In Situ Analysis of Chromatin Proteins During Development and Cell Differentiation Using Flow Cytometry

General remodeling of chromatin is associated with events determining cell fate and the expression of specific genetic programs (1 ,2 ). In almost every case there is a tight link between these chromatin remodeling events and a drastic modification of the cell cycle parameters. One of the most striking examples of this phenomenon is early embryonic development. Indeed, transition periods have been defined during development characterized by the modification of both chromatin constituents and the proliferative capacities of cells (3 –6 ). For instance, during Xenopus laevis early development, the midblastula transition (MBT) is characterized as a period after which the somatic replication-dependent type of H1 starts to accumulate and the embryonic H1 (B4) and HMG1 decrease dramatically (7 ). Moreover, in vertebrates, later during development, other subtypes of H1 accumulate:H1^o in almost all vertebrates, H1t in mammalian spermatogenic cells and H5 in avian erythrocytes, again associated with a drastic modification of proliferative capacities of cells (7 ). To better understand the significance of these transitions, it is important to correlate chromatin remodeling events with cell cycle modification events (8 –10 ). For instance, we showed previously that during early Xenopus development, the type of H1 expressed appeared to be more related to the frequency of cell division than any other cellular event (6 ,7 ).