It is not known why XO mouse embryos, which develop more slowly than XX embryos until early mid-gestation, reach the same stage in their growth and development as their XX littermates at the mid-gestation stage. It is hypothesized that there is an effect of 'litter size' that causes an acceleration of the development of XO embryos at mid-gestation. The present study was performed to determine whether the development of XO embryos is retarded compared with that of their XX littermates at early mid-gestation (day 8 of gestation), before reduction of litter size. The percentage of pre-somite stage XO embryos was greater than the percentage of pre-somite stage XX embryos, and the mean number of somites was greater in XX embryos than it was in XO embryos. These findings indicate that the development of XO embryos was retarded when compared with that of their XX littermates at early mid-gestation. This result is discussed with respect to the compensatory development of XO embryos at mid-gestation and the reduction of litter size shortly after early mid-gestation.
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