Induction of notochord during ascidian embryogenesis.

The tadpole larva of solitary ascidians has 40 notochord cells in its tail. Of these cells, 32 in the anterior and middle part of the tail are derived from the A-line blastomeres, while 8 in the posterior part of the tail originate from the B-line blastomeres. Previous experiments involving continuous dissociation of daughter blastomeres from the first cleavage to the 110-cell stage suggested that cellular interactions may be involved in the formation of notochord cells. In the present study, the presumptive-notochord blastomeres isolated from the 32-cell embryos did not develop features of notochord. These results suggest that cellular interactions may be required for the fate specification of notochord, that is to say, notochord formation occurs as a result of inductive interaction between blastomeres. In order to confirm the involvement of induction in the determination of notochord and to identify the inducer blastomeres, the presumptive-notochord blastomeres at the 32-cell stage were coisolated or recombined with one of the surrounding blastomeres in a series of experiments. The results suggested that, for the A-line precursors, notochord differentiation occurs as the result of an inductive influence from vegetal blastomeres that include the presumptive-endoderm blastomeres and the presumptive-notochord blastomeres themselves. It was also suggested that induction of notochord is complete by the 64-cell stage and that inductive interactions have to be initiated before the decompaction of blastomeres during the 32-cell stage. Ascidians are Urochordata and are closely related to vertebrates. In vertebrates, it is well known that inductive interactions play a crucial role in the determination of notochord. It appears, therefore, that induction of notochord is common throughout the phylum Chordata.