Oviduct secretion contributes to the establishment of species specific barrier preventing penetration of oocytes with foreign spermatozoa

Ovulated oocytes can be fertilized in vivo almost exclusively with spermato zoa of its own species only, while the species specificity of zona pellucid a of in vitro matured oocytes is less restrictive. Our present experiments were undertaken to determine whether estrous oviductal fluid modifies the i nteraction between gametes of unrelated species. After incubation of in vit ro matured sheep oocytes with bull spermatozoa, the penetration rate was 75 .0%, whereas when the oocytes were matured in medium supplemented with 15% sheep oviductal fluid collected using the permanent indwelling oviductal ca nnulae, the penetration rate decreased to 4.8% (4/84), In reverse combinati on, 70.4 % (38/54) cattle oocytes matured in vitro were penetrated with ram spermatozoa, The addition of oviductal fluid caused a drop in penetration by ram sperm to 38% (19/50). In parallel experiments, no penetration was re corded when in vivo matured sheep oocytes were incubated with bull spermato zoa; high fertilization rates (79.4% - 27/34) were recorded when such eggs were incubated with ram spermatozoa, irrespective to the presence or to the absence of oviductal fluid in the medium. The results suggest that the pro perties of zonae pellucidae of ovulated and in vitro matured oocytes are no t identical and may be modified by contact with estrous oviductal fluid.