Gamma irradiation-induced apoptosis in murine pre-B cells prevents the condensation of fibrillar chromatin in early S phase.

Local changes in chromatin structure leading to temporally distinct geometric forms were characterized in nuclei of reversibly permeabilized cells. Reversal of permeabilization was tested by 3H-thymidine incorporation and trypan blue dye exclusion. Apoptotic changes were visualized in a cell cycle dependent manner at the chromatin level by fluorescent microscopy in non-irradiated cells and after 400 rad Co60 irradiation. Fluorescent microscopy of chromatin structures belonging mainly to the interphase of the cell cycle confirmed the existence of specific geometric forms in nuclei of non-irradiated cells. In this control population, the following main transitory forms of condensing chromatin were distinguished: decondensed veil-like structures and fibrous structures in early and mid S phase (2.0-2.5 average C-value), chromatin bodies, semicircles later in mid S phase (3.0-3.5 C), precondensed chromosomes in late S (3.5-3.7 C) and metaphase chromosomes at the end and after S phase (3.7-4.0 C). Our results show that upon gamma-irradiation (a) the cellular and nuclear sizes were increased, (b) the DNA content was lower in each elutriated subpopulation of cells, (c) the progression of the cell cycle was arrested in the early S phase at 2.4 C value, (d) the chromatin condensation was blocked between the fibrillar chromatin and precondensed elongated chromosomal forms, and (e) the number and size of apoptotic bodies were inversely correlated with the progression of the cell cycle, with many small apoptotic bodies in early S phase and less and larger apoptotic bodies in late S phase.