This study was conducted to investigate the possibility of using bovine oocytes for a heterologous fertility test by intracytoplasmic sperm injection (ICSI) and to compare the pronuclear formation of ram, bull and minke whale spermatozoa after injection into bovine oocytes. Bovine oocytes were cultured in vitro for 24 h and those with a polar body were selected for ICSI. Frozen-thawed semen from the three species were treated with 5 mM dithiothreitol for 1 h and spermatozoa were killed by storing them in a -20 °C refrigerator before use. ICSI was performed using a Piezo system. Three experiments were designed. In experiment 1, a higher (p < 0.05) male pronuclear formation rate was found in the oocytes injected with ram (52.6%) or bull (53.4%) spermatozoa than with minke whale spermatozoa (39.1%). In experiment 2, sperm head decondensation was detected at 2 h after ICSI in the oocytes injected with a spermatozoon of each species. Male pronuclei were first observed at 4 h in the oocytes injected with ram or bull spermatozoa and at 6 h in oocytes injected with minke whale spermatozoa. The mean diameters of male pronuclei derived from both whale and bull spermatozoa were larger than those from ram spermatozoa (30.4 μm and 28.3 μm vs 22.4 μm, p < 0.005). The mean diameter of female pronuclei in the oocytes injected with whale spermatozoa was also larger than with ram spermatozoa (29.3 μm vs 24.7 μm, p < 0.05). The development of male and female pronuclei was synchronous. In experiment 3, ethanol-activated oocytes injected with a spermatozoon from any of the three species achieved significantly higher (p < 0.05-0.001) cleavage rates than control oocytes. Blastocyst formation was only observed when bull spermatozoa were used. The results of this study indicate that dead foreign spermatozoa can participate in fertilisation activities in bovine oocytes after ICSI.
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