Cationized ferritin binding and internalization during in vitro aging of mouse embryonic fibroblasts

The anionic surface sites of mouse embryo fibroblasts in primary culture, aging in vitro, have been studied using cationized ferritin (CF) as a marker. We have not observed significant differences between early and late cells with respect to CF binding; the labelling is uneven and patchy in both cases. Due to the increase of their surface area with the mitotic age, the total amount of negative charges, per cell, must however be much higher in the late passage cells. Virtually no differences in the rates of CF endocytosis via coated pits, with subsequent delivery to large vacuoles and lysosomes, have again been observed, but there was a cisternal formation from apposed plasma membranes in the late cells. New findings with regard to previous studies with CF concern the capping of the labelled surface components, followed by phagocytosis of the cap material, and the organization of the extracellular matrix in a rather regular network at the periphery of the cells.