Evidence that the conceptus contributes to the inhibition of follicular growth in the ewe

The present study was designed to determine the contribution of the gravid uterus and the conceptus in the inhibition of antral follicular growth. All antral follicles were counted and measured on day 48 post mating in ovaries of pregnant ewes (Group 1, n = 5) and in ovaries of ewes from which the uterus (Group 2, n = 5) or the conceptus (Group 3, n = 5) had been surgically removed on days 32-33 post mating. The secretory activity of the corpora lutea was maintained after removal of the uterus or the conceptus until slaughter of ewes. No difference was found between the three groups of ewes in the mean (+/-SEM) total number (non-atretic and atretic) of follicles per ewe, being 59.8 +/- 7.6, 35.0 +/- 7.6 and 50.8 +/- 4.6, respectively for Groups 1, 2 and 3. The mean (+/-SEM) number of follicles 2 mm or more in diameter was greater (P < 0.01) in Group 2 ewes (3.4 +/- 0.5) and Group 3 ewes (4.2 +/- 0.5) than in Group 1 ewes (1.6 +/- 0.4). Of all follicles counted which were 2 mm or larger, 90% in Group 1 were atretic, this value being higher (P < 0.05) than in Group 2 (68%) and Group 3 (55%). The mean (+/-SEM) diameter of the largest (F1) and second largest (F2) follicles was greater (P < 0.01) in Group 2 (F1, 4.0 +/- 0.4; F2, 3.2 +/- 0.4) and in Group 3 (F1, 3.9 +/- 0.2; F2, 3.3 +/- 0.2) than in Group 1 (F1, 2.6 +/- 0.1; F2, 2.2 +/- 0.1). The mean diameter (+/-SEM) of the largest non-atretic follicles was greater (P < 0.01) in Group 2 (4.0 +/- 0.4) and Group 3 (3.9 +/- 0.2) than in Group 1 (1.7 +/- 0.3). In conclusion, the results from this in vivo experiment provide evidence that the conceptus prevents the development of large healthy follicles by inducing atresia during the first trimester of pregnancy. The inhibition of follicular growth would reduce oestradiol production and permit the maintenance of luteal function during early pregnancy.