Activin A and follistatin during the oestrous cycle and early pregnancy in ewes

Implantation is a critical step for survival of the blastocyst and requires acceptance of the semi-allogeneic conceptus by the maternal immune system. One molecular pathway regulating this process in the maternal system involves activin. The aim of the current study was to determine whether the concentrations of activin A and follistatin (FST), an activin binding protein, differed between ewes with a lifetime history of enhanced or reduced embryonic survival. The mRNAs encoding FST and activin A (inhibin beta A subunit, INHBA) were present in the uterus and particularly abundant in the uterine luminal or glandular epithelia by day 18 of gestation. Concentrations of activin A in the systemic circulation were elevated in animals with reduced embryonic survival during the oestrous cycle and early gestation. Concentrations of activin A in uterine fluid were approximately two-fold greater on day 16 of gestation in ewes with reduced embryonic survival compared to enhanced embryonic survival. No consistent differences in FST were observed between these groups. In conclusion, FST and INHBA mRNA were both synthesised by the uterine epithelia, with strong expression observed around the time of implantation. Ewes with reduced embryonic survival had increased concentrations of activin A in both plasma and uterine fluid providing evidence for a role of activin A in embryonic survival in sheep.