A mammalian cell was studied as an independent microorganism at the University of Colorado Medical Center. Single mammalian cells were plated so that each cell would produce a discrete colony. When 100 single cells of the HeLa S3 clonal strain were inoculated into a petri dish, each cell multiplied about a 1,000-fold and produced a macroscopic colony in about 10 days. These colonies could be counted and picked for the establishment of new clonal stocks. These mammalian cell stocks could be cultivated with genetic stability comparable to that of microorganisms like Escherichia coli provided the molecular environment was carefully controlled. The method makes it possible to sample normal cells from human subjects and grow these indefinitely without change in their chromosomal constitution.
Puck1 and his associates studied the genetic behavior of mammalian cells in vitro. Tjio and Levan demonstrated in 1956 that the human karyotype contains 46 and not