Regulation of Gene Expression in the Oocyte and Early Embryo: Implications for Transcriptomic and Proteomic Embryo Assessment

Oocytes and early preimplantation embryos use distinct mechanisms to achieve timely expression of genes that regulate meiotic reactivation, oocyte maturation, and embryonic cleavage divisions. Transcription becomes suppressed upon stimulation of oocyte maturation. Thereafter, gene expression is regulated by translational activation and suppression of maternally derived mRNAs. The activation of embryonic transcription, called zygotic genome activation (ZGA), occurs at 4- to 8-cell stage in human. Therefore, during oocyte maturation, fertilization, and early cleavage divisions, total mRNA content does not reflect gene expression. Instead, an accurate profile of translated mRNAs may be determined by identifying polysome-bound transcripts. Conversely, cumulus and granulosa cells have active transcription, and their gene expression utilizes common pathways shared by other somatic cells.