The path of the sperm to the egg

LA SPINA, FLORENZA A.; BUFFONE, MARIANO G

MOLECULAR REPRODUCTION AND DEVELOPMENT

WILEY-LISS, DIV JOHN WILEY & SONS INC

Año: 2017 vol. 84 p. 543 - 543

1040-452X

Not long ago, it was broadly accepted that sperm undergo acrosomal exocytosis (AE) upon interaction with the zona pellucida (ZP) of the egg, and many of the advances in our knowledge of this process were derived from in vitro studies using solubilized ZP. Recent evidence demonstrated that most fertilizing mouse sperm undergo acrosomal exocytosis before binding to the zona pellucida of the eggs (Jin et al, 2011. PNAS, 1018202108). These findings raised the question of where fertilizing spermatozoa initiate AE (Buffone et al, 2014. BOR 24671881). An invaluable tool to determine physiological sites of AE is by the use of double transgenic mouse sperm, which carries EGFP in the acrosome and DsRed2 fluorescence in mitochondria (Nakanishi et al, 1999. FEBS Lett. 10338148). By using real time imaging of sperm inside the oviduct, we observed that mouse sperm do not initiate AE close to or on the ZP, and that a significant proportion of sperm initiate AE in the upper segments of the oviductal isthmus (La Spina et al, 2016. Dev. Biol. 26872876). In this image, it is observed sperm migrating in the lower isthmus of the oviduct after mating (also considered as the sperm reservoir). The determination of the precise sperm location within the oviduct is greatly facilitated by the DsRed2 fluorescence of the flagellums which are clearly visible through the thin oviductal walls. Most sperm in the lower segments of the oviduct are acrosome-intact (as judged by the presence of EGFP in their acrosomes).