Role of the Corpus Luteum and Progesterone in the Evolution of Vertebrate Viviparity

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Abstract
For the past 25 years we have used a comparative strategy designed to identify anddescribe the endocrine parameters of the oviparous-vivparous transition and subsequent gradual reduction in hepatic yolk protein precursor (vitellogenin) synthesis associated with placental viviparity. Our approach has been to study vertebrate groups in which both oviparous and viviparous modes are common (reptiles, elasmobranchs). We have provided evidence for the control of follicular (granulosa/theca) and luteal steroidogenesis, and the cellular basis of gonadal steroid hormone action on the key target tissues (oviduct, liver). Our results, some of which are summarized below, have led us to suggest that ovarian progesterone (follicular or luteal in origin) has a dual role in the evolution of viviparity: 1. To inhibit myometrial contractions, thus providing a primary condition for egg retention and viviparity. 2. To inhibit estrogen-induced hepatic vitellogenin synthesis as part of both normal oviparous cycles and as a concomitant of placental evolution.

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