ABSTRACT
The ability to create cryo-banks of viable fish sperm, eggs and embryos would be one of the most powerful facilitating tools in aquaculture and species conservation. Preservation of haploid nuclear genomes of viable male and female reproductive cells, such as sperm cells or unfertilized eggs or of viable embryos, is vital if fish populations are to be maintained. While no specific problem is encountered in preservation of fish spermatozoa, the preservation of the maternal genome of fish remains a challenge. In contrast to mammalian species where methods for cryopreservation of embryos and oocytes are well established (reviewed in Ambrosini et al., [1]), successful cryopreservation methods for yolk-laden fish embryos have remained elusive. Unlike mammalian species, fish oocytes and eggs contain yolk that consists of proteins, lipids and in several species also lipid droplets. Fish oocytes are smaller than their respective eggs, constitute a single compartment and do not have permeability barriers such as the yolk syncytial layer found in embryos [2,3]. Some earlier studies showed lower membrane permeability of oocytes when compared to fully matured eggs [4-6], although more recent studies showed ovarian follicles to be more permeable to water and cryoptectants than embryos [7-9] with differences between freshwater species such as medaka and zebrafish and the marine gilthead seabream. However, fish oocyte membrane permeability may still not be high enough or their cryopreservation. The high water content of pelagic floating eggs is an additional problem for most cultured marine fish species. The term “oocyte” refers to, in most of the cases, ovarian follicles that inclue the oocyte with its attached theca and granulose cells. The term oocyte will be used here to indicate an ovarian follicle.
