Pregnancy and fetal characteristics after transfer of vitrified in vivo and cloned bovine embryos.

This study was conducted to examine pregnancy progression and fetal characteristics following transfer of vitrified bovine nuclear transfer versus in vivo-derived embryos. Nuclear transfer (NT) was conducted using cumulus cells collected from an elite Holstein-Friesian dairy cow. Expanding and hatching blastocysts on Day 7 were vitrified using liquid nitrogen surface vitrification. Day 7 in vivo embryos, produced using standard superovulation procedures applied to Holstein-Friesian heifers (n=6), were vitrified in the same way. Following warming, embryos were transferred to synchronized recipients (NT: n=65 recipients; Vivo: n=20 recipients). Pregnancies were monitored by ultrasound scanning on Days 25, 45 and 75 and a sample of animals were slaughtered at each time point to recover the fetus/placenta for further analyses. Significantly more animals remained pregnant after transfer of in vivo-derived embryos than NT embryos at all time points: Day 25 (95.0 versus 67.7%, P<0.05), Day 45 (92.8 versus 49.1%, P<0.01) and Day 75 (70.0 versus 20.8%, P<0.0). There was no significant difference (P=0.10) in the weight of the conceptus on Day 25 from NT transfers (1.14+/-0.23 g, n=8) versus in vivo transfers (0.75+/-0.19 g, n=8). On Day 45, there was no significant difference in the weight of either fetus (P=0.393) or membranes (P=0.167) between NT embryos (fetus: 2.76+/-0.40, n=12; membranes: 59.0+/-10.0, n=11) or in vivo-derived embryos (fetus: 2.60+/-0.15, n=6; membranes: 41.8+/-5.2, n=4). However, on Day 75 the weight of the fetus and several of the major organs were heavier from NT embryos. These data suggest that morphological abnormalities involving the fetus and the placenta of cloned pregnancies are manifested after Day 45.