Karyotopic and cell number analysis of 3.5 day post coitum preimplantation mouse embryos was used to determine whether XY embryos had more cells than XX embryos at the late morula/early blastocyst stage. This proved to be the case for the CD1 strain (for which it had previously been shown that XY embryos form a blastocoel earlier than XX embryos) and for the MF1 strain. However, this increased cell number was not seen in MF1 embryos carrying an RIII strain Y in place of the MF1 Y. Furthermore, interstrain crosses between CD1 and the MF1, YRIII strain showed that the cell number increase segregated with the CD1 Y but not with the RIII Y. It is concluded that the CD1 and MF1 Y chromosomes carry a factor that accelerates the rate of preimplantation development.

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