Roadmap to pregnancy during the period of maternal recognition in the cow: Changes within the corpus luteum associated with luteal rescue.

A viable corpus luteum (CL) producing an adequate amount of progesterone is absolutely essential for establishment and maintenance of pregnancy. One function of embryonic signaling to the mother is to ensure that the CL is maintained. In ruminants, the secretion of uterine prostaglandin F2alpha (PGF2A) is the signal that initiates luteolysis. Despite many studies to determine if PGF2A secretion from the uterus is altered in early pregnancy, conflicting interpretations have led to controversy regarding the exact mechanisms by which maternal recognition of pregnancy is achieved. In addition to alteration of uterine prostaglandin metabolism, changes within the CL itself may facilitate the establishment of a successful pregnancy. These changes include alteration of luteal blood flow, prostaglandin metabolism, sensitivity to prostaglandin actions, increased steroidogenic capacity, significant intracellular molecular changes and modification of the immune cells that are within the CL. Whether these intraluteal changes are necessary to establish pregnancy is undetermined. The focus of this review will be to provide a brief historical perspective on the utero-ovarian relationships that regulate luteal lifespan and review current knowledge of the CL of pregnancy in sheep and cattle.