Duration of competence and inducing capacity of blastomeres in notochord induction during ascidian embryogenesis.

Notochord cells in ascidian embryos are formed by the inducing action of cells of presumptive endoderm, as well as neighboring presumptive notochord, at the 32-cell stage. Studies of the timing of induction using recombinations of isolated blastomeres have suggested that notochord induction must be initiated before the decompaction of blastomeres at the 32-cell stage and is completed by the 64-cell stage. However, it is not yet clear how the duration of notochord induction is strictly limited. In the present paper, the aim was to determine in detail when the presumptive notochord blastomeres lost their competence to respond, and when the presumptive endoderm blastomeres produced inducing signals for the notochord. Presumptive notochord blastomeres and presumptive endoderm blastomeres were isolated from early 32-cell embryos, and were heterochronously recombined at various stages ranging from the early 32-cell stage to the 64-cell stage. Presumptive notochord blastomeres could respond to inductive signals at the early 32-cell stage, and started to lose their responsiveness at the decompaction stage. By contrast, the presumptive endoderm blastomeres persisted in their inducing capacity even at the 64-cell stage. These observations suggest that the loss of competence in presumptive notochord blastomeres limits the duration of notochord induction in intact ascidian embryos.