Early embryo loss, morphology, and effect of previous immunization against androstenedione in the ewe

Using transrectal ultrasound to examine the effect of exogenous progesterone on early embryonic loss in sheep

“…Indeed, low levels of prenatal loss have been reported to occur throughout ovine gestation [ 11 , 32 ], yet they were not detected in this study. Nevertheless, these results agree with previous studies in sheep, who report that the majority of embryonic loss occurs prior to day 30 [ 1 , 6 , 18 , 38 ], with most loss concentrated to the period immediately preceding day 19 [ 39 ]. Implantation in the ewe is generally perceived to occur between days 14–16 [ 7 , 27 , 40 – 42 ] but has been reported to occur as early as Day 10 [ 43 ] and as late as day 18 [ 44 ].…”

“…This could be due to an unsupportive maternal environment [ 6 ] owing to imbalanced hormone levels [ 6 , 16 , 26 ], maternal nutrition [ 12 , 13 ] or age [ 14 ]. In regards to the current study, it is likely that a combination of these factors are at play, increasing consideration of the idea that the quality of the oocyte and its developmental potential plays a more pivotal role in influencing the success of early embryonic development [ 38 , 45 ]. Often fertilisation is achieved, yet errors or lethal effects do not occur until downstream, such as during implantation.…”

“…Unfortunately in the current study, it wasn’t possible to cull ewes and collect embryos following identification by TRUS, as this would have provided a more detailed comparison of the characteristics of viable and non–viable embryos. Fortunately, O’Connell et al, [ 38 ] described such a comparison when they saw an embryonic loss of 12% during day 4–14 of gestation. Here the author hypothesises that the adhesive or signalling properties of degenerate embryos could differ from viable ones and could therefore be subjected to high rates of phagocytosis by the maternal immune system [ 38 ].…”

“…Fortunately, O’Connell et al, [ 38 ] described such a comparison when they saw an embryonic loss of 12% during day 4–14 of gestation. Here the author hypothesises that the adhesive or signalling properties of degenerate embryos could differ from viable ones and could therefore be subjected to high rates of phagocytosis by the maternal immune system [ 38 ]. In a more applied study, Bari et al, [ 46 ] reported that during embryo transfer, grade 1 and 2 embryos survived significantly better than grade 3 or 4 embryos, again confirming that successful implantation is not solely influenced by the maternal environment.…”

The Role of Oocyte Organelles in Determining Developmental Competence

“…In sheep and cattle, between 20% and 40% of embryos are lost during the first two weeks of pregnancy [ 8 , 9 ]. O’Connell et al [ 9 ] reported the majority of early embryo loss in sheep occurs between Days 4 and 14 of gestation (12%), with 6% lost before Day 4 of gestation and minimal losses between Days 14 and 30. This suggests the importance of both oocyte quality and the oviductal environment for embryo survival prior to implantation.…”