The nature of the sperm surface receptors that mediate the binding of sZP is unknown. Topfer-Petersen and Cal-vete have proposed that the class of small proteins known as “spermadhesins” may play an important role in zona binding of boar sperm; these proteins, which interact strongly with zona components, are produced by the male reproductive tract and bind to the sperm head surface, in particular to the acrosomal region, at ejaculation. However, it has been found that most of the spermadhesin activity dissociates from the sperm during capacitation , a characteristic that does not accord with the zona binding changes we have observed. buy ortho tri-cyclen online
On the other hand, the individual members of the spermadhesin family are not released uniformly, and it may be that those with greatest affinity for the zona may be retained or even unmasked. Several other putative zona receptors have been described in different sperm species . The boar sperm membrane protein APZ identified by Peterson and Hunt is apparently distributed over the whole sperm surface , and its involvement in the changes we have described thus seems doubtful. Hardy and Garbers have isolated a boar sperm membrane protein that confers specific binding ability with respect to the pig zona pellucida; this protein is expressed primarily in haploid spermatids and contains a region that may be involved in oligomerization, as well as mucin-binding regions , but its distribution on the sperm cell has not yet been defined.