Origin and development of the avian tongue muscles.

The musculature of the vertebrate tongue is composed of cells recruited from the somites. In this paper we have investigated the migration and organisation of the muscle cells that give rise to the tongue muscle during chick embryogenesis. At the molecular level, our data suggests that a population of Tbx-3 expressing cells migrate away from the occipital somites prior to the migration of muscle precursors that express Pax-3. Both populations take the same pathway and form the hypoglossal cord. The first signs of muscle cell differentiation were not detected until cells had migrated some distance from the somites. We have determined the contribution of single somites to the musculature of the tongue and show in contrast to previous data that somites 2-6 take part in the formation of all glossal and infrahyoid muscles to the same extent but do not contribute to suprahyoid muscle. This is particularly interesting since glossal and infrahyoid muscle differ from the suprahyoid muscles not only in their morphology, but also in their developmental origin. Furthermore we show that myocytes cross the midline and contribute to the contralateral glossal and infrahyoid muscles. This is supported from our molecular data, which showed that the migratory precursor population was maintained primarily at the rostral tip of the developing hypoglossal cord.