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Intense angiogenesis, vascular remodelling as well as regression of its vasculature are prerequisites for ovarian function with its cyclically developing and regressing follicles and corpora lutea. So far neither a stringent explanation for the enormous angiogenic potential of the ovary nor its cellular origins have been suggested. In an earlier study of our work group, endothelial cells were isolated from the bovine corpus luteum and cultivated in vitro. They performed vasulogenesis in vitro and showed properties of progenitor cells. The present study aimed at in situ identification of endothelial progenitor cells (EPCs) in the bovine ovary. Immunohistochemical examinations, based on the detection of KDR and CD34 co-labelled cells - a marker combination that amongst others is commonly accepted as typical for EPC identification - were performed. Hormonal cycle dependent expression varieties were analysed by the measurement of mRNA amounts of CD34 and KDR as well as the stem cell marker CD133 (Prominin-1). Ovarian samples comprising corpora lutea of varying stages (developing and mature corpus luteum, corpus luteum in regression, corpus luteum of pregnancy) from 17 adult cows were examined. Results show that specific mRNA of CD133, CD34 and KDR was expressed in ovaries of all luteal stages. Expression data analysis revealed significant differences in CD133 and CD34 expression levels between the luteal stages but no significant differences in KDR expression. CD34/KDR co-immunoreactive cells were predominantly situated within the media of arterial vessel wall. The detection of ovarian EPCs represents an important step towards further understanding of the mechanisms involved in the reproductive biology and pathophysiology of the ovary.