Studies on chilling sensitivity of zebrafish (Danio rerio) oocytes

Human activity in the last few decades has had a devastating effect on the diversity of fresh water and marine fish. Further decline of fish population may have serious economic and ecological consequences. One of the most promising techniques to preserve fish population is to cryopreserve their germ cells. Cryopreservation has been successfully applied to fish sperm of many species, but there has been no success with fish embryo cryopreservation and fish oocyte cryopreservation has never been studied systematically. The aim of this study is to investigate the chilling sensitivity of fish oocytes. Experiments were conducted with zebrafish stage III (vitellogenic) and stage V (mature) oocytes, which were chilled at 10, 5, 0, -5 or -10 degrees C for 15 or 60 min using a low temperature bath. Control oocytes were kept at room temperature at 22 degrees C. Oocyte viability was assessed using three different methods: trypan blue staining (TB), thiazolyl blue tetrazolium bromide (MTT) staining and observation of germinal vesicle breakdown (GVBD). The results showed that zebrafish oocyte are very sensitive to chilling and their survival decreased with decreasing temperature and increasing exposure time periods. Normalised survivals assessed with TB staining after exposure to 0, -5 or -10 degrees C for 15 or 60 min were 90.1+/-6.0, 77.8+/-7.6, and 71.2+/-9.3%, and 60.2+/-3.8, 49.6+/-6.7, and 30.4+/-3.0%, respectively. The study found that the sensitivity of viability assessment methods increase in the order of MTT < TB < GVBD. It was found that stage III oocytes were more susceptible to chilling than stage V oocytes, and that individual female had a significant influence (p < 0.0001) on oocyte chilling sensitivity. Zebrafish oocyte chilling sensitivity may also be one of the limiting factors for development of protocol of their cryopreservation.