S phase, an evolutionary chromatin condensation state from G1 to G2, in a breast epithelial cell line

In order to better understand the changes in DNA organization during the cell cycle, we quantified the chromatin texture of breast epithelial cells and followed its evolution through a cell cycle. The diversity of quiescent cell states led us to limit this study to proliferating cell phases, and to choose a cell line with no G0 cells, the MDA AG cell line. We recently developed a methodology for characterizing in situ the cell cycle of breast epithelial cell lines using a cell image processor. This method is based on 15 densitometric and texture parameters computed on individual Feulgen-stained nuclei and on multiparametric analysis of the resulting data. Chromatin pattern assessment is based on nine texture parameters measured from grey-level co-occurrence and run-length section matrices. In the present study, texture parameter computation showed gradual and progressive modifications of nuclear texture. While discrimination of G1, G2 and M phases was possible, we could not discriminate G1 from S and S from G2. The chromatin pattern (defined by these nine parameters) in the G1 and early S phases, on the one hand, and in the late S and G2 phases, on the other hand, were similar. The parameter values of cells in the S phase progressively increased from G1 to G2. Two interphase chromatin condensation states were distinguished in these breast cells: a base state characteristic of a prereplicative stage and a very granular state characteristic of a postreplicative stage. We hypothesized that S cells are a blend of these two states, the evolution of a non-duplicated state toward a duplicated one.