COMMITMENT OF MESODERM CELLS IN HENSENS NODE OF THE CHICK-EMBRYO TO NOTOCHORD AND SOMITE

Hensen's node in the chick embryo contains prospective notochord cells in a DELTA-shaped midline region of the mesoderm, and prospective medial half somite cells in the lateral portions of the node mesoderm, whilst lateral somite cells are derived from the rostral part of the primitive streak (Selleck and Stern, Development 112, 615-626, 1991). In the present study, we have investigated the commitment of these mesoderm cells to their fates by grafting between these regions of the node and primitive streak. By challenging the cells in this way, we have attempted to discover whether prospective notochord and somite cells can change their fates. We find that the mesoderm cells in the midline DELTA-shaped portion of Hensen's node, when grafted into a different region, continue to integrate into the notochord, or form a separate notochord-like structure. By contrast, the prospective somitic cells do not appear to be committed to a somitic fate.