Outgrowth by fin motor axons in wildtype and a finless mutant of the Japanese medaka fish.

The outgrowth of motor axons to the developing pectoral fin of the Japanese medaka fish (Oryzias latipes) was investigated both in wildtype embryos and in the pectoral finless (pl) mutants in which adults are missing pectoral fins. Late in embryogenesis the pectoral fin is a simple limb which contains two antagonist muscles which are innervated by presumptive motor neurons from the first four spinal segments (S1-4). The pectoral fin develops from a fin bud located in S1 and S2 centered on the border between S1 and S2 and, as with other limbs, one of the earliest signs of differentiation is the apical ectodermal ridge (AER). By the time the AER is well formed the growth cones of the presumptive motor neurons have reached the base of the fin bud and formed a plexus by extending toward the fin bud upon emergence from the spinal cord. This is especially evident on the ventral surface of the metamerically arranged axial muscles. For example, growth cones from S2 extend in a diagonal direction (both anterior and lateral) towards the fin bud. One hypothesis which can account for the pattern of motor outgrowth is that growth cones are attracted to the base of the fin bud, perhaps via a long distance cue. This hypothesis was tested by examining outgrowth of segmental nerves in pl embryos in which the fin buds arrest early in development following the initial appearance of the AER. In pl, nerves from S1-4 converged to form a plexus at the base of the abnormal fin bud, but the pattern of outgrowth varied from wildtype in a way consistent with a diminished capacity of the fin bud to attract segmental nerves to it.