Types of mammalian ovary

Abstract

We can distinguish among mammals at least three types of ovaries. They are represented among the rodents by the ovaries of the guinea pig, rat and rabbit.

(1). In the first type, that of the guinea pig, in the oestrus period, preceding ovulation by a number of hours, the large majority of all follicles—all but the very small ones—begin to degenerate. In the larger follicles the granulosa becomes karyorrhectic en masse and connective tissue begins to grow into the follicular cavity after the destruction of the granulosa epithelium has progressed still further. In smaller follicles the process of destruction of the granulosa is less obvious and the granulosa cells disappear and here also connective tissue soon begins to invade the cavity. In the week following ovulation the very small follicles develop again and pass through their cycle; about eight days after ovulation the follicles have again reached full size and maturation on the one hand and atresia on the other hand can set in; this atresia usually affects only the large follicles.

(2). In the second type, that of the rat, ovulation is not preceded by a general atresia of follicles, but the large follicles mature, rupture and become converted into corpora lutea. There takes place throughout the cycle a limited atresia of follicles. In the rat the ingrowth of connective tissue into the atretic follicles is less active than in the guinea pig, in consequence of which the cavity of the degenerating follicle remains preserved during a longer period of time in the former species. In both guinea pig and rat ovulation is spontaneous. It occurs as soon as follicles have had a chance to mature and the inhibiting influence of the corpus luteum has ceased to exert itself. In the guinea pig a considerably longer period is necessary for the maturation of the follicles than in the rat, because in the former additional time is needed to allow follicles to develop from very small to large size, while in the rat only the very large follicles are destroyed at the time of ovulation and therefore maturation of a new set of follicles can take place in a very short time.