Cell lineage relationships in the development of the mammalian CNS

Genetically mosaic animals are valuable tools in the study of cell lineage relationships during development. The work described here is a quantitative analysis of large motor neurons of the facial nerve nucleus of the mouse. Aggregation chimeric mice were made from embryos which differed at the structural locus for the enzyme beta-glucuronidase. This difference is an intrinsic property of each cell and is easily detected by histochemical means. The numbers of cells in each of 10 facial nuclei from six glucuronidase chimeras were determined and each cell was scored for its embryo of origin using the glucuronidase marker. Unlike cerebellar Purkinje cells, this analysis revealed no evidence of numerical quanta of invariant size in the facial nucleus. Determination of the fraction of the total cells comprised by the descendants of each embryo, however, revealed evidence of quanta. These values suggest that the neurons of each facial nucleus descend from 12 progenitors. In agreement with similar calculations performed on Purkinje cells, the facial nucleus progenitors appear to be selected as the sole source of the facial nucleus neurons some time during the neural plate to neural groove stage of development. The absence of rigidly specified numerical quanta in the nucleus suggests that factors extrinsic to these cells contribute to the final adult number of neurons.