Nucleus : cell volume ratio directs the timing of the increase in blastomere adhesiveness in starfish embryos

Abstract

Blastomeres of starfish embryos begin to increase in adhesiveness after the eighth cleavage and form a monolayered hollow blastula. To investigate factors that affect the timing of the adhesiveness increase, we changed the volume of the cytoplasm or the ploidy of embryos and examined the morphologic changes in the descendent blastomeres during early cleavage stages. In parthenogenetic embryos, in which the ploidy is doubled, the timing of the increase in adhesiveness was accelerated by one cell cycle. In contrast, the timing was delayed by approximately one cell cycle in a large-sized embryo formed by the fusion of an egg and a non-nucleate egg fragment. These two sets of observations are in accord with the expectation from the classical concept that the DNA: cytoplasmic ratio may direct the timing of events in early development. However, observations of small-sized embryos with a reduced amount of cytoplasm were contradictory to the expectation based on the DNA: cytoplasmic ratio; the timing of the increase in adhesiveness in half-sized embryos was almost the same as in control embryos and the timing was delayed by only one cell cycle in quarter-sized embryos. Measurement of the diameters of nuclei showed that the size of nuclei was variable, depending on the stage of development, the volume of cytoplasm and ploidy. We calculated a volume ratio of nucleus to cytoplasm (N: C volume ratio) for tetraploid, large-, half- and quarter-sized embryos. We found that the embryonic cells begin to adhere always when their N: C volume ratio reaches 0.06. A plausible model for the cellular timing mechanism of cell contact is proposed.