Chromatin structure, DNA synthesis and transcription through the lifespan of human embryonic lung fibroblasts.

The presence in terminal embryonic fibroblasts of small molecular weight (MW) DNA independent of bulk DNA could be ascertained by three different techniques performed in parallel. This alteration was not artifactually induced, either by high pH and the detergent used or by the release of cellular enzymes. An increased thermolability of old chromatin was also observed. Cells with altered chromatin synthesized DNA and RNA according to a pattern similar to young type nuclei. Long-term treatment with hydrocortisone significantly increased the cell yield but did not prevent, in the late passages, the occurrence of old-type chromatin; the nucleolar filamentous masses, however, maintained a 'young' pattern. Short-term treatment induced only a moderate reversion in the appearance of chromatin lesions. Direct evidence was obtained of increased gene expression in the presence of hydrocortisone.