BOVINE EMBRYONIC-DEVELOPMENT AFTER IN-VIVO AND IN-VITRO FERTILIZATION

A mammalian embryo originates from the fusion of a sperm cell with a m ature oocyte. This event usually takes place inside the maternal repro ductive tract, but since the development of Assisted Reproductive Tech nologies (ART), fertilization has been successfully performed in vitro for different mammalian species, in relatively simple media. Embryos developing after in vitro fertilization are placed back into the ovidu ct or uterus at different points of time after insemination, depending on the species. In humans for instance, embryos are routinely transfe rred to the uterus 2 or 3 days after fertilization (at which time the 4- to 8-cell embryo normally resides in the oviduct). In the bovine sp ecies, uterine transfer is postponed until the physiological time of e mbryo arrival into the uterus, which is one week after fertilization. This means that the bovine embryo has to be held in vitro during a sub stantial period of time, including during its first differentiative ev ents, compaction and blastocyst formation. Although at first sight no major differences are present between these in vivo and in vitro deriv ed embryos, more detailed research during the last five years has thro wn a new light on the effects the in vitro environment has on the fine ly tuned events which take place during fertilization and early embryo nic development. It is the purpose of this review to highlight these d ifferences and influences by describing the bovine fertilization and e mbryogenesis process as it happens in vivo and in vitro. By doing this , we will hopefully indicate new strategies for research, leading towa rds an improvement of the quality of in vitro produced bovine embryos.