In approximately 7% of men undergoing microsurgical epididymal sperm aspiration (MESA), spermatozoa retrieval is not possible. In these patients in whom the epididymal sperm reservoir is inaccessible, recent work has shown that whole, motile sperm can be obtained from the testicle itself. Critical for the success of testicular sperm extraction (TESE) is determination that the testis is making mature sperm.