Role of Estrogen and the Placenta in the Maintenance of the Rabbit Corpus Luteum

The rabbit is among those species in which the placenta secretes low or physiologically insignificant quantities of progesterone, and therefore, the corpora lutea must remain steroidogenically active throughout gestation (Hilliard, 1973; Thau and Lanman, 1974). If the young embryo is to survive, it must transmit a signal that in some way halts or overrides incipient luteal regression, reflected in declining serum progesterone values by 15 days after ovulation in non-pregnant animals (Keyes et al., 1983a). The subject of this manuscript is an exploration of the mechanisms that are responsible for placental maintenance of luteal function. We begin with a brief summary of the literature, setting the stage for the specific hypotheses and experiments reported herein.