Transcriptional code of mechanoreceptors in gastropods and vertebrates. (a-d) Whole-mounts of the pleural (a,b) or cerebral ganglion (c,d) (ventral view) of Aplysia californica (Ac) hybridized with the indicated probes. In the cerebral ganglia, Brn3 is expressed in the Sensorin+ J and K sensory clusters (white arrowheads)  but not in the scattered Sensorin+ cells. Projections from the sensory clusters contain the Sensorin mRNA and are detected by in situ hybridization . (e-l) Consecutive sagittal sections of the pleural ganglion of Lymnaea stagnalis (Ls) showing coexpression of the indicated genes. (m-p) Consecutive sagittal sections of a dorsal root ganglion in Mus musculus (Mm) showing coexpression of the indicated genes. In Lymnaea pleural ganglia, Islet and VGluT are also expressed in large Brn3— neurons at the ventral pole. (q-s): schematic of the nervous system of Aplysia californica (q), Lymnaea stagnalis (r) and Mus musculus (s). A, anterior; Ab, abdominal ganglion; C, cerebral ganglion; DRG, dorsal root ganglion; HB, hindbrain; L, left; P, pleural ganglion; Pe, pedal ganglion; PSC, pleural sensory cluster; RP, right parietal ganglion; SC, spinal cord; T, trigeminal ganglion; V, visceral ganglion. Scale bars, 100 μm (a,b); 200 μm (c,d); 100 μm (e,g,i,k); 100 μm (f,h,j,l); 200 μm (m-p).