Cryopreservation of germ cells from bovine fetal ovaries

Bovine fetuses at stages required for studies of female germ cells (primordial germ cells and oogonia) become available from the abattoir at unpredictable times. To alleviate this logistical problem, a procedure to cryopreserve these ovarian germ cells has been devised. Fetal ovarian cells were dispersed and suspended in 1.5 M dimethyl sulfoxide (DMSO) prepared in modified TCM 199 medium. The suspensions were aspirated into plastic semen straws, cooled, seeded to induce ice formation at -7 degrees C, and then cooled at 1 degree C min-1 to -70 degrees C before being plunged into liquid nitrogen at -196 degrees C for storage. The straws were thawed at a moderate rate of approximately 250 degrees C min-1, the DMSO was diluted 28-fold with culture medium, and then the cells were cultured for > 2 h before their viability was tested or they were used for nuclear transfer. No statistically significant difference in viability before and after cryopreservation was detected by vital staining with fluorescein diacetate (P > 0.05). When frozen-thawed germ cells were fused to cytoplasts, the cleavage rate of the resultant reconstructed embryos 44 h after fusion was 31%, although none developed into blastocysts. It is concluded that cryopreservation of bovine fetal ovarian germ cells is feasible and can play a major role in facilitating future experimentation.