Total follicular populations in ewes of high and low ovulation rates

Abstract

The total ovarian follicular populations were studied in 2 breeds of ewes which differed greatly in their ovulation rates. Thus 8 Romanov (mean ovulation rate 3.1) and 12 Ile-de-France ewes (mean ovulation rate 1.4) were ovariectomized at estrus during the breeding season. Each right ovary and 3 left ovaries were sectioned at 7 .mu.m and examined microscopically. The number of small follicles, i.e., with 2 or less layers of granulosa cells, was estimated by a tested sampling procedure while all larger follicles were measured and arranged into classes. There were half as many small follicles but 1.5-2 times more large follicles in the ovaries of the Romanov ewes compared to those of the Ile-de-France ewes. The number of atretic follicles was approximately the same in both breeds and does not explain the difference observed in ovulation rate. The higher ovulation rate in the Romanov ewe apparently is due to the greater number of large follicles available to be stimulated for ovulation.