Selective expression of proprioceptor markers in the DRG and trigeminal system. The sensory ganglia of wild-type embryos were examined at E13.5 and E16.5 for the expression of transcription factors Brn3a, Etv1, Runx3 and Islet2. (a-d) At E13.5, Etv1 expression is restricted to the DRG, while Runx3 is expressed in both the DRG and TG. (e-h) At E16.5, Etv1 is expressed in both the DRG and the TG, but in the TG Etv1 positive neurons with large nuclei consistent with 1a proprioceptors are rare (large arrow). Instead, the majority of Etv1-positive neurons in the TG have nuclei of intermediate size and co-express Islet2; similar cells are also found in the DRG (small arrows). The 1a proprioceptors of the DRG co-express Brn3a, but at relatively low levels. (i) Etv1 expression in the mesV of an E18.5 embryo expressing a tauLacZ transgene integrated into the Brn3a locus , which is thus heterozygous for Brn3a, but phenotypically normal. Numerous neurons that co-express Etv1 and the Brn3a-LacZ marker are noted. The caudal location and large size of these neurons are consistent with proprioceptors innervating the muscles of mastication. Cb, cerebellum; chp, choroid plexus; fr, fasciculus retroflexus; IP, interpeduncular nucleus; mes5, mesencephalic trigeminal. Scale 50 μm (a-h), 400 μm (i), 50 μm (i, inset).