The effects of platelet-derived growth factor and plasma components on saturation density in cultures of 3T3 cells were investigated. Both of these components of whole blood serum affect saturation density; however, when 3T3 cells become quiescent at high density in medium containing whole blood serum, only platelet-derived growth factor and fresh whole blood serum are capable of stimulating proliferation. Addition of fresh plasma- derived serum has little effect on cell growth. These results suggest that the platelet factor is the major determinant of saturation density in cultures of 3T3 cells maintained in medium supplemented with whole blood serum.

Experiments were performed to investigate the mechanism by which platelet-derived growth factor regulates saturation density. We investigated the possibilities of inactivation of growth factors by proliferating cells, and the effects of cell density on the response of 3T3 cells to platelet-derived growth factor. The amount of platelet- derived growth factor required to initiated DNA synthesis increases with increasing cell density. Some inactivation of growth factors by growing cells was detected, but this depletion was only evident at high cell density. We propose that density-dependent inhibition in cultured 3T3 cells is the result both of an increased requirement for the platelet- derived growth factor as the cultures become more crowded and of inactivation of growth factor activity by growing cells.

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