Relationships between the completion of first cleavage and the chromosomal complement, sex, and developmental rates of bovine embryos generated in vitro.

One thousand eighty-four two-cell bovine embryos produced from 1,574 oocytes matured and fertilized in vitro were cultured as groups separated according to the time when they completed their first cleavage (24, 30, 40, 48, or 62 hr postinsemination; hpi). At 5 days after insemination, the proportions of each group that had progressed to the eight-cell stage or beyond were determined and the 350 that had done so were fixed and examined cytogenetically for cell number, chromosomal abnormalities, and sex. Embryos in the "early" cleaving (24 and 30 hpi) and "late" cleaving (40-62 hpi) groups were compared. Early cleaving embryos were more likely to have developed to the eight-cell stage or beyond (52.2% vs. 20%), contained more cells (22 vs. 17), and were more likely to be male (3.6:1 vs. 0.93:1). It is suggested that these phenotypic differences between the sexes begin before the embryonic genome is generally thought to become activated and are due either to differential processing of X- and Y-bearing sperm within the zygote or to very early differential expression of genes derived from X- and Y-bearing sperm.