ABSTRACT
Normal embryonic cells in culture show a characteristic growth pattern as described in the classical work of Todaro and Green for rodent embryo cells, and by Hayflick for human embryonic cells. Normal mammalian embryo cells in culture exhibit a series of growth properties which distinguish them unequivocally from cells of established lines. Those considered as most characteristic are the restriction at low cell density and the limited life span in culture. Analysis of the process of immortalization definitely relies on the predictable growth pattern of the indicator cells, usually primary embryonic rodent cells. Immortal clones are usually obtained by cotransfer of appropriate oncogenes and a selectable marker gene, and subsequent cultivation in a selection medium. This demands three simultaneously exerted cellular functions: the efficient expression of the resistance gene, that of the oncogene, and the ability to grow alone in a surrounding mass of dying cells.
