During prolonged incubation with oAMV, rabbit sperm agglutinated head-to-head in groups of approximately 2-20 (Fig. 1). Rabbit sperm heads were stacked on one another with the oAMV apparently forming the “glue” that held them together. No such regular, head-to-head agglutination pattern was observed in the presence of rabbit kidney AMV (tissue control) or in the absence of AMV (medium control). When equine sperm were incubated in the presence of oAMV and antigen-binding fragments (Fab) derived from polyclonal antiserum raised to the periacrosomal plasma membrane of equine sperm, oAMV binding was greatly diminished. These data suggest that binding between sperm and oAMV involves specific molecular interactions.
When rabbit, equine, and human sperm were incubated in vitro with homologous oAMV from either preovulatory does and mares or premenopausal women, a significantly higher number of sperm remained viable in culture compared to those incubated in the presence of kidney AMV, anovulatory oAMV, postmenopausal oAMV, or culture medium alone. These experiments suggest that direct interaction with the isolated apical plasma membrane, in the absence of intact oviductal cells and/or their secretions, is sufficient to maintain sperm viability in vitro in a manner analogous to that occurring in vivo.
FIG. 1. Transmission electron micrograph of agglutinated rabbit spermatozoa after 1-h incubation in the presence of apical plasma membrane vesicles isolated from preovulatory oviductal epithelial cells. Vesicle binding is restricted to the rostral region of the sperm head. X27 500. Adapted from Smith and Nothnick, Biol Reprod 1997; 56:87, Figure 3.