Lipid accumulation in the stationary phase of strain L cells in suspension culture.

Strain L (Earle) cells in suspension tissue culture exhibit a logarithmic growth period followed by a stationary or plateau phase. Lipid to cell and lipid to protein ratios were found to be at minimal levels during logarithmic growth and increased 13 per cent and 42 per cent, respectively, in cultures in the stationary phase. The lipid accumulation was due to elevated cellular levels of phospholipid and cholesterol, which each increased in nearly the same ratio. Cellular triglyceride and free fatty acid content was not significantly altered. There was a loss of cellular protein in the older cultures that largely accounted for the greater increase in the lipid to protein ratio. Electron microscopic examination of cells from the stationary phase revealed numerous autophagic vacuoles and dense bodies, many of which contained membranous debris and other cytoplasmic components in different stages of autodigestion. These varied and complex membrane-bound structures localized acid phosphatase activity on cytochemical examination, establishing them as autophagic lysosomes. The present correlative biochemical and morphologic studies indicate that the observed elevation of phospholipid and cholesterol in stationary phase cells occurred as a result of autophagocytosis and demonstrate the role of cell injury in the accumulation of lipid of this type.