Control of motor axon guidance in the zebrafish embryo

The zebrafish neuromuscular system has been an exemplary model for studying motor axon guidance since its detailed characterization almost two decades ago. In particular, characterization and detailed analysis has focused on the development and axogenesis of early developing primary motoneurons, Dur ing the first day of development, neuromuscular connections are limited to three primary motoneurons per spinal cord hemisegment innervating three dis creet myotome territories. Observations of dye labeled primary motor axons in living embryos revealed that axogenesis is highly stereotyped with each primary motor axon extending along specific pathways and displaying particu lar characteristics. Exploiting the unique attributes of zebrafish, notably the ability to analyze motoneurons in living embryos and the capability to induce mutations, has allowed a comprehensive cellular, molecular and gene tic approach to discerning the mechanisms that control the formation of neu romuscular connectivity, Knowledge gained from this body of work not only r elates to zebrafish, but to vertebrate axon guidance in general. (C) 2001 E lsevier Science Inc.