Angiogenesis in the human corpus luteum: Localization and changes in angiopoietins, Tie-2, and vascular endothelial growth factor messenger ribonucleic acid

In the menstrual cycle, extensive angiogenesis accompanies luteinization. During luteolysis, endothelial cells die, whereas in a conceptual cycle, the corpus luteum (CL) persists, and endothelial cell survival is extended. A main stimulator for angiogenesis is vascular endothelial growth factor (VEGF), while the angiopoietins (Ang-1 and Ang-2) may be important modulators. The aim of this study was to investigate the localization of Ang-1, Ang-2, their common receptor Tie-2, and VEGF messenger ribonucleic acid (mRNA) at the different stages of the functional luteal phase and after rescue by hCG. Ang-1 mRNA was uniformly expressed at a low level throughout the CL. The signal was highest during the early luteal phase. In contrast, Ang-2 mRNA expression was localized strongly to individual granulosa and thecal luteal and endothelial cells. Administration of hCG was associated with an increase in the Ang-2 mRNA area of expression and grain density in individual luteal and endothelial cells. The Tie-2 receptor mRNA was localized in endothelial cells, and the area of expression was highest during the early luteal phase and during luteal rescue. VEGF mRNA was found exclusively in granulosa luteal cells, and the area of expression was highest in corpora lutea during simulated pregnancy. These results begin to characterize the molecular regulation of the divergent processes involved in luteal angiogenesis during luteinization, luteolysis, and rescue in the human and imply that the angiopoietins are involved during the initial angiogenic phase and in luteal rescue.