[The nature of a proliferation block in differentiated cells with heterokaryons as a model: various types of absence of proliferation in cells in terminal differentiation].

Heterokaryons obtained by fusion of proliferating and terminally differentiated cells were studied. The data obtained suggest that mechanisms of proliferation arrest are different in macrophages on one hand and nucleate erythrocytes and polymorph leukocytes on the other. Macrophages appeared to be devoid of factors preventing replication in nontransformed and spontaneously immortalized cells. Inhibition of proliferation was probably due to certain modifications of macrophage genome which arise during differentiation and can be compensated by the effect of "immortalizing" oncogenes. On the contrary, nucleate erythrocytes and polymorphs evidently contain some factors mediating negative control of proliferation. For reactivation of DNA synthesis in these cell types after fusion with other cells the latter did not have to be immortalized. After cell fusion macrophages specifically inhibit DNA synthesis in cells containing active oncogenes.