Early regionalization of the somitic mesoderm as studied by the development of the axial skeleton of the chick embryo

Abstract

The excision of a portion of unsegmented somitic mesoderm from the thoracic region has no or little effect on the development of vertebrae and ribs. Its orthotopic replacement by thoracic somitic mesoderm obtained from another embryo does not cause important deficiencies of the rib basket. But its replacement by nonsomitic tissue (neural tube, midgut, somatic mesoderm) not only causes the formation of a costal gap within the thoracic region, but also leads to the production of a defective spine on the operated side. Segmented or unsegmented somitic mesoderm from the cervical region, implanted in place of unsegmented thoracic somitic mesoderm, differentiates according to its origin: no ribs develop in the host embryo at the operation level. Conversely, segmented or unsegmented somitic mesoderm from the thoracic region, implanted in place of unsegmented cervical somitic mesoderm, also differentiates according to its origin, giving rise to supernumerary ribs in front of the host's rib basket. These extra ribs, however, always lack a sternal component. It is concluded that the level-specific morphogenetic capacity of the cervical and thoracic somitic mesoderm is determined before metamerization occurs. Despite this early determination, the unsegmented somitic mesoderm is endowed with regulative properties.