Distinct regulatory cascades for head and trunk myogenesis.

Most head muscles arise from the pre-otic axial and paraxial head mesoderm. This tissue does not form somites, yet expresses the somitic markers Lbx1, Pax7 and Paraxis in a regionalised fashion. The domain set aside by these markers provides the lateral rectus muscle, the most caudal of the extrinsic eye muscles. In contrast to somitic cells that express Lbx1, lateral rectus precursors are non-migratory. Moreover, the set of markers characteristic for the lateral rectus precursors differs from the marker sets indicative of somitic muscle precursors. This suggests distinct roles for Lbx1/Pax7/Paraxis in the development of head and trunk muscles. When grafted to the trunk, the pre-otic head mesoderm fails to activate Lbx1, Pax7 or PARAXIS: Likewise, somites grafted into the region of the lateral rectus precursors fail to activate the lateral rectus marker set. This suggests that distinct regulatory cascades act in the development of trunk and head muscles, possibly reflecting their distinct function and evolution.