An investigation of the fertilizing characteristics of pyriform-shaped bovine spermatozoa

Two breeding trials were done to determine the effect of pyriform-shaped bo vine spermatozoa on fertility. In breeding trial (1), heifers were supersti mulated by injecting follicle-stimulating hormone (FSH) and randomly allott ed to an affected Bull A (n = 21) or a Control Bull, C (n = 18). Semen from Bull A contained 85% pyriform spermatozoa while that from the Control Bull C contained 91% normal sperm and 2% pyriform spermatozoa. Fertilization ra te was lower (P = 0.01) for Bull A (total of 63 ova/embryos; 68.5%) than fo r the Control Bull, C (total of 81 ova/embryos; 84.4%). In breeding trial ( 2), 37 oestrus-synchronized heifers were randomly allotted to Bull A (n = 1 9) or Bull C (n = 18). Pregnancy rates at Day 60 (37% and 61% for Bulls A a nd C, respectively; P = 0.22) and rate of embryo/fetal loss between Days 22 and 60 of pregnancy (23% and 8% for Bulls A and C, respectively; P = 0.55) were not different. In vitro experiments involving the same Bull A and ano ther Control Bull, B, were designed to explain the mechanism of infertility caused by pyriform sperm. The mean (+/- SEM) number of sperm bound to the zona pellucida was lower (P < 0.05) for Bull A (24.6 +/- 1.2) than for the Control Bull, B (46.6 +/- 1.9) and the percentage of zonae penetrated by sp erm from Bull A (56.0%) and Bull B (82.8%) was also different (P < 0.05). T he percentage of pyriform sperm from Bull A bound to (53%), and penetrating (49%) the zona pellucida was lower(P < 0.05) than that in the inseminate ( 85%). Although fertilization rates (64.1% and 72.8%) were not different (P = 0.5), cleavage rates (48.2% vs. 74.1%) and morula production rates (24.8% vs. 37.7%) were less (P < 0.05) for Bull A than for Bull B, respectively. In summary, pyriform sperm had reduced capability to bind to and penetrate the zona pellucida, and zygotes (resulting from the fertilization of oocyte s by pyriform sperm) appeared to have a reduced ability to initiate cleavag e. (C) 1999 Elsevier Science B.V. All rights reserved.