Int. J. Environ. Res. Public Health 2010, 7 2039 Figure 4. Effect of HU on Ovulation Rate in Female Mice Treated IP with 30 mg HU/kg for 28 days; n = 5 per Group. Results are expressed as mean ± SE (HU = treated mice; Control = vehicle treated mice. Asterisks indicate a significant difference from controls (P < 0.05). Fertilization rate of oocytes recovered from HU-treated mice was 93%, comparable with that of recovered oocytes from control mice (95%). The ability of apparently normal 2-cell embryos recovered from HU-treated mice to progress to 4-cell stage of development in 36h of in vitro culture was comparable with their counterparts recovered from control mice. However, in vitro culture of in vivo generated 2-cell embryos (60 embryos/group) revealed that fewer (P < 0.05) in vivo HU-exposed embryos developed to blastocyst stage (19 [32%]); Figure 5 compared with 36 (60%) of their control counterparts that progressed to blastocyst stage of development in vitro (Figure 6). Of the remaining embryos, more embryos were arrested at the 8-cell stage of development among in vivo HU-exposed (73%; P < 0.05) compared with controls (29%) during the 5 day in vitro culture. Figure 5. Effect of in vivo Exposure of Embryos to HU on Their Ability to Develop to Blastocyst Stage; n = 60 per Group. Results are expressed in percentages (HU = treated mice; Control = vehicle treated mice. Asterisks indicate a significant difference from controls (P < 0.05).