Regulation of the number of follicles maturing and ovulating during the 5-day cycle in the rat

In mammals the number of follicles, rupturing and extruding their ovum during an oestrous cycle (ovulation rate, Land, 1970), varies widely from species to species. In many species, including man, bats and elephants, as a rule only one ovum is extruded from the ovaries at each oestrus. On the other hand, in rodents, insectivores, pigs and carnivores a number of eggs is released simultaneously at each oestrus. Within any given species there is a tendency to release a constant number of ova during each cycle (Brambell, 1966). The study of the mechanisms controlling the number of ova shed per cycle is not only of academic interest. It may also lead to practical applications in at least two important fields. Firstly, it might provide further improvements in treatment of that category of infertile women, in which follicular maturation and ovulation fails to occur. Treatment of these patients with exogenous gonadotrophins results, at present, rather frequently in overstimulation of the ovaries, followed by ovarian cyst formation or multiple pregnancies (Gemzell & Roos, 1967; Gemzell & Johannsen, 1970). Secondly, control of the number of eggs ovulated has potential uses in animal husbandry, such as the multiplication of offspring from selected animals (Hammond, 1961).