Cryopreservation of Rainbow Trout (Oncorhynchus mykiss) Blastomeres: Influence of Embryo Stage on Postthaw Survival Rate.

The cryopreservation of isolated fish blastomeres is likely to provide a valid alternative to embryo cryopreservation, the results of which are still discouraging. A repeatable technique for the cryopreservation of rainbow trout blastomeres has been established and the effect of embryonic developmental stage on freezing tolerance evaluated. Embryos at Ballard 6A, 6B, and 6C stages were dechorionated and left to dissociate in a Ca2+- and Mg2+-free medium. Cryoprotection was provided by step-wise addition of 1.4 M 1,2-propanediol. Cells were loaded into the middle of 250-µl straws and slowly frozen to -80 degreesC before being plunged into LN2. A low thawing rate was adopted, followed by step-wise removal of the cryoprotectant. Morphological evaluation was by microscopy and video recording. Metabolic activity and survival rate were determined by FDA and PI staining, by recovery of the ability to reassociate after 24 h culture in Leibovitz L15 + 2% Ultroser, and by measuring DNA synthesis in 6B cells by the method of BrdU incorporation. Survival rates were 53 +/- 9.3, 88 +/- 1.7, and 95 +/- 0.5% for stage 6A, 6B, and 6C cells, respectively. While 6A cells reassociated into clumps of cells, 6B and 6C cells formed holoblastic morulas in 24 h; proliferation of 6B cells was comparable to fresh control cells. The relationship between freezing tolerance and the physiological events occurring during early embryonic development is discussed in light of these results and conclusions are drawn that envisage the transfer of frozen-thawed blastomeres into recipient embryos. Copyright 1998 Academic Press.