Differences in trophectoderm mitotic rate and P450 17alpha-hydroxylase expression between late preimplantation...

We have previously demonstrated that Meishan preimplantation conceptuses contain fewer cells and produce less estradiol-17beta (E2) than do Yorkshire conceptuses on the same day after ovulation. We have also shown that the apparent rate-limiting enzyme for biosynthesis of E2, P450 17alpha-hydroxylase/17-20 lyase (P450c17), is maximal on Day 11.5 of gestation in both Meishan and Yorkshire conceptuses, with the level of enzyme expression being significantly greater in Yorkshire as compared to Meishan conceptuses. Other authors have suggested that insulin-like growth factor-I (IGF-I) from the pig endometrium is stimulated by conceptus E2 that may function to stimulate additional conceptus steroidogenic enzyme activity. In order to better understand developmental differences between these two breeds, conceptuses 2, 4, 6, and 8 mm in diameter were collected and fixed in 10% neutral buffered formalin. Uterine flushings associated with these conceptuses were used for quantification of E2 and IGF-I. Techniques were developed in processing the fixed conceptuses so as to eliminate excessive convolutions of the trophectoderm. Conceptuses could then be embedded in paraffin in an "inflated" state, which allowed orientation of the conceptus with respect to the embryonic disk. Sections of these conceptuses were then immunocytochemically stained for proliferating cell nuclear antigen (PCNA) and P450c17. At all sizes examined, Meishan conceptuses exhibited a reduced trophectoderm mitotic rate when compared to Yorkshire conceptuses as measured by PCNA immunostaining. Meishan conceptuses were first observed to stain positively for P450c17 at a diameter of 4 mm, whereas Yorkshire conceptuses were first observed to stain positively for P450c17 when they had reached a diameter of 6 mm. E2 in the uterine flushings from both breeds was low and similar if the average littermate conceptus diameter was 2 or 4 mm, but was markedly increased in association with litters of 6- or 8-mm conceptuses, with E2 in flushings from Yorkshire females increasing more than Meishan females. The concentration of IGF-I was consistently elevated in flushings from Yorkshire as compared to Meishan females. Embryonic E2 production is thought to influence conceptus survival; therefore, the reduced trophectoderm mitotic rate and/or uterine luminal IGF-I concentration of the Meishan, as compared to Yorkshire, may limit E2 synthesis--a limitation that could lead to decreased conceptus mortality in this breed.