Ectopic mesodermal expression promoted by the eight-cell stage Bufo arenarum subequatorial cytoplasm

Mesodermal determinants were investigated by cytoplasmic transfer and blastomere isolation in the eight-cell stage of Bufo arenarum. Their existence was confirmed by assaying the subequatorial cytoplasm's ability to respecify the developmental potency of animal quartets. The gray subequatorial cytoplasm, but not animal cytoplasm, is able to divert the ectodermal fate of animal quartets to several mesodermal components. The source of the transplanted cytoplasm was important in determining the category of the resulting structures. Ventral subequatorial cytoplasm from ventrovegetal blastomeres generated ventral derivatives, namely erythrocytes and mesenchyma. Dorsal subequatorial cytoplasm from dorsovegetal blastomeres produced dorsolateral derivatives, such as notochord, muscle, nephric tubules, and coelomic epithelium, including mesenchyma. On the other hand, transfer of vegetal pole cytoplasm to animal quartets resulted in the formation of groups of endoderm-like cells dispersed among epidermal cells. However, the presence of such cells did not cause any mesodermal induction. The present findings suggest the existence of cytoplasmic information responsible for mesodermal specification. The alternative hypothesis that animal blastomeres become mesoderm due to vegetal induction is questioned.