Anterior trunk muscle shows mix of axial and appendicular developmental patterns.

Background: Skeletal muscle in the trunk derives from the somites, paired segments of paraxial mesoderm. Whereas axial musculature develops within the somite, appendicular muscle develops following migration of muscle precursors into lateral plate mesoderm. The development of muscles bridging axial and appendicular systems appears mixed.

Results: We examine development of three migratory muscle precursor-derived muscles in zebrafish: the sternohyoideus (SH), pectoral fin (PF), and posterior hypaxial (PHM) muscles. We show there is an anterior to posterior gradient to the developmental gene expression and maturation of these three muscles. SH muscle precursors exhibit a long delay between migration and differentiation, PF muscle precursors exhibit a moderate delay in differentiation, and PHM muscle precursors show virtually no delay between migration and differentiation. Using lineage tracing, we show that lateral plate contribution to the PHM muscle is minor, unlike its known extensive contribution to the PF muscle and absence in the ventral extension of axial musculature.

Conclusions: We propose that PHM development is intermediate between a migratory muscle mode and an axial muscle mode of development, wherein the PHM differentiates after a very short migration of its precursors and becomes more anterior primarily by elongation of differentiated muscle fibers.