1. Section for Reproductive Biology, Department of Animal Breeding and Genetics, Danish Institute of Agricultural Sciences, 8800 Tjele, Denmark; Fertility Clinic, Holbaek Hospital, Holbaek, Denmark
2. Institute of Animal Reproduction, Guangxi University, Nanning, China
3. Section for Reproductive Biology, Department of Animal Breeding and Genetics, Danish Institute of Agricultural Sciences, 8800 Tjele, Denmark