On the origin of lipofuscin; the iron content of residual bodies, and the relation of these organelles to the lysosomal vacuome. A study on cultured human glial cells.

Cultured human glial cells constitute a suitable model system for the study of lipofuscinogenesis in vitro. These cells, although not post-mitotic, can be kept for several months in stable monolayers due to their display of very pronounced density-dependent inhibition of cell growth. Residual bodies, or lipofuscin pigment granules, accumulate over time in this "pseudo" post-mitotic cell system. I. In early dense cultures, exposed to purified rat liver mitochondriae, it was possible to follow the uptake of mitochondriae and their degradation, which was found to be incomplete and result in the formation of numerous residual bodies containing lipofuscin-type material. It was concluded that incomplete degradation of mitochondriae may be an important origin of lipofuscin. II. Dense, older cultures exposed to electron dense marker particles (colloidal thorium dioxide) accumulated these markers within endosomes, and later in secondary lysosomes of various types, including residual bodies. It was concluded that residual bodies constitute an integral part of the lysosomal vacuome system. III. Phase III glial cells were cultured on formvar-coated gold EM-grids and studied by whole cell transmission electron microscopy using TEM and STEM techniques in combination with energy dispersive X-ray microanalysis. It was found that residual bodies contained iron. This fact was taken as a further indication that lipofuscin has its origin in autophagocytosed mitochondriae and ER-material rich in metallo-enzymes. Due to their high concentration of iron, residual bodies may constitute unstable structures within the cells. Since iron is a well known catalyst of various peroxidative processes, the surrounding lysosomal membrane might be damaged, e.g. by oxidative stress, with risk for leakage of degradative lysosomal enzymes into the cell sap.