In vitro production of bovine embryos: A progress report and the consequences on the genetic upgrading of cattle populations

Abstract

In vitro bovine embryo production (IVP) has improved tremendously during the past decade. Repeatable and reliable systems for mass production of embryos for use in cattle breeding schemes are now available in many laboratories all over the world. The different steps involved in IVP are discussed in the present review. In vitro maturation of oocytes (collected at the abattoir or by transvaginal ultrasound-guided aspiration from live animals) for 24–27 h results in a maturation rate of 82%. In vitro fertilization with heparin capacitated spermatozoa leads to a normal penetration rate of 56%, an incidence of polyspermia and parthenogenetic activation of 6% and 12%, respectively. By in vitro culture in a cell-free medium or a medium containing bovine oviduct epithelial cells (or conditioned medium) of presumptive zygotes it is possible to obtain compact morula and blastocyst rates of 33% and 26%, respectively. The quality of the embryos is inferior to in vivo produced and even to zygotes cultured in sheep oviducts. This is reflected in their morphology (darker, early blastoceol formation, fewer cells per embryo) and their increased susceptibility to freezing. A higher incidence of embryonic loss is likewise registered after non-surgical transfer. Possible causes are unknown as of today, but placental defects may be suspected. This is a serious obstacle to a more efficient use of IVP in cattle breeding schemes, which includes: twinning, cloning, transgenesis, other research purposes, studies on embryonic mortality; use of oocytes from calves and fetuses; alleviation of certain problems in heifers and cows (genital problems, poor superovulatory response) and bulls (treatment of certain types of infertility) and finally more efficient use of semen. In the final part of the paper a model for an advanced embryo technology center is presented. This center will be based on the conventional embryo transfer methodology, but will also include newer techniques such as in vitro production of embryos (recovered at slaughter or by ultrasonography), cloning and preimplantation diagnosis of embryos (sexing). The future of cattle breeding depends heavily on these new technologies and it is the responsibility of the scientist to address one essential issue: to improve the viability of in vitro produced embryos in order to be able to achieve satisfactory survival rates after freezing and transfer of both fresh and frozen in vitro produced embryos.