Ret is required for the maintenance, but not initiation, of Er81 expression in RA mechanoreceptors. A, The number of S100+ Pacinian corpuscles per limb was quantified in serial sections of P14–P21 wild-type (38.50 ± 2.36 Pacinian corpuscles per limb), RetCFP/+ (31.50 ± 2.23 Pacinian corpuscles per limb), Er81+/− (35.17 ± 2.40 Pacinian corpuscles per limb), and RetCFP/+;Er81+/− (22.33 ± 0.71 Pacinian corpuscles per limb) hindlimbs. The number of Pacinian corpuscles in RetCFP/+;Er81+/− double heterozygotes is significantly less than in all other genotypes. B–D, Double-fluorescent in situ hybridization against CFP (red) driven from the Ret locus and Er81 (green) in L4/L5 DRGs of RetCFP/+ and RetCFP/CFP embryos at E13.5. The percentage of CFP+ neurons expressing Er81 is similar between controls and mutants (40.55 ± 3.74% in RetCFP/+ DRGs, 44.19 ± 2.65% in RetCFP/CFP DRGs; p = 0.47). E–G, At E16.5, CFP/Er81 double-fluorescent in situ hybridization shows a decrease in the proportion of RA mechanoreceptors that express Er81in Ret mutants relative to controls (51.40 ± 0.50% in RetCFP/+ DRGs, 32.12 ± 6.03% in RetCFP/CFP DRGs; p = 0.03). H–J, CFP/Er81 double-fluorescent in situ hybridization of P0 Ret mutants and controls in an apoptosis-deficient Bax-null background shows a significant decrease in the percentage of RA mechanoreceptors that express Er81 in Ret mutants (58.69 ± 0.26% in RetCFP/+;Bax−/− DRGs, 38.24 ± 3.40% in RetCFP/CFP; Bax−/− DRGs; p = 0.004). N = 6 limbs (A), 3 animals (B–G), or 2 animals (H–J) per genotype. Scale bar, 50 μm. *p < 0.05. **p < 0.01. ***p < 0.001. Not significant, p > 0.05. Error bars indicate SEM.