and development. A recent study indicates that certain bovine embryo culture media can alter

the abundance of DNA methyltransferase 3a (Sagirkaya et al., 2006).

 As mentioned earlier, it is clear that the differences observed between in vitro produced

and in vivo derived blastocysts can affect cryosurvival (Rizos et al., 2002). Changes in gene

expression (Rizos et al., 2002, 2003) as well as ultrastrucure (Abe et al.,1999; Fair et al., 2001;

Abe et al., 2002) have been associated with a reduced capacity to survive following

cryopreservation. In particular the accumulation of lipid droplets in the cytoplasm of embryos

produced in vitro appears to reduce cryotolerance. Reduction in lipid content, either by

centrifugation (Diez et al., 2001) or by using metabolic inhibitors (De La Torre-Sanchez et al.,

2006a, 2006b), can improve survival following cryopreservation.

 Alterations in sex ratio caused by in vitro embryo production appear to be related to

embryo culture conditions rather than the preferential survival of male embryos after transfer. In

vitro, male embryos develop faster than female embryos (Avery et al., 1991; Xu et al., 1992;

Gutierrez-Adan et al., 2001). There is some indication that this effect may be the result of

glucose in the culture medium (Larson et al, 2001). However, even in medium without glucose,

68% of day 7 expanded blastocysts were male and glucose-free medium did not alter the sex

ratio in favor females until day 9-10 after fertilization (Gutierrez et al., 2001). The ability to

used sexed semen efficiently for in vitro embryo production as mentioned above offers one

strategy to overcome this problem (Wilson et al., 2005; Wheeler et al., 2006; Wilson et al.,

2006).

 Strategies to Improve Post-Transfer Survival of Bovine Embryos Produced In Vitro

 In general, there are 3 strategies for improving the post-transfer survival of in vitro

produced bovine embryos: 1) alter the recipient to improve fertility, 2) identify markers for

embryo survival and 3) modify embryo culture to enhance post-culture viability.