Evaluation of the reorganization in the high-order structure of DNA occurring during cell senescence.

Electron microscopic pictures of loosened chromatin obtained from human fibroblasts aged in vitro were analyzed with an image processor. It was found that the density of the 30-nm chromatin fibers decreased progressively through the cell population lifespan. The spacing between the fibers increased abruptly during the last divisions (phase IV). The evolution of these 2 parameters, density and spacing, was identical, respectively, with that of the decline in the rate of initiating DNA synthesis after seeding the cells and with that of the maximal percent of cells synthesizing DNA during a 24-h period between seeding and resting phase. Cells from 2 human donors (30 and 60 years old) with Werner's syndrome were compared, using the same methodology, with those of 9 normal donors ranging from 29 to 98 years old. The spacing between chromatin fibers in the Werner's cells was significantly increased as compared to the age-matched normal controls while the density was within the normal range. The results are discussed with regard to other modifications in DNA structure previously reported in the same cell system.