dILA neurons in the dorsal spinal cord are the product of terminal and non-terminal asymmetric progenitor cell divisions, and require Mash1 for their development.

dILA and dILB neurons comprise the major neuronal subtypes generated in the dorsal spinal cord, and arise in a salt-and-pepper pattern from a broad progenitor domain that expresses the bHLH factor Mash1. In this domain, Mash1-positive and Mash1-negative cells intermingle. Using a Mash1(GFP) allele in mice, we show here that Mash1+ progenitors give rise to dILA and dILB neurons. Using retroviral tracing in the chick, we demonstrate that a single progenitor can give rise to a dILA and a dILB neuron, and that dILA neurons are the product of asymmetric progenitor cell divisions. In Mash1-null mutant mice, the development of dILA, but not of dILB neurons is impaired. We provide evidence that a dual function of Mash1 in neuronal differentiation and specification accounts for the observed changes in the mutant mice. Our data allow us to assign to Mash1 a function in asymmetric cell divisions, and indicate that the factor coordinates cell cycle exit and specification in the one daughter that gives rise to a dILA neuron.