98 EXPERIMENTAL TRANSFER OF BOVINE IVF-DERIVED 32-CELL STAGE EMBRYOS INTO THE UTERUS: ENVIRONMENTAL EFFECTS ON DEVELOPMENTAL CHARACTERISTICS

Abstract

Usually, in vitro-produced bovine embryos are cultured in vitro in static culture systems for 7 to 9 days in media composed according the oviducal fluid although it is well accepted that around Day 4.5–5 the bovine embryo enters the uterine cavity, providing environmental conditions different from the oviduct. Therefore, one has to raise the question whether changing culture media properties after Day 5 of culture could have beneficial effects on early development of bovine embryos. To answer that question, we transferred bovine IVF derived 32-cell stage embryos into the uterine cavity of synchronized recipients. All embryos had been matured and fertilized under routine standard conditions and were cultured in synthetic oviducal fluid supplemented with essential and nonessential amino acids (SOFaa) supplemented with either 0.3% fatty acid free bovine serum albumin (BSAfaf/Uterus) or 10% serum (serum/uterus) at 38.5°C, 5% O₂, and 5% CO₂ in humidified air prior transfer into the uterine environment, allowing further development to the blastocyst stage within the physiological environment prior recollection at Day 7 by routine uterine flushing followed by comparison with statically in vitro-developed embryos cultured in media supplemented with serum (serum/serum group) or BSAfaf (BSAfaf/BSAfaf group). All in all, a total of 1031 in vitro-derived 32-cell stage embryos were transferred to 21 synchronized Simmental recipient heifers. Of these, a total of 680 embryos (66%) could be recollected at Day 7. Embryos of the serum/serum group reached a higher blastocyst rate compared with embryos of the BSAfaf/BSAfaf group (68% v. 41%; P < 0.05, ANOVA, Tukey test), whereas the developmental rate to the blastocyst stage did not differ after 9 days of in vitro culture, indicating higher developmental kinetics of bovine 32-cell stage embryos when culture media is supplemented with serum. Moreover, embryos of the serum/uterus group reached significantly lower developmental rates to the blastocyst stage until Day 7 compared with embryos of the serum/serum group (12.9% v. 68.4%). Likewise, embryos in the BSAfaf/uterus group reached significantly lower developmental rates to the blastocyst stage until Day 7 compared with embryos in the BSAfaf/BSAfaf group (16.0% v. 40.1%). When allowed to develop for additional 48h in vitro, developmental rates to the blastocyst stage at Day 9 were still higher in BSAfaf/BSAfaf treatment compared with the BSAfaf/uterus treatment (91.4% v. 74.4%) and the serum/serum treatment compared with the serum/uterus treatment (92.5% v. 56.0%). Taken together, the results of our study demonstrate that uterine transfer of bovine 32-cell stage embryos results in reduction of developmental kinetics as well as lower developmental rates compared with embryos statically cultured in vitro. That might indicate, that a considerable proportion of bovine 32-cell stage embryos might not be able to adapt to the uterine environment.