Onset of competence to respond to activin A in isolated eight-cell stage Xenopus animal blastomeres

Single animal hemisphere blastomeres isolated from the eight-cell stage Xenopus embryos differentiate into mesoderm when treated with activin A, whereas when cultured without activin they form atypical epidermis. The mesoderm tissue induced by activin is different between dorsal and ventral blastomeres. In the present study, the duration and timing of activin treatment was varied, in order to identify the critical stage when animal blastomeres acquire competence to respond to activin A. It was shown that the critical time was 45 min after blastomere isolation, which corresponds approximately to NF stage 6 (32-cell stage) of normal development. The competence gradually increased during the morula stages.