Aim: To study the histologic changes of the vas deferens following Nd: YAG laser irradiation. Methods: Intravasal laser irradiation was given to (i) 52 segments of rabbit (laser dosage: 2 seconds at 40 W ~ 50 W) and 16 segments of human (3 seconds at 45 W ~ 55 W) vas deferens in vitro, (ii) 25 rabbit vasa (2 seconds ~ 2.5 seconds at 40 W ~ 45 W) in vivo and (iii) 2 human vasa (3 seconds at 55W) in vivo. Segments of vasa were removed from the in vivo irradiated vasa deferentia 15 days ~ 180 days (rabbit) or 15 days (man) after the exposure. All vas segments were embedded in methacrylate resin. Serial sections (thickness 25 mm ~ 30 mm) were obtained and observed under a light microscope. Results: (i) Laser-induced damage reached the muscularis layer in 27 % and 94 % of the rabbit and human vas segments in vitro, respectively. (ii) Fourteen of the 25 in vivo rabbit vasa were completely occluded by fibrous tissue and the longer the time interval after treatment, the more likely was the vas occluded. Those unoccluded vasa had either a normal histology or a mucosal damage. (iii) One in vivo human vas was almost completely occluded by the fibrous tissue but the other had a relatively large lumen packed with sperm granulomatous tissue and partial destruction of the smooth muscle layer. Conclusion: Laser irradiation can induce long-term vas occlusion; for rapid occlusion, laser doses just completely destroying the mucosal layer will be advisable

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