ANALYSIS OF BLOOD-VESSEL MATURATION PROCESSES DURING CYCLIC OVARIAN ANGIOGENESIS

Cyclic angiogenic processes in the ovarian corpus luteum (CL) of monov ulatory species are characterized by distinct phases of blood vessel g rowth, vessel maturation, and vessel regression. To characterize molec ular and cellular systems that may play a role in regulating blood ves sel maturation, we have (a) analyzed the spatiotemporal expression of vascular endothelial growth factor (VEGF) and its receptors VEGF-R1 (F lt-1) and VEGF-R2 (Flk-1) throughout the ovarian cycle, (b) examined t he recruitment of pericytes during vessel maturation, and (c) quantita tively measured the ratio of angiopoietin-2 (Ang-2) to angiopoietin-1 (Ang-1) throughout the ovarian cycle. The data indicate that the VEGF/ VEGF-receptor system is expressed not only during ovarian angiogenesis , but also with similar intensity in the nonangiogenic midstage CL. In fact, VEGF is expressed through most of the ovarian cycle, only being down-regulated during luteolysis, which leads to regression of the CL neovasculature. Pericytes are recruited soon after the induction of C L angiogenesis following the front of invading endothelial cells. Base d on a double-staining immunohistochemistry technique, we developed a microvessel maturation index (MMI) that reflects the percentage of the capillary neovasculature that is associated with pericytes. The MMI i n the angiogenic corpus rubrum is approximately 0.60. This value is no t significantly higher in the nonangiogenic midstage CL but increases to close to 0.90 during CL regression. Lastly, an RT-PCR analysis of A ng-1 and Ang-2 expression revealed that both molecules are expressed t hroughout the ovarian cycle. The quantitative Ang-2/Ang-1 ratio does, however, change from 1.34 in the angiogenic CL and 1.07 in the midstag e GL to 7.59 during CL regression, reflecting the strong overexpressio n of Ang-2 over Ang-1 during blood vessel regression. Taken together, the data support a model of a transiently maturated vasculature in the midstage CL, which is characterized by VEGF and pericyte contact-medi ated endothelial cell survival and an induction of blood vessel regres sion during luteolysis that is characterized by the down-regulation of VEGF and the up-regulation of Ang-2.