By Karl Maramorosch

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1980). Neurons from senescent rats have been found to bind significantly more fluorescamine-labeled Con A t h a n young rat neurons (Bennett and Bondareff, 1977), and the bound lectin is present in patches or caps on old rat cells instead of in a uniform coat as in young. Scanning and transmission electron microscopy have revealed that the small, spindle-shaped active dividers of young WI-38 cultures possess areas of cytoplasmic ruffling with large bundles of microfilaments lying immediately below the membrane (Bowman and Daniel, 1975).

Recently, decreased synthesis of ribosomal RNA has been demonstrated in senescent WI-38 cells, and in heterokaryons derived from senescent and presenescent cells of this line (Rener and Nardone, 1980). Minor differences in the mRNA sequences in growing versus stationary mouse fibroblasts have been reported, but no similar comparisons exist for young versus old diploid cells in culture (Williams and Penman, 1975). Schneider et al. (1975) have reinterpreted the apparent decrease in RNA synthesis in senescent h u m a n fibroblast cultures on the basis of RNA synthesized per cell number, or per cellular DNA, and found that the net RNA synthesis was slightly increased in old cultures.

Chang (1962) has shown age-related increases in the incorporation of palmitate and cholesterol into cell lipids in h u m a n amnion cells during aging, but no age-related changes in the regulation of cholesterol synthesis has been found in skin and fibroblasts from old h u m a n subjects (Shakespeare and Postle, 1979). , 1970), although two enzymes of the hexose monophosphate shunt (6-phosphogluconate dehydrogenase and transketolase) show age-associated reduction (Cristofalo, 1970). The rate of proliferation of WI-38 cells parallels the rate of glucose utilization (Cristofalo and Kritchevsky, 1965), and mannose, fructose, and galactose may substitute for glucose.

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