Endoderm patterning by the notochord: development of the hypochord in Xenopus

The patterning and differentiation of the vertebrate endoderm requires sign aling from adjacent tissues. In this report, we demonstrate that signals fr om the notochord are critical for the development of the hypochord, which i s a transient, endodermally derived structure that lies immediately ventral to the notochord in the amphibian and fish embryo. It appears likely that the hypochord is required for the formation of the dorsal aorta in these or ganisms, We show that removal of the notochord during early neurulation lea ds to the complete failure of hypochord development and to the elimination of expression of the hypochord marker, VEGF, Removal of the notochord durin g late neurulation, however, does not interfere with hypochord formation, T hese results suggest that signals arising in the notochord instruct cells i n the underlying endoderm to take on a hypochord fate during early neural s tages, and that the hypochord does not depend on further notochord signals for maintenance. In reciprocal experiments, when the endoderm receives exce ss notochord signaling, a significantly enlarged hypochord develops. Overal l, these results demonstrate that, in addition to patterning neural and mes odermal tissues, the notochord plays an important role in patterning of the endoderm.