreds of migrating neurons. Three general classes of receptor systems were analyzed, the neuron-glial adhesion ligand astrotactin, the neural cell adhesion molecules of the IgG superfamily, N-CAM, L1 and TAG-1, and the beta 1 subunit of the integrin family. In the absence of immune activities, migrating cerebellar granule neurons had an average in vitro migration rate of 12 microns h-1, with individual neurons exhibiting migration rates over a range between 0 to 70 microns h-1. The addition of anti-astrotactin antibodies (or Fabs) significantly reduced the mean rate of neuronal migration by sixty-one percent, resulting in eighty percent of the neurons having migration rates below 8 microns h-1. By contrast, blocking antibodies (or Fabs) against L1, N-CAM, TAG-1 or beta 1 integrin, individually or in combination, did not reduce the rate of neuronal migration. By video-enhanced contrast differential interference contrast microscopy the effects of anti-astrotactin antibodies were seen to be rapid. Within fifteen minutes of antibody application, streaming of cytoplasmic organelles into the leading process arrested, the nucleus shifted from a caudal to a central position, and the extension of filopodia and lamellopodia along the leading process ceased. Correlated video and electron microscopy suggested that the mechanism of arrest by antiastrotactin antibodies involved the failure to form new adhesion sites along the leading process and the disorganization of cytoskeletal components. These results suggest astrotactin acts as a neuronal receptor for granule neuron migration along astroglial fibers.

Animals↗

PubMed1990

Neuronal lineages in chimeric mouse forebrain are segregated between compartments and in the rostrocaudal and radial planes.

On the basis of neuronal phenotypes and the mode of development of the mammalian forebrain, the cerebral cortex can be subdivided into deep versus superficial layers, and the striatum into patch versus matrix compartments. Interspecific chimeric Mus musculus----Mus caroli mice were used to determine the contribution of lineage to cellular position within these forebrain compartments. Statistical analysis revealed evidence of both spatial and compartmental lineage segregation. A significant difference in genotype ratio depending on chimeric specimen was observed between areas (regardless of compartment) that were separated by greater than 300 microns in the rostrocaudal plane. Differences were observed between early-born (striatal patch and deep cortex) versus late-born (striatal matrix and superficial cortex) neurons, but not between neurons of cortex as a whole versus neurons of striatum as a whole. The difference between early- and late-born neurons was primarily due to the difference between deep and superficial cortical neurons. On a finer scale of analysis, differences in genotype ratios were seen between radially aligned deep versus superficial cortical compartments, in both the neuronal and glial populations. This evidence is consistent with an early positional and compartmental segregation of forebrain progenitor cells.

Analysis of Variance↗

PubMed1989

Pattern formation in the striatum: developmental changes in the distribution of striatonigral projections.

The mammalian striatum (the major subcortical structure in the telencephalon) can be divided into two compartments, the patch and the matrix, on the basis of various neurochemical and hodological markers expressed in the adult. The primary efferent target of striatal neurons is the substantia nigra. We have previously shown that the patch compartment sends projections to the substantia nigra embryonically; whereas the matrix does not form a similar projection until the early postnatal period (Fishell and van der Kooy, J. Neurosci., 7 (1987) 1969-1978). The projection of patch neurons to the substantia nigra is the earliest developmental marker for the patch compartment. Here we ask about the early distribution of patch projections and their possible relation to striatal compartmentalization. Embryonic anterograde axonal tracing of the striatonigral pathway can take advantage of the temporal separation of patch versus matrix projections to reveal the terminal distribution of patch striatonigral neurons independent of the nigral terminal distribution from the striatal matrix. The anterograde tracer rhodamine isothiocyanate was shown in a model system to persist in labeled neurons for more than a week, but to be available for uptake into these neurons for a few days after injection at the most. These properties of rhodamine isothiocyanate were combined experimentally with short and long term survival periods. This allowed assessment of the changing developmental distribution of nigral fibers from specifically the striatal patch compartment. In all experimental cases the anterogradely labeled sections of the substantia nigra were also stained with antibodies to tyrosine hydroxylase, which permitted differentiation of the dopamine cell rich pars compacta from the dopamine cell poor pars reticulata. The results show that in the adult the majority of patch and matrix striatonigral projections are confined to the substantia nigra pars reticulata. Furthermore, their fiber distribution within the pars reticulata is overlapping rather than complementary. Most interestingly, in the late embryonic period (most noticeably at embryonic day 19) there is a marked overlap between patch striatonigral fibers and nigral dopamine perikarya. By early postnatal times, when the matrix compartment begins to form its striatonigral projection, the overlap of patch striatonigral fibers and dopamine cells has largely disappeared. The results suggest that a transient interaction between patch striatonigral fibers and dopamine neurons (which is concomitant with the formation of striatal compartments), may be an important developmental event in the phenotypic maturation of striatal pa

Animals↗

PubMed1987

Neuronal birthdate underlies the development of striatal compartments.

The striatum of the mammalian forebrain is composed of two complementary functional compartments, the patches and the matrix. By injecting [3H]thymidine at different embryonic times and sacrificing the rats as young adults, we found that the earliest neurons to leave the mitotic cycle were restricted to the patch compartment. Neurons that became postmitotic at later times preferentially joined the matrix compartment. Distinctive periods of cell proliferation may underlie pattern formation throughout the developing forebrain.

Animals↗

PubMed1987

The development of laterality in the forebrain projections of midline thalamic cell groups in the rat.

Bilateral forebrain (caudoputamen, nucleus accumbens and frontal cortical areas) injections of two different fluorescent retrograde tracers demonstrated that labeled cells situated in the midline nuclei of the thalamus and midbrain each project only unilaterally to the forebrain, regardless of the laterality of their perikarya. Thus, these intermingling midline perikarya send their axons primarily ipsilaterally and to a lesser degree contralaterally, but never bilaterally to the forebrain. At embryonic day 19, these midline nuclei exist as two bilaterally situated, independent structures, each projecting only ipsilaterally to the forebrain. By postnatal day 2, these perikarya fuse into a single mass on the midline. Upon fusion, many of the perikarya of the two developing subnuclei cross the midline, intermingle with each other, and thus some neurons come to have contralateral forebrain projections. These observations suggest that neurons are able to maintain their axonal projections while migrating short distances.

Amidines↗

PubMed1987

Pattern formation in the striatum: developmental changes in the distribution of striatonigral neurons.

The striatum of the mammalian forebrain can be divided into 2 compartments, the patches and the matrix. We have investigated embryonic events involved in the formation of these compartments in rats. Early in development, dopamine fibers from the substantia nigra selectively innervate the patches. In the perinatal striatum, we observed a close match between the distributions of striatal cell bodies with axonal projections to the substantia nigra and patches of afferent dopamine fibers. Striatal cells projecting to the nigra are first seen in the ventrolateral striatum at embryonic day (E) 17. Striatonigral cell bodies are distributed homogeneously through the striatum from E18 to 19. At E20 and until postnatal day 4, these cell bodies are organized into discrete patches. After this time, striatonigral cell bodies assume the dense and homogeneous distribution characteristic of the adult striatum. A retrograde tracer injection in the nigra at E18 (during the early period of homogeneous striatonigral distribution) produces a patchy striatonigral distribution if the embryo is not sacrificed until E21. The number of retrogradely labeled striatonigral cell bodies in a midstriatal section, at times immediately before and after the early homogeneous to patchy changeover did not differ significantly. We suggest that the neurons of the patch compartment of the striatum are born first and project to the substantia nigra first. The patch neurons only become restricted to "patchy" areas as the later-born matrix neurons migrate out into the striatum.

Animals↗