Equine endometrial vascular pattern changes during the estrous cycle examined by Narrow Band Imaging hysteroscopy

The aim of this study was to evaluate the uterine blood supply and endometrial vessel architecture, during the equine estrous cycle. Narrow Band Imaging (NBI) hysteroscopy was used for evaluating changes in the endometrial vasculature during the estrous cycle [six mares, d 0 (representing the day of ovulation), d 6 and 11 in four locations]. In addition, endometrial biopsy samples were used for immunodetection of markers for angiogenesis (Vascular Endothelial Growth Factor A, its receptor 2, as well as angiopoietin-2 and its receptor-tyrosine-kinase Tie2) during the estrous cycle (three mares, d 0, 5 and 10; one biopsy per mare). Detailed analysis of hysteroscopic images revealed an increase in the vascular density from estrus towards diestrus. In contrast, microscopic specimens prepared from biopsies revealed no evidence for changes in the endometrial vessel number during the estrous cycle. Studies on expression of angiogenesis markers indicated that cyclic changes in the endometrial vascular density observed by NBI-hysteroscopy were not due to formation of new vessels. It is concluded that vessels are involved in blood supply of a smaller area during diestrus, facilitating better distribution of nutrients during this phase.