1. Human and Animal Physiology, Wageningen University, P.O. Box 338, Wageningen, The Netherlands
2. National Engineering Research Center For Breeding Swine Industry, College of Animal Science, South China Agricultural University, Guangzhou, Guangdong, P.R. China
3. Reproductive Biology Laboratory, Academic Medical Center, University of Amsterdam, Q3.119, Meibergdreef 9, Amsterdam, the Netherlands