THE HOMEOBOX GENE GOOSECOID AND THE ORIGIN OF ORGANIZER CELLS IN THE EARLY CHICK BLASTODERM

The chick homeobox gene goosecoid (gsc) is first expressed in a barely noticeable cell population near the posterior margin (Koller's sickle ) of the unincubated egg. Then it is detected in Hensen's node, tradit ionally considered the chick organizer. Later, gsc-expressing cells le ave the node with the prechordal plate. Fate mapping indicates that th ese three regions are related by cell lineage, and transplantation exp eriments suggest that they all have inducing activity. Quail posterior margin and anterior primitive streak grafts (gsc expressing) induce g sc transcription in neighboring chick host cells. We propose that deve lopment of the chick organizer starts earlier than previously thought and that gsc marks this changing cell population.