[The frequency of sister chromatid exchanges in preimplantation mouse embryos]

The frequency of sister-chromatid exchanges (SCE) was analysed in spontaneously and superovulated morulae and blastocyst of CBA, C57Bl, F1 CBA x C57Bl mice in culture with bromodeoxyuridine (BrdU). The background level for SCE was found to be 4-5 times higher in early embryos than in fetal or adult tissues of the mouse. The phenomenon of spontaneous sister-chromatid differentiation (SCD) of blastocyst chromosomes without any treatment with BrdU was observed. The SCE frequency was calculated on such chromosomes and was the same as with BrdU. A possible mechanism of this phenomenon is suggested.