Distinct parameters are involved in controlling the number of rounds of cell division in each tissue during ascidian embryogenesis.

We counted cell numbers during embryogenesis of the ascidian, Halocynthia roretzi, every hour. Cell numbers were determined by counting the numbers of nuclei in squashed embryos. The cell number of a larva just after hatching was approximately 3000. Our study addresses the question of what factors control the number of rounds of cell division during development. Three kinds of egg fragments were prepared by cutting unfertilized eggs to alter the volume of cytoplasm and the amount of DNA. After the egg fragments were fertilized, the cell numbers were estimated at the hatching stage. The cell numbers of the resulting larvae differed from those of normal larvae. Precursor blastomeres of various tissues were then isolated from normal and manipulated embryos, and cultured as partial embryos. The cell numbers of the resulting partial embryos were counted to estimate the number of cell divisions in each larval tissue. The results suggested that the number of cell divisions is controlled by a distinct mechanism in each tissue. We propose that the number of rounds of cell division during ascidian embryogenesis is controlled by three mechanisms: the first depending on the volume of cytoplasm; the second on the nucleo-cytoplasmic ratio; and the third depending on neither of these parameters. J. Exp. Zool. 284:379-391, 1999.