33 Post-thaw evaluation of in vivo- and in vitro-derived bovine embryos

Assisted reproductive technology (ART) enables producers to reach reproductive, financial, and genetic goals, while increasing cattle productivity. ART also facilitates offspring of the desired sex, in addition to embryo and calf production from donors that cannot produce in vivo-derived (IVD) embryos through conventional means. Cryopreservation is a key component of ART, and it enables genetics to be preserved and marketed in the form of a frozen embryo; however, most frozen embryos are thawed and transferred directly, so there is scant morphological information about frozen/thawed bovine embryos. The objective of this experiment was to evaluate the post-thaw development of various types of donated bovine embryos frozen in ethylene glycol (n = 32 IVF, n = 32 Sexed, and n = 48 IVDs [CON]). All embryos were thawed (5 s air, 30 s in 30°C water), rinsed in holding medium, and cultured individually in in vitro culture medium (5 μL) to compare quality and embryonic developmental changes. Embryo grade and stage (IETS guidelines) were recorded (from the straw label) and evaluated morphologically after being thawed and at 24 and 48 h of culture. Embryo quality and developmental data were analyzed with one-way ANOVA. The quality of embryos before freezing was similar among the CON, IVF, and Sexed embryos. Upon thaw, on average, the quality of all embryos (CON, IVF, and Sexed embryos) was negatively affected (P < 0.05) by the freeze-thaw cycle. The quality of the CON embryos was similar after thaw and at 24 and 48 h of in vitro culture, whereas the quality of the Sexed embryos tended to be lower (P < 0.1) at 24 h compared with controls, and the quality of the IVF embryos declined (P < 0.05) at 24 and 48 h of culture relative to CON. The IVF embryos were developmentally advanced (P < 0.05) upon freezing and after thaw; however, they demonstrated little advancement at 24 h (0.3 stages) and 48 h (0.4 stages). In contrast, the CON embryos advanced (P < 0.05) substantially at 24 h (1.3 stages) and at 48 h (0.6 stages), and the Sexed embryos advanced (P < 0.05) substantially at 24 h (1.1 stages) and 48 h (0.6 stages). There was no difference in developmental capacity of the CON and Sexed embryos. The embryo transfer (ET) industry does not normally evaluate embryos after thaw as they are transferred directly, thus there is little information about freeze-thaw damage or post-thaw developmental capacity. In these data, the effects of freezing and thawing negatively affected all embryos (more so in the IVF group); however, the CON and Sexed embryos recovered quickly and continued to develop in culture, while the IVF embryos did not, indicating a loss in viability in the IVF group. Taken together, these data indicate that IVD embryos tolerated the freeze-thaw cycle, while IVF embryos did not. Perhaps a post-thaw short-term culture strategy may allow for identification of higher quality embryos and thus improve ET success rates.