DEVELOPMENTAL BIOLOGY 146,62-U (1991) Alteration of Pectoral Fin Nerves following Ablation of Fin Buds and by Ectopic Fin Buds in the Japanese Medaka Fish HITOSHI OKAMOTO*~~ AND JOHN Y. KUWADA*+$ *Department of Biology, tNeuroscience Program, und *Institute of Gerontology, University of Michigan, Ann A&n-, Michigan @IO9 Accepted March .SV 1991 The role of the pectoral fin bud for outgrowth by fin axons was assessed by ablation of pectoral fin buds and by transplantation of fin buds to ectopic sites in the embryos of the Japanese medaka fish (&&as latipes). Normally nerves from segments 1-4 (Sl-4) and less frequently the S5 nerve converged at the base of the fin bud by extending toward the fin bud on the ventral surface of the axial muscles (H. Okamoto and J. Y. Kuwada, 1991, Dev. BioL 146). Following ablation of the fin bud before motor growth cones have begun to extend laterally, nerves in Sl-5 followed a trajectory down the middle of each segment parallel to the borders of the metamerically arranged axial muscles rather than converging. This trajectory was similar to that of more posterior segmental nerves which do not converge toward the fin bud. When fin buds were transplanted to more posterior segments, nerves from Sl-5 often changed their trajectories and extended to the base of ectopic buds. Furthermore, motor nerves from segments posterior to S5, which normally do not innervate the fin bud, also extended to the ectopic fin bud. When faced with both the host and ectopic fin bud, motor nerves extended to either fin bud or branched and extended to both fin buds. These results demonstrate that the early fin bud is necessary for correct outgrowth of fin nerves and suggest that the fin bud normally attracts fin nerves to its base. One possible mechanism for the attraction of motor growth cones by the fin bud is a long distance cue emitted by the fin bud. a 1991 Academic PRSS, IIIC. INTRODUCTION bud ablations usually some shoulder muscles developed. This indicates that the ablations were often incomplete and that some brachial plexus targets were left intact. Second, it is possible that segmental nerves take a de- fault posterior pathway, which was unmasked by limb bud ablations, in the absence of the attraction normally provided by the limb bud. The role of the limb bud for outgrowth to and forma- tion of a plexus at the base of the limb bud by motor axons has been most extensively examined by a combina- tion of ablation and transplantation studies in amphib- ians and chicks. Transplantation of limb buds to ectopic sites suggested that axons are attracted to limb buds (Detwiler, 1934, 1936; Hamburger, 1939; Platt, 1956; Hollyday and Hamburger, 1976) and that a long dis- tance cue may be involved in guidance of axons to the limb. Ablation of limb buds, on the other hand, has given results which appear to be either contrary to an attractive influence of limb buds on axons or are incon- clusive. For example, in chicks a plexus forms following ablation of the limb bud (Shorey, 1909; Hamburger, 1934,1958; Oppenheim et al, 1978), but axons within the plexus run uniformly in a posterior direction, including those which normally run anterior (Tosney and Land- messer, 1984). These results are not inconsistent with the finding that limb buds attract segmental nerves. First, the brachial plexus contains the axons which in- nervate structures at the base of the wing, e.g., shoulder muscles, as well as in the wing proper. Following wing ’ Present address: National Institute for Basic Biology, 38 Nishigon- aka, Myodaiji-cho, Okazaki, Aichi 444, Japan. The pectoral fin and fin motor neurons of the Japa- nese medaka fish make up a simple system which pos- sesses features that make it attractive for uncovering mechanisms of growth cone guidance (Okamoto and Kuwada, 1991). The pattern of outgrowth by motor axons in wildtype and finless mutant (~2) embryos sug- gested the hypothesis that fin axons converge and form a plexus at the base of the fin bud because they are attracted by the fin bud. Given the uncertainty asso- ciated with past limb bud ablations, we have reexam- ined this question in two ways. First, we analyzed motor outgrowth following ablation of the pectoral fin bud. The Japanese medaka embryo is well suited for fin bud ablations since the location and orientation of the fin bud with respect to the body trunk make it possible to completely remove it without damaging the substrates motor axons extend upon to get to the base of the fin bud. Second, we analyzed motor outgrowth following transplantation of fin buds to ectopic sites. The pattern of outgrowth by segmental nerves in both cases support 0012.1606191 $3.00 62 Copyright $, 1991 by Academic Press, Inc. All rights of reproduction in any form reserved.