Xenopus dorsal pattern formation is lithium-sensitive.

Frog embryos that are treated with lithium during cleavage stages produce increased amounts of anterodorsal structures and decreased amounts of posterior and ventral structures. This alteration in pattern formation involves at least two changes in cell fate: the blastomeres that normally produce ventral structures populate anterodorsal structures, and the blastomeres that normally produce posterodorsal structures populate anterodorsal structures. The dorsalization of the ventral mesoderm has been shown to occur because Li alters the response to the ventral component of mesoderm induction. The posterior to anterior changes can be attributed to a reversal of embryonic polarity caused by a marked reduction in the dorsal involution of chordamesoderm. However, it is not known whether the reduction of chordamesodermal involution is produced secondarily by the abnormal migratory properties of the dorsalized ventral mesoderm, or whether it is caused by Li directly. In order to distinguish between these possibilities, chimeras of normal and Li-treated embryos were produced at the beginning of gastrulation. Chimeras with Li-treated ventral halves and normal dorsal halves developed into embryos with two heads and a single normal trunk, confirming the conclusion that the dorsalization of ventral mesoderm is produced directly by the Li. Chimeras with normal ventral halves and Li-treated dorsal halves developed into embryos that lacked postcephalic dorsal structures. These results indicate that the Li-produced lack of postcephalic dorsal structures does not depend on the altered ventral morphogenetic movements. The fate changes on the dorsal side of the embryo suggest that Li also alters the dorsal component of mesoderm induction.