Early Embryonic Development Of Crossbreeding Between Yak And Ordinary Cattle In Vitro

Through studying the factors affecting early embryonic development of crossbreeding between yak and ordinary cattle in vitro, the productive system for embryo of cattle-yak in vitro was established preliminarily. On the base of the system, the development for zygote of different species gametes and the reason of low pregnant rate of crossbreeding between yak and ordinary cattle could be investigated. The studies provided scientific evidence for crossing improvement of yak, whose meaning was significant to economic development of animal production in west China. The results were as follows:1. No. of useful oocytes from each ovary of yak or native yellow cattle in Gansu province by scoring the ovarian surface with a set of surgical blades followed aspiration of follicular oocytes (6.33, 7.28 VS 4.45, 4.67) were significantly higher than that by only aspiration of follicular oocytes ( P<0.01).2. Cumulus-oocyte complexes (COCs) from surface follicles of yak ovary were cultured in medium A (M199 + 10% FBS + 75 mg/L Penicillin G + 50 mg/L Streptomycin.), the rate of maturation (75.56%) was higher than that (72.73%) cultured in medium B (medium A + 1.0mg/L FSH), and lower than that (78.57%) medium C (medium A + 0.5mg/L FSH);The rate of maturation (81.43%) cultured in medium D(medium C + 5.0 mg/L LH + 1.0mg/L 17β-E₂ ) was lower than that (82.76%) cultured in medium E (medium D + 55 mg/L sodium pyruvate), but there weren't significant difference between them (P> 0.05). This showed optimal concentration of FBS and sodium pyruvate could accelerate the maturation of yak's oocytes.3. Maturation rates of cumulus-oocyte complexes (COCs) from surface follicles of yak ovary were significantly higher than that from interior follicles of yak ovary (81.29% VS 67.74%, P<0.01).4. In BO medium, cryopreserved semen of Holstein fertilized IVM matured yak oocytes in vitro, the rate of cleavage with final sperm concentration 0.4 ×10⁶⁰.6X 10⁶/ml (28.49% VS 49.15%, 50.34%) was significant lower that with final sperm concentration 4×10⁶⁶×10⁶/ml or 40×10⁶⁶0×10⁶/ml (P<0.01);the rate of morula with final sperm concentration 40×10⁶⁶0×10⁶/ml was significant lower that with final sperm concentration 0.4×10⁶⁰.6×10⁶/ml or 4×10⁶⁶×10⁶/ml (P<0.01);the rate of cleavage at 6h of sperm-oocyte co-culture (35.97% VS 49.15%, 49.32%) was significant lower that at 24h or 48h of sperm-oocyte co-culture (P<0.05);the rate of morula at 24h of sperm-oocyte co-culture (3.53% VS 2.00%, 2.82%) was higher that at 6h or 48h of sperm-oocyte co-culture, but there wasn't significant difference between them. (P>0.05). Therefore, at 24h of sperm-oocyte co-culture, with final sperm concentration 4×10⁶⁶×10⁶/ml, the result of cryopreserved semen of Holstein fertilized IVM matured oocytes of yak in vitro was best.5. zygotes that cryopreserved semen of Wild yak fertilized IVM matured oocytes of yak in vitro were cultured in Co-culture system using IVC medium with oviducal epithelial cells or Granulosa Cells, the rates of morula were 10.40% and 7.71% respectively, there wasn't significant difference between them. The zygotes were cultured in SOF and CRlaa media, whose rates of morula were all zero. Co-culture using IVC medium with oviducal epithelial cells was the best method to break 8-to 16-cell block of yak's early embryo.6. In the system, cryopreserved semen of Holstein, Simmental or Wild yak fertilized IVM matured oocytes of yak in vitro, the rates of cleavage were 49.15%, 38.41% and 67.29% respectively, and the rates of morula were 3.53%, 2.48% and 10.63% respectively;cryopreserved semen of Wild yak fertilized IVM matured oocytes of native yellow cattle in Gansu province, the rates of cleavage and morula were 41.07% and 2.75% respectively.