Patterning of the axial musculature in the developing chick embryo (Gallus)

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While the development and patterning events of the skeletal, myogenic and connective tissues of the developing limb buds of the chick have been relatively well studied, there is little known about the formation of the epaxial muscles and tendons. The epaxial muscles form the postvertebral muscle groups and develop from the myotome of the somite. The myotome develops from myogenic precursors migrating from the dorsomedial lip of the dermomyotome. These myogenic cells differentiate in a cranial to caudal sequence with the muscle fibres also orientated in a cranial to caudal direction.Here we use immunohistological staining both in whole mount and in transverse sections of chick embryos from various stages of development. Antibodies to the myosin heavy chain or tenascin were used to visualise the development of the epaxial muscles and tendons.The first myotomal muscle fibres are differentiated by Hamburger and Hamilton (H & H) stage 10 in the cranial somites, and differentiation proceeds caudally until at least H & H stage 22–23. Further development of the epaxial muscles does not take place until H & H stage 26–27 with the splitting of the myotome into the individual muscles. We demonstrate how the myotome splits into the individual muscles and how some muscle fibres become reorientated into a more oblique orientation. This delayed development and reorientation of muscle fibres is unique to the epaxial muscles.