Microsurgery on bovine embryos at the morula stage to produce monozygotic twin calves.

Mature Brangus donor cows were superovulated with follicle stim-ulating hormone administered twice daily in intramuscular injections. On day 6.5 to 7 post-estrus, embryos were collected non-surgically using a phosphate-buffered saline medium. A total of 37 ova was collected, of which 28 were advanced morulae and early blastocysts. Twenty of these embryos were selected for micromanipulation with a radial-type Leitz micromanipulator. While the embryos were in a holding medium containing 10% fetal calf serum, three glass microinstruments were used to open the zona pellucida, remove the mass of blastomeres and bisect the embryo on a vertical plane. Halved embryos were inserted into bovine zonae and placed either as single half-embryos or twin half-embryos in 0.25 ml French straws with fresh holding medium. The micromanipulated embryos (demi-embryos) were then non-surgically transplanted, either as a single demi-embryo or as a twin demi-embryo pair, into the uterine horn of day 6.5 to 8 recipient beef females ipsilateral to the existing corpus luteum. Of the 14 micromanipulated embryos that were transplanted to recipients, pregnancy rates were 16.6% for the single demi-embryos and 62.5% for the twin demi-embryos. No pregnancies resulted from bisected blastocysts.