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Management of male infertility has recently shifted from treatment of the subfertile man towards techniques of assisted reproduction (ART). This study aimed to evaluate the possible role of the ultramorphological status of the spermatozoon with respect to sperm selection in vivo and prediction of ART success. Ultramorphological sperm parameters were assessed retrospectively for 92 males with sufficient sperm density (10(7) spermatozoa ejaculate-1) whose wives conceived following a stepwise discarding of the female genital tract barriers, using intra-uterine insemination (IUI) (n = 26), in vitro fertilization (IVF) (n = 45) or intracytoplasmic sperm injection (ICSI) (n = 21). In parallel, sperm samples of 71 fertile males were examined. Normal ultramorphology of all head and tail subcellular organelles was found to be essential for the ability of spermatozoa to pass the lower female genital tract. The ultramorphological migration threshold for this barrier is apparently higher than that essential for oocyte fertilization. No specific indication associated with passage through the upper genital tract was found. A high prevalence of axonema defects was found to impair the ability of sperm cells to penetrate the oocyte investment. The natural fertility index, based on routine sperm parameters and the ultrastructural status of the spermatozoon's subcellular organelles was confirmed to be beneficial for directing patients to ART. A discriminative score based on axonema integrity was found to contribute additional information for the first choice decision between conventional ART and ICSI (75% prediction ability). Thus it may be helpful in finding the simplest and least expensive procedure with the greatest long-term chance for pregnancy.

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