The word “transformation” is used to describe the permanent acquisition of new characters by cultured cells. Of the many different types of transformation that are known, the best studied is neoplastic transformation. Here, cells that have been exposed to certain viruses, chemicals (or X-rays) change their social behavior and take on many of the attributes of tumor cells. All mammalian cells are maintained in culture by the periodic replacement of medium and serum. Populations of untransformed cells divide while they remain sparse, but as the culture becomes more crowded, the growth rate decreases dramatically; in fact the cells of some lines stop dividing altogether once they have formed a confluent monolayer. It is not understood how this growth control is mediated. At one time it was thought that the growth of cells in tissue culture is controlled entirely by “contact inhibition”–the individual cells in a culture responding to the close proximity of other cells by ceasing to multiply. © 1974, Elsevier Inc. All rights reserved.
Sambrook, J., & Pollack, R. (1974).  Basic Methodology for Cell Culture-Cell Transformation. Methods in Enzymology, 32(C), 583–592. https://doi.org/10.1016/0076-6879(74)32058-7