Mesoderm formation by isolated and cultivated 8-cell stage blastomeres of the teleost, Leucopsarion ptersii (shiro-uo)

Isolation of cleavage-stage blastomeres and the study of their developmenta l potential has been used extensively for analyzing the mechanisms of embry ogenesis in vertebrates, including amphibians and echinoderms. We devised a method to isolate 8-cell stage blastomeres in the teleost, shiro-uo, by ut ilizing its unique cleavage pattern of the horizontal 3rd cleavage plane. R emoval of all the upper blastomeres at the 8-cell stage allowed almost norm al embryogenesis from the remaining lower blastomeres and yolk cell mass. I solated upper or lower blastomeres formed vesicles and spherical bodies, wh ich later showed morphological changes during cultivation. Mesoderm formati on was detected not only in the cultivated lower blastomeres or whole blast omeres but also in the upper blastomeres isolated from the yolk cell mass a t the 8-cell stage, although at a lower frequency than the lower blastomere s. These results indicated the presence of very early signaling for mesoder m induction, which is independent from the currently postulated signals fro m the yolk syncytial layer at later stages. This also indicated non-equival ence or differentiation of the blastomeres from the very early cleavage sta ge in teleost embryos.