The legacy from the oocyte and its role in controlling early development of the mouse embryo

Mouse embryogenesis from ovulation to the early two-cell stage is controlled at a post-transcriptional level by components of the egg assembled during oogenesis and, in some cases, sequestered in an inactive form. These molecular changes appear to be under dual control. The terminal stages of oogenesis initiate a programme of sequential activation of previously untranslated mRNAs and post-translational modifications of pre-existing and newly synthesized polypeptides. Superimposed upon this programme is a sequence of events set in train by fertilization. The earliest of these events that we have detected is a post-translational modification(s). This is followed closely by the activation and translation of new species of mRNA(s). We suggest that the oocyte programme controls the general 'housekeeping' functions of the cell and that the transition to the fertilized state may be initiated at the post-translational level and lead to other 'fertilization-specific' changes that influence processes later in development. The transition from maternal (post-transcriptional) to embryonic (transcriptional) control of development occurs at the early two-cell stage and involves two periods of transcriptional activity closely coupled to translation. In contrast, the maternal mRNAs show an abrupt decline in stability during the two-cell stage. However, many of their polypeptide products appear to persist undegraded until at least the morula stage.