Endothelial mitosis during the initial stages of corpus luteum neovascularization in the cycling adult rat

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Abstract
The initial stages of neovascularization of the corpus luteum were studied in cycling adult rats using light‐microscopic autoradiography. The aim of this analysis was to determine whether endothelial mitosis is a factor in this vascular growth and whether there are differences in the amount of mitotic activity in various regions of the ovary. Ovaries were examined at two time intervals: 1–2 hr and 7–8 hr following ovulation. Animals received an intraperitoneal injection of tritiated‐thymidine 20 min prior to perfusion fixation of the ovaries. Autoradiographic demonstration of tritiated‐thymidine labeling in endothelial nuclei was considered an indication of DNA synthesis preceding mitosis. The percentage of labeled endothelial cells in the ovaries at both time intervals varied according to the region of tissue examined and the stage of differentiation of that region. Stromal vessels were less heavily labeled than thecal vessels. Thecal vessels surrounding growing follicles were more heavily labeled than those surrounding atretic follicles. The heaviest labeling was seen in the developing corpora lutea 7–8 hr following ovulation. Minimal labeling was evident in the corpora lutea which were formed in previous cycles. A regional difference was also detected in the ovarian mesothelium. The portion of the mesothelium overlying ovulated follicles and developing corporalutea had the greatest percentage of labeled cells. The major findings of this study were: (1) endothelial mitosis was elevated in the initial stages of luteal neovascularization; (2) the heightened endothelial labeling was confined to specific regions of the ovary; and (3) mesothelium in close proximity to the developing corpora lutea also displayed heightened DNA synthesis.