**5.3 Permeability of teleost embryos**

The low permeability of the chorion and the perivitelline is evident from retarded permeation of radio-labeled DMSO from the external medium into the embryos by several folds (Harvey et al., 1983). Even after removal of the outermost barrier, the embryos were poorly permeated by cryoprotectants. By chemical shift selective magnetic resonance microscopy and magnetic resonance spectroscopy, kinetics of permeation of cryoprotectants methanol, DMSO and propylene were measured. While methanol can permeate the entire six-somite zebrafish (*Brachydanio rerio*) embryos in 15 minutes, DMSO and propylene are relatively poor in permeating into the embryos when applied to the medium (Hagedorn et at., 1996). Similar findings were obtained by osmometric measurements of volume changes in the embryos tested (Hagedorn et al., 1997c). Also, magnetic resonance imaging on the distribution of cryoprotectants, delivered to the external medium or injected into the yolk, in the three-somite stage *Brachydanio rerio* embryo revealed that the yolk is far less permeable than the blastoderm and the yolk syncytial layer is the major barrier to the cryoprotectants (Hagedorn et at., 1996; Hagedorn et al., 1997a).

To artificially promote permeation of cryoprotectants into fish embryos, ultrasound of 175V was used to increase the permeability by methanol to *Danio rerio* 50% epiboly (Wang et al., 2008). A high-intensity femtosecond laser was also used to introduce transient pores on blastomeres and the blastoderm-yolk boundary. Successful delivery of large molecules such as fluorescein isothiocyanate, streptavidin-conjugated quantum dots and DNA plasmid was detected on pec-fin stage (Kohli et al., 2007). Whether the physically induced permeation method can help cryopreservation of the whole teleost embryos requires experimental verification.
