Macromolecule synthesis and determination in sea urchin blastomeres at the sixteen-cell stage.

Three size classes of blastomeres, micro-, meso-, and macromeres, form at the fourth cleavage of sea urchin embryos. These cells are the first for which there is unequivocal evidence of restricted developmental potential. Overall patterns of protein synthesis are qualitatively similar in all three, but distinct quantitative differences exist, particularly but not exclusively in the ratio of histone to nonhistone synthesis. These differences are reproduced in heterologous cell-free translation directed by RNA purified from each of the three cell types. They are abolished by treatment of the embryos with actinomycin, in the presence of which all cells synthesize the same kinds and very nearly the same amounts of proteins, at least at the level of resolution available with gel electrophoresis in one dimension. Divergence of the protein synthetic patterns in normal embryos appears to be due to the addition of similar amounts of newly transcribed messenger RNA to variable maternal messenger pools, the sizes of which are proportional to the volume of cytoplasm.