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Neuron production takes place continuously in the rostral migratory stream (RMS) of the adult mammalian brain. The molecular mechanisms that regulate progenitor cell division and differentiation in the RMS remain largely unknown. Here, we surveyed the mouse genome in an unbiased manner to identify candidate gene loci that regulate proliferation in the adult RMS. We quantified neurogenesis in adult C57BL/6J and A/J mice, and 27 recombinant inbred lines derived from those parental strains. We showed that the A/J RMS had greater numbers of bromodeoxyuridine-labeled cells than that of C57BL/6J mice with similar cell cycle parameters, indicating that the differences in the number of bromodeoxyuridine-positive cells reflected the number of proliferating cells between the strains. AXB and BXA recombinant inbred strains demonstrated even greater variation in the numbers of proliferating cells. Genome-wide mapping of this trait revealed that chromosome 11 harbors a significant quantitative trait locus at 116.75 +/- 0.75 Mb that affects cell proliferation in the adult RMS. The genomic regions that influence RMS proliferation did not overlap with genomic regions regulating proliferation in the adult subgranular zone of the hippocampal dentate gyrus. On the contrary, a different, suggestive locus that modulates cell proliferation in the subgranular zone was mapped to chromosome 3 at 102 +/- 7 Mb. A subset of genes in the chromosome 11 quantitative trait locus region is associated with neurogenesis and cell proliferation. Our findings provide new insights into the genetic control of neural proliferation and an excellent starting point to identify genes critical to this process.

The EphA5 receptor tyrosine kinase plays key roles in axon guidance during development. However, the presence of EphA5 protein in the nervous system has not been fully characterized. To examine EphA5 localization better, mutant mice, in which the EphA5 cytoplasmic domain was replaced with beta-galactosidase, were analyzed for both temporal and regional changes in the distribution of EphA5 protein in the developing and adult nervous system. During embryonic development, high levels of EphA5 protein were found in the retina, olfactory bulb, cerebral neocortex, hippocampus, pretectum, tectum, cranial nerve nuclei, and spinal cord. Variations in intensity were observed as development proceeded. Staining of pretectal nuclei, tectal nuclei, and other areas of the mesencephalon became more diffuse after maturity, whereas the cerebral neocortex gained more robust intensity. In the adult, receptor protein continued to be detected in many areas including the olfactory nuclei, neocortex, piriform cortex, induseum griseum, hippocampus, thalamus, amygdala, hypothalamus, and septum. In addition, EphA5 protein was found in the claustrum, stria terminalis, barrel cortex, and striatal patches, and along discrete axon tracts within the corpus callosum of the adult. We conclude that EphA5 function is not limited to the developing mouse brain and may play a role in synaptic plasticity in the adult.

Cells within the subventricular zone (SVZ) express basic Fgf (Fgf2) and Fgf receptor proteins. We show that the absence of Fgf2 gene products reduces by 50% the dividing progenitor population of the anterior SVZ (SVZa) without changing their cell cycle time. Every 2-3 cell cycles of the SVZa progenitor cell population, 30,000 newly generated neurons capable of long-term survival are added to the glomerular layer of the olfactory bulb. Fgf2 knockout mice have smaller olfactory bulbs due to decreased output of these newly generated cells into the bulbs. A population of slow-dividing neural stem cells (NSCs) residing in the SVZa is identified by its slow cell cycle kinetics (cell cycle approx. 20 days); these cells, called 'S' cells, are negative for glial fibrillary acidic protein and occasionally express brain-lipid-binding protein, a molecular marker of radial glia. The number of these dividing NSCs is reduced from about 13,000 in wild-type to 8,500 cells in Fgf2 knockout mice. Thus, FGF2 regulates the number of proliferative cells and olfactory bulb neurogenesis by maintaining a slow-dividing stem cell pool within the SVZa., (Copyright 2004 S. Karger AG, Basel.)

