Lumbar spinal cord motor axon projections to the ventral flank. (a-d) E13.5 normal and BmprIaflox/-embryos carrying an Hb9-GFP transgene imaged via indirect fluorescence. The embryos were eviscerated and the ventral abdominal wall was reflected to allow examination from a peritoneal aspect. Images of rostral (a,c) and caudal (b,d) regions are of different embryos, and in the caudal images the more rostral (for example, genitofemoral) nerves were removed during the dissection. In control embryos three major motor nerves (white arrows) project abdominally via the ventral flank from rostral segments of the lumbar spinal cord (a), while none project abdominally from more caudal segments (b). The limb nerves (f, femoral nerve; o, obturator nerve; S, sciatic plexus) turn along dorsal or ventral limb trajectories into the plane of the image. In mutant embryos, additional flank-projecting nerves (yellow arrows) are present at both rostral (c) and caudal (d) levels. (e) Quantification of nerve trajectories at E11.5–11.75. Transverse sections of normal and BmprIaflox/-embryos carrying an Hb9-GFP transgene were scored for the presence of a GFP+ dorsal limb, ventral limb or ventral flank nerve branch. The combined continuous anteroposterior extent of sections containing nerves following each trajectory is presented. gf, genitofemoral nerve; ih, iliohypogastric nerve; il, ilioinguinal nerve. M, medial; L, lateral; A, anterior; P, posterior.