How to cryopreserve fish embryos, bring them back to life

Cryopreservation has been used to save sperm, oocytes and even embryos of many species, including humans, cattle and lab animals. Preserving the embryos of most fishes, however, has remained an elusive goal. The embryos are relatively large with big yolks and are divided by multiple compartments. These traits make the embryos difficult to cool and warm uniformly without damage and ice formation. A few techniques, including microinjection of cryoprotectants and laser irradiation for re-warming, have shown promise toward achieving this long-sought goal. John Bischof and colleagues wanted to tweak the methods to see if they could finally make cryopreserving fish a reality.

The researchers injected a cryoprotectant, along with plasmonic gold nanoparticles to serve as a laser absorber, directly into zebrafish embryos. Plunging the embryos in liquid nitrogen rapidly cooled them to a cryogenically stable state in less than a second, according to modeling results. The researchers then used laser irradiation to heat up the nanoparticles, which were uniformly distributed inside the embryos, at an ultra-fast rate (1.4 x 107 degrees Celsius per minute). Not all of the embryos made it, but many were revived -- a feat that is currently not possible by other techniques. Their hearts, eyes and nervous systems developed through at least the next 28 hours -- and they started to wiggle. As more fish populations shrink and become threatened, the researchers say the cryopreservation method could help establish banks of frozen fish germ cells and embryos that could one day help replenish the oceans' biodiversity. The technique could also be applied to amphibian, reptile and bird species with similar embryonic sizes and structures.