Studies on membrane permeability of zebrafish (Danio rerio) oocytes in the presence of different cryoprotectants

Investigation into fish oocyte membrane permeability is essential for developing successful protocols for their cryopreservation. The aim of the present work was to study the permeability of the zebrafish (Danio rerio) oocyte membrane to water and cryoprotectants before cryopreservation protocol design. The study was conducted on stage III and stage V zebrafish oocytes. Volumetric changes of stage III oocytes in different concentrations of sucrose were measured after 20min exposure at 22 degrees C and the osmotically inactive volume of the oocytes (V(b)) was determined using the Boyle-van't Hoff relationship. Volumetric changes of oocytes during exposure to different cryoprotectant solutions were also measured. Oocytes were exposed to 2M dimethyl sulphoxide (DMSO), propylene glycol (PG), and methanol for 40min at 22 degrees C. Stage III oocytes were also exposed to 2M DMSO at 0 degrees C. Oocyte images were captured on an Olympus BX51 cryomicroscope using Linkham software for image recording. Scion Image was used for image analysis and diameter measurement. The experimental data were fitted to a two-parameter model using Berkeley Madonna 8.0.1 software. Hydraulic conductivity (L(p)) and solute (cryoprotectant) permeability (P(s)) were estimated using the model. The osmotically inactive volume of stage III zebrafish oocytes was found to be 69.5%. The mean values+/-SE of L(p) were found to be 0.169+/-0.02 and 0.196+/-0.01mum/min/atm in the presence of DMSO and PG, respectively, at 22 degrees C, assuming an internal isosmotic value for the oocyte of 272mOsm. The P(s) values were 0.000948+/-0.00015 and 0.000933+/-0.00005cm/min for DMSO and PG, respectively. It was also shown that the membrane permeability of stage III oocytes decreased significantly with temperature. No significant changes in cell volume during methanol treatment were observed. Fish oocyte membrane permeability parameters are reported here for the first time. The L(p) and P(s) values obtained for stage III zebrafish oocytes are generally lower than those obtained from successfully cryopreserved mammalian oocytes and higher than those obtained with fish embryos and sea urchin eggs. It was not possible to estimate membrane permeability parameters for stage V oocytes using the methods employed in this study because stage V oocytes experienced the separation of outer oolemma membrane from inner vitelline during exposure to cryoprotectants.