Expression and putative function of fibronectin and its receptor (integrin α5β1) in male and female gametes during bovine fertilization in vitro.

Fibronectin (Fn) is a 440 kDa glycoprotein assumed to participate in sperm--egg interaction in human. Recently, it has been demonstrated that Fn -- when present during bovine IVF -- strongly inhibits sperm penetration. The present study was conducted firstly to evaluate the expression of Fn and its integrin receptor (α₅β₁) on male and female bovine gametes using indirect immunofluorescence and secondly, to determine the function of Fn during bovine IVF. Endogenous Fn was detected underneath the zona pellucida (ZP) and integrin α₅ on the oolemma of cumulus-denuded oocytes. Bovine spermatozoa displayed integrin α₅ at their equatorial segment after acrosome reaction. We established that the main inhibitory effect of exogenously supplemented Fn was located at the sperm-oolemma binding, with a (concurrent) effect on fusion, and this can probably be attributed to the binding of Fn to spermatozoa at the equatorial segment, as shown by means of Alexa Fluor 488-conjugated Fn. Combining these results, the inhibitory effect of exogenously supplemented Fn seemed to be exerted on the male gamete by binding to the exposed integrin α₅β₁ receptor after acrosome reaction. The presence of endogenous Fn underneath the ZP together with integrin α₅ expression on oolemma and acrosome-reacted (AR) sperm cell surface suggests a 'velcro' interaction between the endogenous Fn ligand and corresponding receptors on both (AR) sperm cell and oolemma, initiating sperm--egg binding. [ABSTRACT FROM AUTHOR]