Cell partitioning and mixing in the formation of the CNS: analysis of the cortical somatosensory barrels in chimeric mice.

Abstract

Development of structural units along clonal lines as demonstrated in some invertebrates is an elegantly simple way to proceed in the formation of tissues. The possibility of a related event occurring in mammalian neurogenesis, i.e. that discrete assemblies of cortical neurons may be presorted according to lineage during neuronal development, was examined in chimeric mice. These mice consist of cells of two genotypes. Genetically determined differences in beta-glucuronidase activity, detected by histochemical means, were used as the cell-marker to determine the genotypic origin of neurons. Individual 'barrels' of the mouse somatosensory cortex were examined, and each neuron classified as high (Gusb) or low (Gush) as to the expression of beta-glucuronidase activity. Neurons from the two genotypes were found to be distributed in the same proportion amongst all the barrels examined from a given animal. The distribution of like-genotype neurons in the barrels was similar to neighboring cortical areas. Furthermore, there was no evidence for unusual distributions of neurons from one genotype within a single barrel. Such evidence does not support the notion of a unique founder population of neurons for individual barrels (or groups of barrels), and supports the notion of extensive cell mixing and epi-genetic events in the determination of individual barrels in the mouse somatosensory cortex.