Fig. 8. Asymmetry of Mash1 function in the development of dILA neurons. Schematic diagram of progenitor cell divisions that generate a dILA daughter, and a model of the Mash1 function in their development. (A) dILA neurons are generated from progenitor cells by asymmetric cell divisions in control mice (left). These divisions are either non-terminal (top) and generate one dILA neuron and one progenitor, or are terminal (bottom) and generate one dILA and one dILB neuron. In Mash1^–/– mice, aberrant progenitor cells (P*) are formed at the expense of dILA neurons (right). Supernumerary dorsal progenitors of Mash1^–/– mice can incorporate BrdU and do thus replicate, but many are subsequently eliminated by apoptosis. (B) A model of Mash1 function in the development of dILA neurons that arise by asymmetric terminal divisions. Mash1 is expressed in the progenitor cell that gives rise to a dILA and a dILB neuron. In the dILA daughter, Mash1 exerts essential functions for neurogenesis and lineage specification. Mash1 allows a dILA progenitor (P1) to differentiate (P1N), and to express Ptf1a. By contrast, Mash1 is dispensable for the development of the dILB lineage.