Neurons destined for each region of the neocortex are known to arise approximately in an "inside-to-outside" sequence from a pseudostratified ventricular epithelium (PVE). This sequence is initiated rostrolaterally and propagates caudomedially. Moreover, independently of location in the PVE, the neuronogenetic sequence in mouse is divisible into 11 cell cycles that occur over a 6 d period. Here we use a novel "birth hour" method that identifies small cohorts of neurons born during a single 2 hr period, i.e., 10-20% of a single cell cycle, which corresponds to approximately 1.5% of the 6 d neuronogenetic period. This method shows that neurons arising with the same cycle of the 11 cycle sequence in mouse have common laminar fates even if they arise from widely separated positions on the PVE (neurons of fields 1 and 40) and therefore arise at different embryonic times. Even at this high level of temporal resolution, simultaneously arising cells occupy more than one cortical layer, and there is substantial overlap in the distributions of cells arising with successive cycles. We demonstrate additionally that the laminar representation of cells arising with a given cycle is little if at all modified over the early postnatal interval of histogenetic cell death. We infer from these findings that cell cycle is a neuronogenetic counting mechanism and that this counting mechanism is integral to subsequent processes that determine cortical laminar fate.

The basic principles of the development of the central nervous system (CNS) are reviewed, and their implications for both normal and abnormal development of the brain are discussed. The goals of this review are (a) to provide a set of concepts to aid in understanding the variety of complex processes that occur during CNS development, (b) to illustrate how these concepts contribute to our knowledge of the normal anatomy of the adult brain, and (c) to provide a basis for understanding how modifications of normal developmental processes by traumatic injury, by environmental or experiential influences, or by genetic variations may lead to modifications in the resultant structure and function of the adult CNS.

Neuronogenesis during posthatching development of the procerebrum of the terrestrial snail Helix lucorum was analyzed using bromodeoxyuridine immunohistochemistry to label proliferating cells. Comparison of the distribution of labeled cells in a series of animals which differed in age at the time of incubation with bromodeoxyuridine, in survival time after incubation, and in age at sacrifice reveals a clear pattern and developmental sequence in neuron origin. First, the proliferating cells are located only at the apical portion of the procerebrum. Second, cells which are produced at any particular age remain, for the most part, confined to a single layer in the procerebrum. Third, as development proceeds, each layer of previously produced neurons is displaced toward the basal part of the procerebrum by the production of additional neurons. Our results suggest that the vast majority of the neurons (probably about 70-80%) of the snail procerebrum are produced during the first 1-2 months of posthatching development.

Neurons of neocortical layers II-VI in the dorsomedial cortex of the mouse arise in the pseudostratified ventricular epithelium (PVE) through 11 cell cycles over the six embryonic days 11-17 (E11-E17). The present experiments measure the proportion of daughter cells that leave the cycle (quiescent or Q fraction or Q) during a single cell cycle and the complementary proportion that continues to proliferate (proliferative or P fraction or P; P = 1 - Q). Q and P for the PVE become 0.5 in the course of the eighth cycle, occurring on E14, and Q rises to approximately 0.8 (and P falls to approximately 0.2) in the course of the 10th cycle occurring on E16. This indicates that early in neuronogenesis, neurons are produced relatively slowly and the PVE expands rapidly but that the reverse happens in the final phase of neuronogenesis. The present analysis completes a cycle of analyses that have determined the four fundamental parameters of cell proliferation: growth fraction, lengths of cell cycle, and phases Q and P. These parameters are the basis of a coherent neuronogenetic model that characterizes patterns of growth of the PVE and mathematically relates the size of the initial proliferative population to the neuronal population of the adult neocortex.

Neocortical neuronogenesis occurs in the pseudostratified ventricular epithelium (PVE) where nuclei of proliferative cells undergo interkinetic nuclear movement. A fraction of daughter cells exits the cell cycle as neurons (the quiescent, or Q, fraction), whereas a complementary fraction remains in the cell cycle (the proliferative, or P, fraction). By means of sequential thymidine and bromodeoxyuridine injections in mouse on embryonic day 14, we have monitored the proliferative and post-mitotic migratory behaviors of 1 and 2 hr cohorts of PVE cells defined by the injection protocols. Soon after mitosis, the Q fraction partitions into a rapidly exiting (up to 50 microns/hr) subpopulation (Qr) and a more slowly exiting (6 microns/hr) subpopulation (Qs). Qr and Qs are separated as two distributions on exit from the ventricular zone with an interpeak distance of approximately 40 microns. Cells in Qr and Qs migrate through the intermediate zone with no significant change in the interpeak distance, suggesting that they migrate at approximately the same velocities. The rate of migration increases with ascent through the intermediate zone (average 2-6.4 microns/hr) slowing only transiently on entry into the developing cortex. Within the cortex, Qr and Qs merge to form a single distribution most concentrated over layer V.

