Maternal-conceptus signalling during early pregnancy in mares: oestrogen and insulin-like growth factor I

Embryonic production of oestrogen is thought to play an important role in conceptus-maternal signalling during early pregnancy in mares, and may be regulated in an autocrine or paracrine fashion by insulin-like growth factor I (IGF-I). In this study, the hypothesis that IGF-I stimulates embryonic oestrogen synthesis, which in turn stimulates uterine IGF-I secretion was tested. Specific sources of IGF-I in the uterine lumen were characterized. Preimplantation embryos, uterine biopsies, and uterine flush fluids were collected on day 13 of pregnancy. Embryos were cultured whole for 24 h, or dispersed and incubated in serum-free culture medium supplemented with androstenedione or testosterone (0-10 microg ml(-1)) and IGF-I (0-100 microg ml(-1)). Oestrogen synthesis was increased by addition of androgen, but there was no dose-dependent effect of IGF-I. Endometrial explants were cultured for 24, 48 and 72 h in serum-free medium supplemented with oestradiol. IGF-I was measured by radioimmunoassay in embryo-conditioned medium, explant culture medium, blastocoelic fluid, concentrated (x 100) uterine flush fluid and endometrial-tissue homogenate. Both the embryo and endometrium produced significant quantities of IGF-I, indicating a role for this growth factor in autocrine-paracrine signalling during early pregnancy. However, secretion of IGF-I by endometrial explants was not modulated by oestrogen.