Insights into conceptus elongation and establishment of pregnancy in ruminants

This review integrates established and new information on the factors and pathways regulating conceptus–endometrial interactions, conceptus elongation and establishment of pregnancy in sheep and cattle. Establishment of pregnancy in domestic ruminants begins at the conceptus stage (embryo or fetus and associated extra-embryonic membranes) and includes pregnancy recognition signalling, implantation and the onset of placentation. Survival and growth of the preimplantation blastocyst and elongating conceptus require embryotrophic factors (amino acids, carbohydrates, proteins, lipids and other substances) provided by the uterus. The coordinated and interactive actions of ovarian progesterone and conceptus-derived factors (interferon-τ and prostaglandins) regulate expression of elongation- and implantation-related genes in the endometrial epithelia that alter the uterine luminal milieu and affect trophectoderm proliferation, migration, attachment, differentiation and function. A comparison of sheep and cattle finds both conserved and non-conserved embryotrophic factors in the uterus; however, the overall biological pathways governing conceptus elongation and establishment of pregnancy are likely conserved. Given that most pregnancy losses in ruminants occur during the first month of pregnancy, increased knowledge is necessary to understand why and provide a basis for new strategies to improve pregnancy outcome and reproductive efficiency.

Single or Group Culture of Mammalian Embryos: The Verdict of the Literature

During infertility treatment with IVF, embryos are cultured either in groups or individually. Each approach has potential benefits and detriments, and the purpose of this review is to try to come to a consensus based on the literature as to which approach yields superior results. Group culture of embryos may produce better quality embryos via secretion of embryotrophic factors, while opponents of the approach argue that embryos cultured together may either deplete the media of substrates or negatively affect nearby embryos via the transmission of other secreted factors. In these cases, quantity of embryos, volume of media and proximity and quality of companion embryos are also important factors to consider. While it has long been accepted that group culture is beneficial for embryos from various animal species, emerging data also suggest a similar benefit in the human. Conversely, embryos cultured individually avoid potential substrate depletion, negative impact from factors secreted from companion embryos, while more practically permitting the ability to monitor and track the embryo for identification via morphology or molecular analysis to select and transfer potentially superior embryos. Importantly, advancements in embryo culture platforms now permit tracking of individual embryos, while also offering ability to reap the benefits of group culture. These approaches utilize confined microenvironments immediately surrounding the embryos that may be conducive for periodic sampling/analysis, while also allowing access to a larger media reservoir to avoid substrate depletion. Thus, though questions remain as to optimal embryo density and volume of culture media, group embryo culture in the correct culture platform is likely to be superior to individual embryo culture.

The oviduct and development of the preimplantation embryo

Fertilization and early embryo development take place in the oviduct in vivo. Relative to studies in other reproductive organs, the importance of the oviduct has been ignored for many years because pregnancies can be obtained in assisted reproduction treatment using in-vitro fertilization (IVF) and embryo transfer to the uterus without involving the Fallopian tube. After the reports on the beneficial effect of oviductal cells on embryo development in sheep and subsequently in human, and a practical need to improve the success rates in clinical assisted reproduction, there was a period when more research was performed on the Fallopian tube. Many of these studies used in vitro coculture systems to emulate the in vivo environment in vitro, and to search for oviduct-derived embryotrophic factors. With the recent development of sequential culture to improve embryo development in vitro, the use of coculture in assisted reproduction and its related research declined because routine use of coculture is laborious and experience-dependent.