Monozygotic Twinning in Rhesus Monkeys by Manipulation of In Vitro-Derived Embryos1

The nonhuman primate is a relevant model for human disease that can be used for diverse biomedical investigations. The abil-ity to propagate a founder animal by application of assisted re-productive technologies is pressing, but an even greater need in many studies is access to genetically identical animals. In an effort to create genetically identical monkeys, we evaluated two approaches to monozygotic twinning; blastomere separation, and blastocyst bisection. Embryos were produced by intracyto-plasmic sperm injection of oocytes recovered following con-trolled ovarian stimulation. The quality of demiembryos pro-duced in these efforts was evaluated by quantitating the effi-ciency of creating identical pairs for embryo transfer, by mor-phological assessment, by the allocation of cells to the inner cell mass (ICM) and trophectoderm (TE) in the blastocyst, and by the outcome of embryo transfer to synchronized host animals. Pairs were produced in high yield (85%–95%) by both twinning methods. Demiembryos resulting from blastomere separations at the 2- or 4-cell stage grew to blastocysts at the control frequen-cy. Demiblastocysts contained, on average, half the number of cells of the intact controls while maintaining the same ICM:TE or ICM:total cell ratio. The equivalency of demiblastocysts with-in a set was also evaluated by differential cell counting. Embryo transfers of identical sets led to a 33 % clinical pregnancy rate, with two twin pregnancies initiated. Neither pregnancy resulted in term birth of monozygotic twins, but our results are suffi-ciently encouraging to justify a large-scale twinning trial in the rhesus macaque. assisted reproductive technology, embryo, fertilization, in vitro fertilization, pregnanc