Luteal Function and Estrous Cycles in Mammals

Some rodent species such as rats and mice recur incomplete estrous cycles, which are characterized by the lack of the luteal phase. Among animals that recur complete estrous cycles, ruminants have been shown to exhibit one or two events of follicular growth not resulting in ovulation (latent follicular waves) during the luteal phase. We investigated follicular dynamics ultrasonically in Shiba goats and found that they also showed predominantly two latent follicular waves with the third being the ovulatory wave. When goats were treated with a luteolytic dose of prostaglandin (PG) F2α 3 and 4 days after spontaneous ovulation, dominant follicles of the first wave ovulated 2 or 3 days later. Further, ovulation could be repeated at least four times at the interval of around 6 days by successive PGF2α treatments. Thus, the female goat appear to be furnished with every necessary mechanisms for recurring incomplete estrous cycles like rodents if progesterone levels are depleted shortly after the formation of corpora lutea. In rodents, 20α-hydroxysteroid dehydrogenase (20α-HSD), an enzyme which catabolizes progesterone to a bio logically inactive steroid 20α-dihydroprogesterone, plays a critical role in the maintenance of incomplete estrous cycles. Comparative analysis of genomic DNA for 20α-HSD, especially its promoter regions, will provide molecular basis for the differences in phenotypes of the estrous cycle and the evolution of estrous cyclicity in mammals.