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BACKGROUND: The survival of endometriotic cells in the ectopic site has been investigated from the aspect of susceptibility of endometriotic tissues to apoptosis. In order to investigate the nature of abnormal survival of endometriotic cells in ectopic locations, we compared drug-induced apoptosis in endometrial and endometriotic cells. METHODS: Endometrial stromal cells were obtained from normal endometrium in 11 patients who underwent hysterectomy for leiomyoma without endometriosis. Endometriotic cells were isolated from the chocolate cyst linings of the ovary in 13 patients who underwent laparoscopic surgery. Cells were cultured in the presence or absence of staurosporine. Apoptotic cell death was evaluated by staining nuclei with propidium iodide and phosphatidylserine (a marker of early apoptotic events) with Annexin V as well as by DNA fragmentation assay. The number of viable cells was estimated by modified MTT [3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide WST-8] assay. RESULTS: After 3 h of exposure to staurosporine, >50% of the endometrial stromal cells became Annexin V positive. In contrast, >30% of the endometriotic cells were Annexin V positive. DNA fragmentation was not clearly induced in the endometriotic cells. Less than 20% of the endometrial cells survived after staurosporine exposure, while >40% of the endometriotic cells survived. Cell death induced by staurosporine was partially blocked by incubation with the caspase inhibitor, N-benzyoxycarbonyl-Val-Ala-Asp(OMe)fluoromethyl-ketone (ZVAD-fmk), suggesting that a caspase cascade may play a role in the cell death process. CONCLUSIONS: Attenuated susceptibility to apoptosis in endometriotic stromal cells may be associated with abnormal survival in ectopic sites in an environment that is probably unfavourable. These results may be implicated in the pathophysiology of endometriosis. [ABSTRACT FROM AUTHOR]