| The work presented here is focused on three major topics in cortical development. (1) Cell dispersion during the earliest stages of cortical development, the preplate stage, was characterized using injection of a retroviral library, allowing the identification of neurons derived from individual clones. (2) scrambler (scm), a gene required for the orderly sequence of development of the cortical plate in the mouse, was mapped, identified, and characterized. (3) The map position of the scm locus was refined using additional markers to anchor current genetic maps of the scm locus and mdab1 and also to determine the relative map positions of mdab1/scm and other genes in the region.; Previous studies in the ferret showed that clones labeled with retrovirus at E33-E35 consisted of single neurons and multineuron clones that were widely dispersed, spanning on average 40% of the rostral-caudal extent of the cortex. Here, we show that clones labeled at the earliest stages of ferret cortical neurogenesis, E27-E29, consisted of small clustered clones (65%), single neurons (13%) and widely dispersed multineuron clones (13%). In striking contrast, 6 of 21 hemispheres labeled at E27-E29 each additionally contained a large funnel-shaped cluster of labeled cells that appear to represent a separate mode of cell migration only seen after early labeling. These results suggest that there are two distinct mechanisms of clonal organization in the early cerebral cortex; some progenitors produce neurons with tight topographical organization, while others produce neurons that disperse widely across the cortex.; The next topic explored involves the identification and characterization of scrambler, a gene required for the orderly interactions between cortical plate and preplate cells. In this work, we have constructed genetic and physical maps of scm and identified yeast artificial chromosomes containing an exon of mdab1, a homolog of Drosophila disabled. We have also reported that mdab1 transcripts showed abnormal splicing in scm homozygotes, with 1.5 kb of intracisternal A particle retrotransposon sequence inserted into the mdab1 coding region in antisense orientation, producing a mutated and truncated predicted protein.; The third topic explored in this work is the refinement of the map of scm and the anchoring of the maps of the scm and mdab1. We have mapped microsatellite makers used previously in the BSS cross in which the position of mdab1 had been previously determined. This has allowed us to determine the map position of scm/mdab1 definitively and also to determine the order of microsatellite markers on mouse chromosome 4 in the area of the scm locus. |
