Progress in cytogerontology.

The finite in vitro lifetime of cultured normal cells is interpreted to be aging at the cellular level. In addition to the inverse relationship between donor age and population doubling potential (PDP), a number of biochemical and physiological increments and decrements occur prior to the cessation of cell division. The reconstruction of replicating normal human cells from the nuclei of "young" cells and the cytoplasm of "old" cells (and the reverse) suggests that the nucleus governs PDP. Several morphological changes were found to occur in late phase III cells held for up to one year in culture. Autoradiography studies show that (1) a cell population may be composed of several subpopulations, each of which is at a different stage in its life history and (2) lipid synthesis is affected much less as cells age than is DNA, RNA and protein synthesis. Changes occurring in the genetic program of individual cells seem to be the most tenable hypothesis to explain fundamental causes of aging.