Int. J. Environ. Res. Public Health 2010, 7 2040 Figure 6. Photomicrograph of a blastocyst with an outer cell mass (OCM), inner cell mass (ICM) encompassed by an intact zona pellucida (ZP; Magnification = 400X). On the contrary, fewer (P < 0.05) in vivo HU-exposed embryos were arrested at the morulla stage (27%) compared to their control counterparts (71%) during in vitro culture. The continuous culture of embryos in bioavailable concentration of HU in vitro resulted in approximately 88% reduction in the number of embryos that attained the blastocyst stage of development compared with controls (Figure 7). However, the intermittent exposure of embryos to bioavailable concentration of HU offered a modest improvement in the percentage of embryos that attained the blastocyst stage of development (Figure 8). This regimen of embryo culture resulted in a 75% reduction in the number of embryos that attained blastocyst stage compared with their control counterparts. Figure 7. Effect of Continuous in vitro Exposure of Embryos to Bioavailable HU (18 µg/mL) on Their Ability to Develop to Blastocyst Stage; n = 50 per Group. Results are expressed in percentages (HU = treated embryos; Control = vehicle treated embryos. Asterisks indicate a significant difference from controls (P < 0.05).