26 Embryo transfer outcomes and placental vascular development in sheep: breed interactions

Abstract Infertility is a major barrier to reproductive efficiency in livestock. Early pregnancy is a critical window for placental vascular development, which is essential for embryonic survival and fetal growth. This study evaluated the individual and combined effects of embryo and ewe breed on pregnancy establishment and placental vascularization in sheep. A total of 85 embryo transfers (ET) were performed using straight-bred and reciprocal combinations of Rambouillet (Rambo) and Romanov (Romo) ewes. Pregnancy was assessed on day 24 of gestation. Straight-bred groups showed the greatest pregnancy rates: Rambo × Rambo (31.3%) and Romo × Romo (26.7%). The reciprocal Rambo × Romo group had the lowest pregnancy rate (11.8%), while Romo × Rambo was intermediate (22.7%), indicating a tendency for breed-dependent effects (P=0.10). To assess vascular development, uterine cross-sections were collected at slaughter on day 25 and stained with CD31 and CD34 to quantify capillary area density (CAD) and capillary number density (CND) in the caruncle (CAR), fetal membrane (FM), and inter-caruncular (ICAR) regions. compared to the other groups, Rambo × Romo had increased CAD in CAR and ICAR regions (P<0.01), while Romo × Romo had reduced CND in CAR (P<0.05), indicating differences in vascular development due to embryo and ewe breed. These vascular differences may underlie reduced pregnancy success after ET. This study highlights a link between fertility and early placental vascularization, offering insight into mechanisms impacting pregnancy maintenance. Understanding these effects could lead to breeding strategies to improve reproductive outcomes in sheep.