Thus, carbohydrate involvement in sperm binding to epithelium appears to be a widespread phenomenon, although the particular carbohydrate moiety that constitutes the binding site varies according to species. In each of the three species studied so far, a different sugar maximally inhibited binding. These species differences may not seem so unusual when one considers that a single amino acid residue can determine the ligand specificity of a lectin and that closely related animal lectins have different carbohydrate specificities.
Other forms of heterotypic binding between cells involve carbohydrate recognition. Examples are the selectins, which mediate leukocyte binding to endothelium, and gly-colipid ligands on ciliated respiratory cells, which are recognized by mycoplasmas. Selectins mediate temporary binding between the two cell types, just as binding between sperm and epithelium is temporary. Carbohydrate recognition is also implicated in sperm-zona binding (reviewed in ) and sperm-Sertoli cell binding. During the course of evolution, lectins with different specificities could have arisen to regulate sperm attachment to these different surfaces. buy asthma inhalers
In summary, the formation of a sperm reservoir in the oviductal isthmus appears to be regulated by carbohydrate recognition between sperm and the oviductal epithelium. The narrowness of the lumen of the isthmus, and perhaps the mucus within the lumen, may enhance sperm binding by slowing their progress and increasing contact with the epithelial surface. There may be a lectin on the surface of sperm that is responsible for binding and that is lost or modified during capacitation, thereby allowing sperm to be released (Fig. 3). Hyperactivation may provide the force to pull sperm away from their attachment sites.
FIG. 3. Diagram of the proposed scheme for formation of the sperm reservoir and the eventual release of sperm for fertilization. Uncapacitated sperm bind to carbohydrate moieties on glycoproteins or glycolipids on the surface of the oviductal epithelium via a lectin-like molecule. This molecule is lost or modified during capacitation, allowing the sperm to release.