Remembering the cell fate during cellular differentiation.

Higher eukaryote contains several hundreds of different cell types, each with a distinctive set of property defined by a unique gene expression pattern, even though every cell (with minor exception) shares the common genome. During cellular differentiation, the committed gene expression pattern is set up and propagated through numerous cell divisions. Therefore, cells must have evolved some elegant and inherent mechanisms to remember their expression states for the requirement of the stability of differentiation and development. Here we speculate a hypothetically cellular memory mechanism. In this hypothesis, the cell-cell variation during cellular differentiation may result from the inherent stochastic gene expression. The evolution of histone and distant regulatory sequences change the parameters of expression stochasticity. S-phase-dependent gene activation and epigenetic marks on chromatin provide means to discriminate transcriptionally active and repressive states. Eventually, mitotic memory mechanisms have been developed through which these expression states are transmitted through numerous cell divisions. 2005 Wiley-Liss, Inc.