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Two experiments in this study were designed to compare the potential for in vitro capacitation and in vitro fertilization of ejaculated sperm among individual rabbit bucks. In experiment 1, for in vitro capacitation, the ejaculated sperm were preincubated in DM for 12 hr or 18 hr after HIS treatment, then 12 hr -or 18 hr- preincubated sperm were incubated with superovulated rabbit ova in a 5% CO2 incubator for 36 hr at 38^{circ}C, and a part of cleaved ova was transferred to the recipient does for implantation of embryo. In experiment 2, effect of lysolecithin addition to preincubation medium on induction of accelerated in vitro capacitation and in vitro fertilization of individual rabbit sperm was studied. Experiment 1; 1. Percent acrosome reaction of sperm, noted after staining, after 12 hr or 18 hr preincubation ranged from 52.5 to 76.0% and from 67.5 to 90.0%, respectively and sperm motility index of these sperm ranged from 20.0 to 47.5 for 12 hr-preincubated sperm and from 15.0 to 37.5 for 18 hr- preincubated sperm. There was no a certain relation between percent acrosome reaction and sperm motility index. 2. In vitro fertilization rate (cleavage rate) of in vitro capacitated sperm varied widely among individual bucks, ranging from 0 to 47.8% for 12 hr - preincubated sperm and from 0 to 60.9% for 18 hr -prein- cubated sperm. Cleavage rate of 18 hr - preincubated sperm was higher and faster than that of 12 hr - preincubated sperm. 3. Eight of 44 in vitro fertilized embryos transferred into 6 recipients were implanted in 4 recipients (66.7%) up to day 15 and implnatation rate was 18.2%. Experiment 2; 1. The percent acrosome reaction of sperm before and after 4 hr preincubation in DM without lysolecithin varied significantly among individual bucks, ranging from 0.4 to 18.4% and from 1.7 to 37.4%, respectively and percent acrosome reaction of sperm at 30 min after addition of 60{mu}g/ml lysolecithin also was significantly different among bucks, ranging from 19.2 to 67.1%. 2. Effect of accelerated acrosome reaction following lysolecithin addition was more considerable in the individuals showed less percent acrosome reaction before and after 4 hr preincubation. Percentage of motile sperm and motility score showed a trendency towards a decrease with increase of preincubation time and time after lysolecithin addition. 3. In vitro fertilization rate (cleavage rate) at 24 hr postinesmination with pooled sperm were treated to 60 mutextrm{g}/ml lysolecithin for 30 min after 4 hr preincubation was 24.6%, a higher rate than 13.2% for control. While 80 mutextrm{g}/ml lysolecithin-added sperm showed a lower cleavage than control and 60mutextrm{g}/ml-added sperm at both 24 hr and 48 hr postinsemination. These results from 2 experiments suggest that more useful preincubation time for the in vitro capacitation of ejaculated rabbit sperm is 18 hr in DM after HIS treatment, although there is wide variation in vitro capacitation and in vitro fertilization rate among individual bucks, and lysolecithin addition to at least 4 hr - preincubated sperm in DM can result in almost same in vitro fertilization rate as that of 18 hr - preincubated sperm in the experiment 1.
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