Selection of highly fertilization-competent bovine spermatozoa through adhesion to the Fallopian tube epithelium in vitro.

Mammalian spermatozoa undergo a marked reduction in number during their journey through the female reproductive tract. One of the checkpoints in the selection of fertilizing spermatozoa may be the transient adhesion to the Fallopian tube epithelium, an event previously shown to play a key role in sperm storage. Bovine spermatozoa adhering to the Fallopian tube epithelium in vitro may be synchronously released by sulphated glycoconjugates. In the present study, experiments were designed to quantify the number of spermatozoa selected through adhesion, and to compare the zona pellucida (ZP) binding and fertilization competence of the initial sperm suspension versus the bound and unbound sperm subpopulations. Results showed that: (1) a fraction accounting for about 30% of the initial sperm suspension was selected by in vitro adhesion to oviductal epithelial cell monolayers; (2) selected spermatozoa, collected after heparin-induced release, had a significantly superior ZP binding and fertilization competence (mean +/- SD: 110 +/- 28 bound spermatozoa per oocyte; % cleavage, mean +/- SEM: 89 +/- 4) compared with both the initial sperm suspension (45 +/- 10 bound spermatozoa per oocyte, P < 0.001; % cleavage: 69 +/- 3, P < 0.05) and the unselected subpopulation (30 +/- 4 bound spermatozoa per oocyte, P < 0.001; % cleavage: 58 +/- 3, P < 0.01). These findings support the hypothesis that binding to oviductal cells is not only beneficial for sperm survival but also represents a crucial step for the selection of spermatozoa endowed with superior fertilization competence.