On the 'clock' Mechanism Determining the Time of Tissue-specific Enzyme Development During Ascidian Embryogenesis. I. Acetylcholinesterase Development in Cleavage-arrested Embryos

During ascidian embryogenesis a tissue-specific enzyme, muscle acetylcholinesterase (AChE) may first be detected histochemically in the presumptive muscle cells of the neurula. In order to investigate the 'clock' or counting mechanism that is determining the time when AChE first appears, Whittaker's experiment (1973) has been repeated using eggs of the ascidian, Halocynthia roretzi. Embryos that had been permanently cleavage-arrested with cytochalasin B were able to differentiate AChE in their muscle lineage blastomeres. The time of first AChE occurrence in embryos that had been cleavage-arrested in the 32-cell stage with cytochalasin B was about the same as in normal embryos. This result indicates that the clock is not apparently regulated by the events of cytokinesis. The early gastrulae which had been arrested with colchicine or with colcemid could develop AChE activity, although no histochemically detectable AChE activity was observed in the cleavage-stage embryos that had been arrested with either drug. Therefore the clock does not seem to be controlled by the mitotic cycle of the nucleus. It is suggested that the cycle of DNA replication may be related to the regulation of the clock that is determining the time of development of histospecific protein.