Ovogenesis in the Ewe and Cow

Abstract

The evidence presented by Allen 1 did much to establish the concept that ovogenesis extends into the postpuberal period in mammals. He found that the germinal epithelium contributed new ova in the adult mouse and, further, that the rate of ovogenesis varied according to the stage of the oestrous cycle. The greatest number of mitotic figures was found in the germinal epithelium during oestrus. Allen and coworkers 2 found in the sow that there was an elimination of most of the larger follicles in the ovaries coincident with the maturation of a set of follicles. Engle, 3 working with the mouse, found a greater number of both small and large follicles in active stages of degeneration during oestrus and early metoestrum than at other stages, although the actual differences found were relatively small. In a study of ovogenesis and follicular atresia in the rat, guinea pig, dog, cat, and man Evans and Swezy 4 concluded that the maturation of one or more follicles involved the destruction of all remaining follicles and that after ovulation a new ovogenetic wave began.

Twenty-five pairs of ewe ovaries and a slightly smaller number of cow ovaries, removed at various phases of the oestrous cycle, have been carefully studied. From some of these complete series; from others only a few sections were made.

No evidence has been found which would indicate that the new ova arise from the germinal epithelium in the sexually mature ewe or cow. Rather, it appears that they arise in the neogenic layer beneath the ovarian tunic in a manner very similar to that described by Simpkins 5 for the human. As clear-cut evidence has been presented by Allen 1 in the mouse and by Evans and Swezy 4 in the rat, guinea pig, dog, and cat that ova do arise from the germinal epithelium in mature animals, it would appear that species or group differences exist in this respect.

Healthy and atretic follicles are found in the ovaries at all times in approximately the same proportions. If there are differences it will be necessary to make actual counts to determine them. Small differences in atresia such as Engle found in the mouse might easily have been overlooked. Marked differences in the number of atretic follicles in the ovaries of different individuals removed during the same stage of the cycle do exist.