Growth enhancement of myogenic tumor cells by conditioned medium from embryo fibroblasts

Growth medium was conditioned by incubation on mouse embryo cells in vitro. Supplementation of agar suspension cultures with conditioned medium from primary cells, but not from established lines, readily enhanced colony development by mouse tumor cells. Only cells with the properties of myoblasts responded to conditioned medium. Other fibroblastoid cells and virus-transformed cell lines were not affected. Myogenic cells in agar cultures grew in the presence of conditioned medium but did not differentiate. Soluble collagen at 400 mug/ml possessed little colony-stimulating activity by comparison with fresh conditioned medium.