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Despite standardized cultivation conditions different morphological forms of vascular development and different angiogenic potency were detected in endothelial cells of the corpus luteum in vitro. The aim of the present study was a comparative investigation of these luteal endothelial cell cultures by light and electron microscopy as well as immunocytochemistry including the use of endothelial and endothelial progenitor markers (CD34, VEGF-R2 and CD31). Screening of cellular proliferation in endothelial cells from regressing corpus luteum (ECreg) showed that vascular development in vitro followed the steps of the angiogenic cascade which starts with a linear alignment of cells, followed by circular side-by-side arrangement. In contrast endothelial cells from developing corpus luteum (ECdev) showed highly active cells and in vitro vascular development was initiated by specific cell groups, so-called "Vascular Initiation Points" (VIP). These cells showed intense proliferation, built typical cellular clusters reminiscent of de novo vasculogenesis and organized into capillary-like tubes with an internal lumen as shown by electron microscopy.