Endothelial cells in the intact adult are, apart from those in the female reproductive organs, believed to be quiescent. Systematic examination of endothelial cell proliferation in male reproductive organs has not been performed and was therefore the aim of the present study. Intact adult rats were either pulse labeled or long-term labeled with bromodeoxyuridine to label proliferating cells. The roles of Leydig cells and testosterone were examined after castration or treatment with the Leydig cell toxin ethane dimethane sulfonate (EDS) and testosterone substitution. After perfusion fixation, all blood vessels remained open and were easily identified. In all male reproductive organs studied, particularly in the testis and epididymis, endothelial cell proliferation was considerably higher than in other tissues such as the liver, brain, and muscle. Proliferating endothelial cells were observed in all types of blood vessels in male reproductive organs, but other characteristics of new blood vessel formation were not seen. High endothelial cell proliferation may reflect a continuous high turnover of endothelial cells rather than classical angiogenesis. In the epididymis, the ventral and dorsolateral prostate lobes, and the seminal vesicles, endothelial cell proliferation decreased after testosterone withdrawal and increased following testosterone treatment. In the testis, endothelial cell proliferation was decreased after Leydig cell depletion but remained low after testosterone substitution. High, hormonally regulated endothelial cell proliferation is not unique to the female but is also seen in the male reproductive organs.
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