Role of platelet factors in the growth of cells in culture.
The studies reported suggest that the principal mitogen(s) present in sera responsible for the proliferation of diploid cells in culture is derived from the physiologic response of platelet adherence, aggregation, and release upon their exposure to factors present in serum, such as thrombin, or in tissue, such as collagen. Since it is impossible to make whole blood serum without platelet release, all sera contain platelet mitogenic factor(s). In contrast, serum made from platelet-free plasma lacks mitogenic activity and permits maintenance of cells in culture in a quiescent state for long periods if the cells are routinely fed. The platelet factor(s) appears to be a heat-stable, basic polypeptide or protein, that upon exposure to the cells recruits them into the cell cycle, DNA synthesis, and mitosis. The factor(s) has been shown to act not only in cell culture but in vivo as well. Maintaining cells in a culture medium containing platelet-free, plasma-derived serum may be more analogous to the quiescence of adult cells in vivo, since quiescent cells in adult tissues are normally exposed to interstitial fluid that is probably more like a filtrate of plasma or lymph rather than to whole blood serum. In contrast, growth of cells in a culture medium containing whole blood serum would be more analogous to the pathologic situation that occurs during tissue injury accompanied by hemorrhage.