Ribosomal RNA gene expression and chromosome aberrations in bovine oocytesand preimplantation embryos

This review focuses on the key features of development of the bovine oocyte and embryo, with comparisons of the developmental characteristics of embry os produced in vivo and in vitro. The oocyte is transcriptionally quiescent in the primordial and primary follicle. In the secondary follicle transcri ption is initiated in the oocyte and a ribosome-synthesizing nucleolus is e stablished in this cell. Transcription and nucleolar activity are enhanced in the tertiary follicle during oocyte growth. When the oocyte reaches appr oximately 110 mum in diameter, corresponding to a follicle of about 3 mm in diameter, transcription ceases and the nucleolus is inactivated, forming a dense spherical remnant. During the final phase of follicular dominance th is remnant becomes vacuolated and, in conjunction with resumption of meiosi s, it disperses. The rRNA genes are apparently re-activated during the four -cell stage, that is, the third cell cycle after fertilization, but a nucle olus is not formed. During the subsequent cell cycle, that is, during the e ight-cell stage, ribosome-synthesizing nucleoli are again established. Bovi ne embryos produced in vitro apparently display the same pattern of nucleol us development as that in embryos developed in vivo. Examination of the plo idy of embryonic cells using fluorescence in situ hybridization has reveale d that the production of bovine embryos in vitro is associated with increas ed chromosome aberrations in the embryos. Blastocysts produced in vitro dis play a significantly higher rate of mixoploidy, that is, when the embryo co nsists of both normal diploid and abnormal polyploid cells, than that in em bryos developed in vivo. The rate of mixoploidy among embryos produced in v itro increases with increasing developmental stage. Moreover, after fertili zation in vitro, initially there is a high rate of 'true' polyploidy, that is, when all cells of the embryos are polyploid. However, the polyploid emb ryos are eliminated before they cleave beyond the eight-cell stage, the sta ge at which major activation of the embryonic genome occurs in cattle.