USDA, ARS, U.S. Meat Animal Research Center, Clay Center, NE 68933, USA.
J Anim Sci. 2013 Jul;91(7):3030-40. doi: 10.2527/jas.2012-6138.
Conceptus losses in livestock occur throughout gestation. The uterus and the embryo-placenta-fetus play interconnected roles in these losses, the details of which depend on the period of gestation and the species. Studies in sheep and pigs have indicated that the uterine glands are essential for full fertility, based on experiments where gland development was reduced through the use of exogenous hormones. In sheep and cattle, normally the uterus is well able to support more than a single fetus although these species differ in the consequences of multiple births. When 2 conceptuses are present, the placentas of cattle often anastomose, putting 1 fetus at risk if the other is lost. One likely reason this does not occur in sheep is because sheep embryos undergo intrauterine migration, similar to pigs. In pigs, the relatively equidistant separation of conceptuses is likely to be essential for optimizing conceptus survival as is the simultaneous and uniform elongation of blastocysts that occurs during the time of maternal recognition of pregnancy. Other studies in pigs have indicated that the size of the uterus influences litter size and therefore fetal losses. In response to crowded intrauterine conditions in the pig, increased conceptus losses begin to occur between d 30 and 40 of pregnancy, and further losses occur sporadically during later gestation. There is evidence that improved fetal erythropoiesis can reduce these losses. Other studies indicated that profound changes in placental development occurred under crowded intrauterine conditions that may contribute to losses during late gestation. Reductions in placental stroma formation may compromise the ability of the pig placenta to adapt to reduced uterine space. Consistent with this, both hyaluronan and hyaluronidase activity are decreased in the placentas of small compared with large fetuses. These results indicate that improvements in placental stroma formation could improve placental ability to compensate for reduced intrauterine space, resulting in increased placental function and reduced fetal losses during late gestation.
家畜的胚胎损失发生在整个妊娠期。子宫和胚胎-胎盘-胎儿在这些损失中起着相互关联的作用，其细节取决于妊娠期和物种。绵羊和猪的研究表明，子宫腺对于充分的生育能力是必不可少的，这是基于通过使用外源激素减少腺发育的实验得出的。在绵羊和牛中，正常情况下子宫能够很好地支持一个以上的胎儿，尽管这两个物种在多胎分娩的后果上有所不同。当有两个胚胎存在时，牛的胎盘通常会吻合，如果另一个胚胎丢失，就会使一个胎儿面临风险。这种情况在绵羊中不太可能发生的一个可能原因是绵羊胚胎会在子宫内迁移，类似于猪。在猪中，胚胎相对等距的分离可能对优化胚胎存活至关重要，就像在妊娠识别期间发生的同时和均匀延长囊胚一样。猪的其他研究表明，子宫的大小会影响产仔数，从而影响胎儿的损失。在猪中，由于子宫内条件拥挤，妊娠第 30 至 40 天之间开始出现更多的胚胎损失，并且在后期妊娠期间会零星地发生进一步的损失。有证据表明，改善胎儿的红细胞生成可以减少这些损失。其他研究表明，在拥挤的子宫内环境下，胎盘发育发生了深刻的变化，这可能导致后期妊娠期间的损失。胎盘基质形成的减少可能会损害猪胎盘适应子宫内空间减少的能力。与这一点一致的是，与大胎儿相比，小胎儿的胎盘透明质酸和透明质酸酶活性均降低。这些结果表明，改善胎盘基质形成可以提高胎盘适应减少子宫内空间的能力，从而提高胎盘功能，减少后期妊娠期间的胎儿损失。
