EFFECTS OF PREGNANCY AND HYSTERECTOMY ON ANTRAL FOLLICULAR POPULATION AND GROWTH IN THE EWE

Antral follicles were counted and measured on Day 120 post-oestrus in ovaries of four pregnant ewes (group P) and in ovaries of four ewes from which the gravid uterus was removed at Day 31 post-oestrus (group H). After hysterectomy, the secretory activity of corpora lutea (CL) was maintained until slaughter of ewes on Day 120 post-oestrus. Plasma progesterone concentrations remained at more than 1.5 ng ml-1 during the whole period of CL maintenance (89 days). No difference was found between Group P and Group H in mean total number of follicles per ewe (70.3 vs. 73.3). The pregnant ewes developed four atretic follicles more than 1.5 mm in diameter, none of which exceeded 1.7 mm. In contrast, the hysterectomised ewes had 57 follicles more than 1.5 mm, 21 of which exceeded 2.0 mm. Of all the follicles more than 1.5 mm counted in hysterectomised ewes, 33% were found to be healthy. The mean (+/- SEM) diameter of the largest non-atretic (3.3 +/- 0.1 mm) and atretic (3.0 +/- 0.2 mm) follicles in Group H ewes was significantly greater (P < 0.001 for the non-atretic and P < 0.002 for the atretic follicles) than that of the largest non-atretic (1.2 +/- 0.1 mm) and atretic (1.5 +/- 0.2 mm) follicles in Group P ewes. These results suggest that the uterus and/or the conceptus prevent the growth of antral follicles beyond a diameter of 2 mm at least during the last trimester of pregnancy.