Fig. 2. Mash1 and Ngn1 are both required for the production of olfactory sensory neurons but only Mash1 is required for the generation of basal OE progenitors. (A-C,A'-C',A'',C'') In situ hybridisation with a probe for the pan-neuronal marker SCG10 on sections of wild-type (A,C), Mash1 mutant (A',B,C') and Ngn1 mutant (A'',B',C'') OE at E12.5 (A-A'',B,B') and on whole-mount olfactory placodes at E10.0 (C-C''). There is a drastic reduction in the number of neurons differentiating in the OE of both Mash1 and Ngn1 mutant embryos at E12.5, apparent in the medial part of the OE (A',A'') and more pronounced in the rostral part of the Mash1 mutant OE (B) and in the caudal part of the Ngn1 mutant OE (B'). Mash1 is not required for the generation of neurons at placodal stage (C'), while these early born neurons are missing in a Ngn1 mutant placode (C''). (D-D'') Immunocytochemistry for BrdU after a short period of incorporation followed by Haematoxylin staining reveals progenitors cells in S phase of the cell cycle (brown nuclei) and in mitosis (mitotic figures indicated by arrowheads) in wild-type (D), Mash1 mutant (D') and Ngn1 mutant (D'') OE at E12.5. Dividing progenitors are present in apical (top) and basal (bottom) positions in wild-type OE (D). Basal progenitors are missing in the Mash1 mutant OE (D'). Because post-mitotic neurons are also absent, apical progenitors are now present in the whole thickness of the OE. In contrast, basal progenitors are present in the Ngn1 mutant OE (D''). Scale bars, 25 µm (A-A'',B',B''); 50 µm (C-C''); 10 µm (D-D'').