Abstract
The objective of this study was to compare in vitro development to the blastocyst stage of bovine embryos obtained by in vitro fertilization within a breed and between different breeds of cattle. The timing of maturaion division did not differ among oocytes of Japanese Black (JB), Japanese Brown (JBr) and Japanese Shorthorn (JS); 91.5 to 97.5% of the oocytes were at the second metaphase after 21 h of culture. The rate of development to the blastocyst stage was also similar (34.6 to 37.5%) among three kinds of embryos (JB, JBr and JS) that were fertilized in vitro and cultured for 6 to 8 days after insemination. However, significantly higher rates of development to the blastocyst stage were observed (55.0 to 67.2%) when spermatozoa from a JB bull were used for in vitro fertilization of JBr, JS and JP (Japanese Polled) oocytes as compared to the rate of development in control JB embryos (36.7%). Development to the 2- to 4-cell stage did not differ significantly. These results suggest that the effect of heterosis becomes apparent at the blastocyst stage, which might be under the control of zygotic gene activity.
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