Prostaglandin F2α and Progesterone Synthesis in Vitro by Bovine Corpora Lutea of Early Pregnancy

The rapid regression of the bovine corpus luteum at the 17th–19th day of the estrous cycle requires the presence of the uterus and appears to be mediated by estrogen and one or more uterine luteolysins acting locally on the ipsilateral corpus luteum (1,2). Little information is available as to how the presence of a viable embryo in the uterus causes maintenance of the bovine corpus luteum. Del Campo et al. (3) suggested a local relationship between a gravid uterine horn and the adjacent ovary in the pregnant cow. Although the corpus luteum is necessary for maintenance of pregnancy in the cow, at least until Day 236 (4), the transition of the cyclic corpus luteum to the corpus luteum of pregnancy occurs before the embryo implants. Staples et al. (5) showed that very rapid elongation of the trophoblast occurs at about Day 15, just prior to the time of cyclic corpus luteum involution. Mesoderm formation also begins at Day 15, and the bi-nucleated giant cells of the trophoblast (6) first appear at this time (5). Greenstein et al. (7) suggested that these cells may play a role in stimulating maintenance of the corpus luteum of pregnancy. These facts suggest that the developing embryo either inactivates the normal uterine luteolytic mechanism, or produces a potent luteotrophin that overcomes this mechanism.