The molecular set-up of the avian head mesoderm and its implication for craniofacial myogenesis

The head mesoderm is the mesodermal tissue on either side of the brain, from forebrain to hindbrain levels, and gives rise to the genuine head muscles. Its relatedness to the more posterior paraxial mesoderm, the somites, which generate the muscles of the trunk, is conversely debated. To gain insight into the molecular setup of the head mesoderm, its similarity or dissimilarity to the somitic mesoderm, and the implications of its setup for the progress of muscle formation, we investigated the expression of markers (1) for mesoderm segmentation and boundary formation, (2) for regional specification and somitogenesis and (3) for the positive and negative control of myogenic differentiation. We show that the head mesoderm is molecularly distinct from somites. It is not segmented; even the boundary to the first somite is ill-defined. Importantly, the head mesoderm lacks the transcription factors driving muscle differentiation while genes suppressing differentiation and promoting cell proliferation are expressed. These factors show anteroposteriorly and dorsoventrally regionalised but overlapping expression. Notably, expression extends into the areas that actively contribute to the heart, overlapping with the expression of cardiac markers.