The present report is an analysis of the proliferative behavior of the secondary proliferative population (SPP) of the dorsomedial region of the embryonic mouse cerebral wall. It is based upon experiments undertaken on embryonic days 14-16 (E14-E16) and exploits methods in which proliferative cells are labeled in S phase with either or both bromodeoxyuridine and tritiated thymidine. The SPP, which arises from the PVE by E13, is principally the progenitor population to the neuroglial population of the mature neocortex and subjacent cerebral wall. By the end of E14 the SPP comes to be distributed diffusely from the outer margin of the ventricular zone throughout subventricular zone and intermediate zone. The length of the cell cycle of the SPP is constant at approximately 15 hr throughout this interval; thus, this population undergoes 1.6 cell cycles/24 hr or 3.2 cycles in the course of the 48 hr period, E14-E16. Over this 48 hr period, the SPP increases from 11% to 35% of the total proliferative population of the dorsomedial cerebral wall. The absolute size of the SPP increases nearly sixfold. With these values taken together it may be estimated that approximately 87% of postmitotic cells of the SPP reenter S phase after each cell division in this interval which means that only approximately 13% of the proliferative population exits the cycle. These findings illustrate the massive expansion of the SPP antecedent to the explosive diffusion of glial cells through the neocortex and subjacent cerebral wall as neuronal migration comes to completion and neocortical growth and differentiation accelerate.

There are two proliferative populations in the developing cerebral wall: the pseudostratified ventricular epithelium (PVE) and the secondary proliferative population (SPP). The present experiments provide on embryonic day 14 (E14) in the mouse direct measures of the values for the proportions of daughter cells that continue to proliferate (proliferative, P fraction, or P) and for those that leave the cell cycle (quiescent, Q fraction, or Q; Q = 1 - P) for both of these populations. The range of values of P for the PVE, 0.62-0.66, would provide for the relatively low rate of neuronal output and for an expansion of the proliferative population appropriate for E14. The even higher values of P for the SPP, 0.73-1.0, would expand this population rapidly in preparation for the high glial cell output to occur later in cerebral histogenesis.

The use of an exogenously administered thymidine analog, 5-bromo-2'-deoxyuridine (BrdU), for studies of the proliferation, migration and time of origin of cells in the cerebral cortex was investigated and compared with [3H]thymidine [( 3H]dT) autoradiography. Pregnant rats or mice were injected with BrdU and/or [3H]dT and processed by standard immunohistochemical techniques using a primary antibody directed against BrdU in single-stranded DNA, autoradiographic methods, or both. In animals that survived only 1 h after the injection, BrdU-positive cells were distributed in the proliferative zones throughout the central nervous system (CNS). In animals killed 1-3 days after the BrdU injection, intensely immunoreactive cells were in the superficial cortical plate and less intensely labeled cells were scattered throughout the deep cortical plate, the intermediate zone, and the germinal zones. In adult animals, 60 days or more after an injection of BrdU on GD 19, BrdU-positive cells were located in layer II/III of neocortex, the hippocampal pyramidal layer, and the granule layer of the dentate gyrus. In the double-labeling studies, the distribution of BrdU-immunoreactive cells was identical to that of autoradiographically labeled cells, and all autoradiographically labeled neurons were BrdU positive. Thus, BrdU immunohistochemistry is suitable for developmental studies of the CNS; moreover, it provides several advantages over [3H]dT autoradiography.

Some basic principles of the development of the central nervous system (CNS) are reviewed and their implications for both normal and abnormal behavioral development are discussed. The goals of this review are: to provide a set of concepts to aid in the understanding of the variety of complex processes occurring during CNS development, to illustrate how these concepts contribute to our knowledge of the normal anatomy of the adult brain, and to delineate how modifications of normal developmental processes by traumatic injury, by environmental or experiential influences, or by genetic variations may lead to modifications in the resultant structure and function of the adult CNS.