Cryopreservation sperm European eel: were are we now?

“A few months ago, we successfully cryopreserved sperm from the European eel,” says Pauline Jéhannet, researcher in fish reproduction and cryopreservation at CGN. “This is a significant milestone for CGN, as it is the first fish species to be added to our genebank. However, this achievement did not come easily,” she adds. “In fact, our initial attempts to freeze eel sperm were unsuccessful, despite having high-quality milt.”

To improve their approach, experienced CGN-colleague Agnes de Wit visited experts at the Polytechnic University of Valencia (UPV) to learn how they cryopreserve eel sperm. Frustratingly, even after applying the protocol they received from UPV, Pauline, Agnes and their colleagues still struggled to successfully freeze eel sperm. However, after making some adjustments, they finally succeeded – and with impressive results, as the post-thaw sperm retained an average motility of 40%!

The next step: cryopreservation of germ cells

“We are currently planning fertilization trials to determine whether our cryopreserved eel sperm can successfully produce larvae,” says Pauline. “In addition to sperm, we are also aiming to cryopreserve germ cells: a promising alternative as oocytes and embryos have not yet been successfully preserved in fish.”

Germ cells, present in both embryos and adult fish, can be cryopreserved and then converted into functional gametes by transplanting them into a recipient of a closely related species. Once maturing, the recipient can produce gametes from the donor species. “Considering its potential in aquaculture and species conservation, this transplantation technology in fish represents a promising path worth pursuing,” says Pauline. “Therefore, we recently tried to visualize germ cells in eel embryos in order to isolate and cryopreserve them.”

In fish, germ cells have been usually labeled by injection of an artificial mRNA into early stage embryos. Despite its success, this technique is costly and time consuming as it requires many steps for the preparation of the mRNA construct. Therefore, Pauline and her colleagues tried to visualize germ cells by using a novel technique that has been developed in sturgeons. Unfortunately, this novel technique has not yet been proven successful in European eel. But, as with the protocol for freezing eel sperm, perhaps collaboration can lead to a breakthrough: in July, fish researchers from CGN will meet germ cell experts during a workshop of COST Action EEL SUPPORT at the University of Hokkaido in Japan.