Ovarian follicular growth during the oestrous cycle in two breeds of ewes of different ovulation rate, the D'Man and the Timahdite.

The population of growing follicles of diameter greater than 60 micron was studied in 24 D'Man and 24 Timahdite ewes aged 2-2.5 years. The ovaries of each ewe were removed on Days 1, 2, 3, 4, 5, 7, 9, 11, 13, 15 or 17 of the oestrous cycle. Two ewes from each breed were ovariectomized. The mean ovulation rate was 2.9 in D'Man and 1.1 in Timahdite ewes and the mean numbers of normal follicles were 276 and 230 respectively. The distribution of the various sizes of follicles did not differ significantly between the two breeds. However, the number of follicles of diameter 118 to 462 micron (follicles beginning to develop an antrum) and the number of follicles of diameter greater than 1285 micron (large follicles possessing an antrum) were significantly greater in the D'Man than in the Timahdite ewes (136 and 106, and 7.1 and 4.5, respectively). The mean number of atretic follicles was similar for the two breeds (13.3 in the D'Man; 12.2 in the Timahdite), but in the D'Man there was a lower rate of atresia (54.9 compared with 66.7%) for follicles of 1084-2141 micron diameter. During the first days of the cycle there was a significant decrease in both breeds in the number of follicles of 60-118 micron diameter. During this same period there was a significant decline in the number of follicles of diameter 119 to 462 micron but in the D'Man ewes only. Therefore, the higher ovulation rate of the D'Man breed is due to the greater number of those follicles from which ovulatory follicles are recruited.