All booted to switch fates

Abstract

In the zebrafish embryo, the differentiation of myoblasts into fast- and slow-type muscle follows a temporal and spatial sequence that is invariant. Adaxial cells in the presomitic mesoderm express myoD and patched 1 (ptc1) in response to hedgehog (Hh) signals from the midline structures, switch on the slow-myosin isoform genes and migrate radially to the surface of the myotome to form a layer of mononucleated slow fibers. They leave behind a few fibers called the muscle pioneers (MPs) in the myotome that express the transcriptional repressor Engrailed. Concomitant with somitogenesis, the remaining cells in the myotome differentiate and fuse to form syncitial fast myofibers. It is not clear which events following the induction of the general muscle regulatory factor, myoD, lead to the choice of slow-fiber fate by the adaxial cells. Ingham and colleagues 1 now get closer to understanding this by using a zebrafish mutant of a gene called u-boot (ubo), which seems to operate downstream of myoD and ptc1 and acts as a switch that regulates the choice of a slow-fiber fate over that of fast-fiber type.