Cellular composition and viability of demi- and quarter-embryos made from bisected bovine morulae and blastocysts produced in vitro.

The cellular composition and viability of intact, IVP embryos were compared with those of demi- and quarter-embryos produced by bisection of IVP morulae and blastocysts. Embryos were produced by established techniques from oocytes harvested from slaughterhouse ovaries. In Experiment 1, morulae at Day 6 or blastocysts at Day 7 were bisected on an inverted microscope using a microsurgical steel blade. Demi-embryos were then cultured without a zona pellucida until Day 8, when they were morphologically assessed for quality (viability). A higher proportion of demi-embryos made from blastocysts than from morulae were classified as viable (381/420, 91% vs 164/267, 61%; P < 0.001). In Experiment 2, only Day 7 blastocysts were bisected, and some of the resulting demi-embryos were bisected a second time 24 h later to produce quarter-embryos. The remaining demi-embryos, the quarter-embryos, and control intact embryos were cultured until Day 9, at which time they were assessed for quality and subjected to immunosurgery and differential staining to count inner cell mass (ICM) and trophectoderm cells. A higher proportion of demi-embryos than quarter-embryos was classified as viable (408/459, 89% vs 223/319, 70%, respectively; P < 0.001). Total cell numbers decreased with successive bisections, but the proportion of surviving cells found in the ICM was significantly (P < 0.05) higher in the best quality demi- and quarter-embryos (35 and 32%, respectively) than in the controls (22%). Transfer of all 12 quarter-embryos derived from 3 blastocysts, in pairs, into 6 recipient heifers resulted in 2 pregnancies, each with a single viable fetus at 90 d of gestation. The fetuses originated from 2 different blastocysts. The results suggest that bisection of intact IVP embryos into demi-embryos and bisection of those into quarter-embryos can increase the number of transferable embryos by as much as 178 and 235%, respectively.