This shows a series of cross sections through 17h to 23h zebrafish embryos, stained with the F59 monoclonal antibody (red) and with a DNA binding dye to show all nuclei (blue).  F59 labels all of the slow muscle precursor cells as they migrate through the myotome and differentiate.  In the segmental plate (upper left), the slow muscle precursors are also known as adaxial cells:  they are cuboidal cells adjacent to the notochord.  Shortly after somite formation (upper right), the adaxial cells elongate parallel to the notochord and begin to move dorsally and ventrally along the medial surface of the myotome.  After reaching the full dorsal/ventral extent of the myotome, the adaxial cells begin to migrate radially away from the notochord (bottom left), until they become superficial muscle cells.  These data are adapted from Devoto, Melancon, et al. (1996a).   
   
 

  

Three different embryonic cell types can be identified in cross sections of 24h zebrafish embryos: Non-Muscle pioneer slow muscle cells (red cells), Muscle pioneer slow muscle cells (red and green cells), and fast muscle cells (white cells).  All of the slow muscle cells develop from cells adjacent to the notochord in the presomitic mesoderm (segmental plate), while all of the fast muscle cells develop from more lateral presomitic cells 
  
 
       
     

This shows our current working model for the specification of muscle fiber type identity in zebrafish (from Du, Devoto, et al. (1997)).  Slow muscle development is triggered by the exposure of paraxial mesodermal cells to hedgehog signaling from the notochord.  The further subdivision of slow muscle precursors into muscle pioneer slow muscle cells and non-muscle pioneer slow muscle cells is a result of competing influences of further hedgehog signaling from the notochord and/or floor plate and members of the TGF-B family of the signaling proteins present in the dorsal and ventral aspects of the myotome.