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Objectives: To examine the expression of annexin A5, a recently emerged multifunctional protein, in the testis after experimental cryptorchidism., Methods: The hemilateral testes of rats were surgically relocated from the scrotum into the abdomen. The annexin A5 content was evaluated by Western blot analysis, and its distribution was determined by immunohistochemistry. Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling (TUNEL) was performed to detect the apoptotic cells., Results: Although annexin A5 content was not changed in the scrotal testis, it was dramatically increased in the abdominal testis during the 4-week observation period. TUNEL-positive apoptotic spermatogonia appeared in some of the seminiferous tubules only 1 day after the surgery. The appearance of TUNEL-positive cells was accompanied by augmented expression of annexin A5. Spermatocytes disappeared by 1 week after cryptorchidism, and annexin A5 expression was significantly increased inside the seminiferous tubules. In the cryptorchid testis, annexin A5 of the interstitial cells disappeared, as did 3beta-hydroxy steroid dehydrogenase. Annexin A5 on endothelial cells was not changed by the cryptorchidism., Conclusions: These data demonstrate first that the expression of annexin A5 is increased in the seminiferous tubules of cryptorchid testes and suggest that it is related to the degenerative response of germ cells.