Reproduction Supplement 58, 47 - 63 Formation and early development of the corpus luteum in pigs B. D. Murphy1, N. Gévry1, T. Ruiz-Cortés1, F.Cote2, B. R. Downey2 and J. Sirois1 'Centre de Recherche en Reproduction Animale, Facultel de Médecine Véterinaire, UniversitE de Montréal, 3200 Rue Sicotte, St-Hyacinthe, Québec J2S 7C6, Canada; and 2 Department of Animal Science, McGill University, Ste-Anne de Bellevue Québec Canada Numerous corpora lutea form from the multiple follicles that ovulate during the oestrous cycle of pigs. Vascular elements invade the follicle from the theca compartment, first centripetally, and subsequently by lateral branching of centripetal veins and arteries. The vessels are the vehicle for dispersion of steroidogenic theca cells throughout the corpus luteum. Mitosis occurs in both the theca and granulosa layers before ovulation, and in luteal cells well into the luteal phase. Luteal cell proliferation undergoes gradual restriction as the corpus luteum matures, but the mechanisms of exit from the cell cycle are unknown. The extracellular ligands that direct luteinization and maintain the corpus luteum include LH, prolactin, insulin and insulin-like growth factors (IGFs). These ligands induce qualitative and quantitative changes in steroid output, with progesterone as the principal product. These changes upregulate the cholesterol synthetic pathways to increase substrate availability. The intracellular regulation of luteinization is complex. A model is presented in which LH stimulates arachidonic and lineoleic acid metabolism to produce ligands for the nuclear proteins of the peripheral peroxisome activator receptor family. These ligands have downstream effects on cell differentiation and exit from the cell cycle. Luteal function is maintained by interactions among ligands, cholesterol regulatory proteins and constitutively expressed and regulated transcription factors. Introduction The success of gestation in the pig is entwined inextricably with the establishment and maintenance of the corpus luteum. This complex transitory organ develops from follicles as a consequence of ovulation and secretes the progesterone necessary for creating a uterine environment hospitable for survival of embryos and fetuses. Pigs differ from ruminants because the corpus luteum is required for the entire 16 weeks of gestation. Its importance is underlined by the fact that its principal synthetic product, progesterone, can, by itself, maintain gestation in ovariectomized gilts. The focus of this review is the process of luteinization and its aim is to identify the principal unanswered questions about luteal differentiation in pigs. Where possible, answers to these questions are proposed. Corner (1919) described both the theca and granulosa precursors of luteal cells and the Email: murphybigniedvaumontreal.ca 02001 Society for Reproduction and Fertility