Fertilization and cleavage of bovine follicular oocytes in rabbit reproductive tracts after maturation in vitro

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Abstract
Bovine follicular oocytes cultured in vitro in Ham's F‐12 based medium for 27 hours were transferred to the oviducts of rabbits pretreated with 75 IU hCG 24 hours previously. Frozen‐thawed bull spermatozoa were inseminated into the uterus of rabbits 0, 2, 4, 6, 9, and 12 hours before oocyte transfer. Fertilization and cleavage were examined in the oocytes recovered from the oviduct and uterus 24 and 48 hours later. The criteria of fertilization were the presence of an enlarged sperm head, sperm tail, and pronuclei in the vitellus. Bull spermatozoa inseminated 4 hours before oocyte transfer showed the highest rates of fertilization at both 24 and 48 hours (35% and 38%) compared to the other preincubation times. The numbers of cleaved oocytes were also greater at these times. The results indicate that the capacitation time of cryopreserved bull spermatozoa in the rabbit reproductive tract is the same as in the bovine tract—i.e., 4–6 hours.