Ovarian follicular dynamics in lines of sheep (Finn, Merinos) selected on ovulation rate.

Ewes from selected lines of sheep from each of two breeds, Finns (high ovulation rate, low ovulation rate and control lines with respective ovulation rates of 5.4, 2.7 and 3.3) and Merinos (T Merinos selected for increased ovulation rate and control Merinos with respective ovulation rates of 1.9 and 1.2) were used to examine how selection to alter ovulation rate had altered follicle development. Ovarian antral follicles were counted, measured, classified as nonatretic or atretic (more than five pyknotic bodies). The growth of ovulatory follicles in vivo, followed by repeated follicle ink marking, also was compared in the three lines of Finns. Regardless of breed, ewes selected for high ovulation rate had a similar number of antral follicles and a similar extent of atresia compared with their controls. Alterations induced by selection were located in the last stages of folliculogenesis. T Merinos exhibited a lower proportion of atretic follicles among follicles greater than 3 mm and a larger diameter of the largest healthy follicle when preovulatory follicles were excluded. High-line Finn ewes recruited more follicles, which produced smaller preovulatory follicles, each containing a smaller number of granulosa cells compared with either the low- or control-line ewes. Hence, physiological selection for high ovulation rate raised it by different methods in Merino than in Finn ewes.