Production of monozygotic (identical) horse twins by embryo micromanipulation.

The blastomeres of 192- to 8-cell embryos recovered surgically 1-3 days after ovulation from 23 Pony mares were mechanically separated and inserted, in various combinations, into evacuated pig zonae pellucidae to make 27 'half' and 17 'quarter' micromanipulated embryos. These were embedded in agar and cultured in vivo in the ligated oviducts of ewes for 3.5-5 days to allow development to the late morula/early blastocyst stage. Subsequent surgical or non-surgical transfer of 13 'half' and 17 'quarter' embryos to mares resulted in 10 established pregnancies, including 2 monozygotic pairs. Surgical transfer to mares that had not been recently used as donors of embryos was more successful (10/20) than surgical or non-surgical transfer to recently operated mares (0/10).