Optimization of embryo transfer protocols for mice

The efficiency of ova transfer and subsequent survivability were explored in this study. The goals of the experiment were to determine the minimum number of ova necessary for pregnancy maintenance, ascertain if the number of zygotes used in ova transfer approaches or exceeds uterine capacity, and establish if location of deposition of ova influences embryo survival. A total of 1647 pronuclear zygotes were transferred in groups of 1, 2, 4, 6, 15 or 25 on Day 1 of gestation either via the oviducal ampulla or ostium to 156 nulliparous ICR pseudopregnant female mice. Pregnancy status was determined on Day 12 or Day 19 of gestation. Results indicated that pregnancy rates were not significantly increased by transferring larger numbers of zygotes (P < 0.1504) and that beyond transfer of 15 zygotes, the progressive increase in fetal numbers per litter declined. However, on Day 19 of gestation, no definitive evidence of limitation of uterine capacity was obtained with the numbers of zygotes transferred (P < 0.0531), and the estimates of numbers of viable and resorbed fetuses differed when determinations were made on Day 12 versus Day 19 of gestation. Mean numbers of developed fetuses per recipient declined (P < 0.0001), whereas the number of resorptions (partially resorbed fetuses or resorption sites) increased (P < 0.0001) over this period, reflecting fetal loss in mid- to late-gestation and possibly the transient nature of resorptions prior to Day 12. Additionally, there was no difference in pregnancy outcome when transferring ova into the oviducal ostium or isthmus (P < 0.5256). Finally, these results illustrated that when large numbers of zygotes were transferred into the oviducal ampulla, equivalent numbers of ova eventually implanted in the uterus; however, proportionally more of them began resorption.