Stage of blastomeres (stage 2)
The zygote begins to cleave, with the first division forming two daughter cells called the blastomeres. Further divisions occur but the total mass does not exceed that of the zygote (stage of blastomeres). The daughter cells are held together by the zona pellucida. The reserves of maternal messenger-RNA (mRNA), ribosomes, transfer-RNA (tRNA) and protein precursors that have been accumulated in the oocyte before the fertilization cascade are still sufficient for the ongoing protein synthesis. Amongst these reserves are special mRNA-molecules that contain code for the most essential proteins in the regulation of the initial functions of the blastomeres. These results suggest that, up until the 4-cell stage, transcription of embryonic DNA is not required, i.e. embryogenesis is still regulated at the post-transcriptional level. The activation of maternal mRNA is stimulated by fertilization. The activation of the embryonic genome is first observed between the 4- and 8-cell stages; it is essential for protein synthesis as well as for further cell divisions.