The fertilization performance in vivo of rabbit spermatozoa capacitated in vitro

Abstract

Although mammalian spermatozoa can be capacitated in vitro appropriately for the accomplishment of fertilization in vitro, it has not been established whether such spermatozoa can fertilize immediately in the Fallopian tube nor whether their fertilizing ability is comparable to that of spermatozoa capacitated in vivo. Rabbit spermatozoa treated here in a way reported to capacitate them for fertilization in vitro (20 min in high ionic strength medium (HIS) of 380 mOsm/kg at 37°C) could not penetrate eggs in the Fallopian tube unless allowed a considerable time for further capacitation in the female tract. Thus, while it may initiate the events of capacitation, HIS treatment clearly does not complete the process. Spermatozoa cultured for 12 h under another set of defined conditions (BSA‐free medium +20% serum under oil equilibrated with 5% CO2+ 8% O2at 37°C) did fertilize eggs in 7 of 14 trials, however, when inseminated 12–15 h post‐hCG. Therefore capacitation that allows fertilization in vivo soon after insemination can be achieved in at least some spermatozoa, largely and perhaps entirely in the absence of factors from the female tract. Nonetheless, the performance of the 12h in vitro spermatozoa (29% of 131 eggs fertilized: 0.3 perivitelline spermatozoa/egg) compared poorly with that of 10 times fewer uterine spermatozoa in the contralateral oviduct (81% of 43 eggs fertilized: 15 perivitelline spermatozoa/egg). Moreover, in a final series, 12 h in vitro capacitated spermatozoa failed to compete directly for ova if inseminated together with many fewer labeled uterine spermatozoa, even though they fertilized significant numbers when inseminated alone into the contralateral oviduct. Current in vivo and in vitro fertilization systems for the assay of rabbit sperm capacitation are not entirely comparable because the penetrable life of rabbit ova is short in the Fallopian tube but prolonged in culture. Since fully capacitated spermatozoa begin to penetrate eggs almost immediately, assay of the capacitation status of spermatozoa of the rabbit or indeed of other species might best be evaluated within about 3 hours of gamete mixing. Such a limitation on the time allowed for sperm‐egg interaction in capacitation assays would minimize the possibility for completion of capacitation by factors present in the fertilization assay system. Copyright © 1981 Wiley‐Liss, Inc., A Wiley Company