Chilling causes perivitelline granule formation in activated zebrafish oocytes

Chilling sensitivity in oocytes of the zebrafish represents a potential obstacle to their successful cryopreservation. Here, we report the first cryomicroscopic observations of the response of zebrafish oocytes to chilling conditions. In activated stage V oocytes that had been exposed to hypothermic temperatures, we observed a latent effect of chilling, manifesting as a granular precipitate that appeared in the perivitelline fluid upon return to 28.5 °C. The granules were visible in unstained oocytes under transmitted light microscopy, and the resulting perivitelline turbidity increased in a dose-dependent manner with decreasing chilling temperature (p < 0.001), as well as with increasing time of hypothermic exposure (p < 0.0001). The change in appearance of the perivitelline space in oocytes that had been chilled and rewarmed became statistically significant after a 7-min exposure to 10 °C and after only 30 s at 1 °C (p < 0.05). Thus, even moderate chilling exposures can lead to detectable changes in activated zebrafish oocytes.