Dynamics of chromatin changes in live one-cell mouse embryos: a continuous follow-up by fluorescence microscopy.

Conditions of minimal dye concentration and minimal irradiation which allow the continuous observation of pronuclei in live unicellular mouse eggs by fluorescence microscopy have been found with the use of Hoechst 33342 as fluorophore and a camera of high sensitivity coupled with an image processing system allowing true integration of weak fluorescent signals and further treatment and analysis. Under these conditions the developmental potential of the embryos is not affected. Using such an approach, which avoids eventual artifacts due to fixation procedure, we describe the changes in the nuclear organization and chromatin structure, from formation of pronuclei to mitosis, with particular attention to the chromatin associated with nucleoli and the timing process of chromatin condensation.