The fates of cels from the anterior region of the ascidian neural plate are described either as neural or as mixed neural and non-neural. In Ciona intestinalis, all cellular progeny are accounted for until a time 60% between the onset of embryonic development and larval hatching. To resolve the issue of their fates in this species, we have examined the later mitotic history of neural-plate cells. Because cessation of cell division in the neural plate has been claimed to occur at 70% of embryonic development, we need to account for cell production from 60% onward, to determine whether more cells are produced than populate the larval CNS, allowing some to adopt non-neural fates. The embryonic incorporation of bromodeoxyuridine (BrdU), 500 muM in seawater, was monitored in 1-h larvae by anti-BrdU immunocytochemistry. The pattern of incorporations indicates that all larval neurons are born before 70% of embryonic development, but that cell division unexpectedly continues to generate ependymal cells until at least 95%. Divisions in the neurohypophysis continue throughout embryonic development. The total number of cells produced appears sufficient only to complete the complement of larval CNS cells, denying non-neural fates for anteriorly migrating neural plate cells, and indicating a general absence of cell death. Consistent numbers of incorporations after the same exposure in different larvae provide evidence for determinacy of neural plate lineages. The last three conclusions confirm those reached previously (Nicol and Meinertzhagen, 1988b).
