MAMMALIAN CELL GROWTH IN TISSUE CULTURE 183 



Most of the individual advances that led to this specific molecular 

 scheme arose from studies of unicelled microorganisms, and viruses, in 

 which the ease of genetic and biochemical manipulation has permitted 

 experimenters to ask precise questions, often in quantitative fashion, 

 of their materials. The mammalian organism would appear to be sus- 

 ceptible to the same kind of experimental analysis, if one could study 

 its component somatic cells as independent microorganisms. In this 

 way one might hope to establish the pathways of the transfer of infor- 

 mation and the concomitant control of biochemical synthesis which 

 make up the totality of each individual cell's chemical potentiality. 

 Such analysis might be expected to: identify individual mammalian 

 genes, most of which are still unknown; map their positions accurately 

 on the chromosomes; establish their equivalent biochemical operations; 

 and permit tests for genetic operations such as transformation, trans- 

 duction, mitotic crossing-over, and mutagenesis. 



Recent developments in the technological aspects of growth and 

 manipulation of mammalian cells in vitro now appear to make possible 

 such an experimental program. For example, refinements in tissue cul- 

 ture now permit the following operations as routine procedures: 



1. Routine establishment of euploid cell cultures from any indi- 

 vidual of a number of animal species, including man (Puck, Cieciura, 

 and Robinson, 1958 ) . 



2. Growth of such cultures for long periods, and storage at low 

 temperatures, without the gross changes in chromosomal constitution 

 which had been the rule in earlier tissue culture procedures (Puck 

 gfflZ., 1958; Puck, 1958-59). 



3. Plating of single cells under conditions where virtually every 

 cell produces a discrete colony. Such colonies can be counted to yield 

 a quantitative measure of the capacity for growth ( Puck, 1959 ) . 



4. Ready isolation of clones, establishment of mutants with desir- 

 able markers, and scoring of mutagenesis ( Puck, 1959 ) . 



5. Routine delineation of the chromosomes of such somatic cells 

 (Tjio and Puck, 1958). 



6. Use of tritiated thymidine to label DNA synthesis in mammalian 

 chromosomes and thus establish the major periods of the cell's repro- 

 ductive cycle ( Painter, Drew, and Hughes, 1958 ) . 



7. Formulation of a defined medium in which at least certain 

 mammalian cell strains can grow as single cells, with \'irtually 100 per 

 cent plating efficiencies ( Fisher, Puck, and Sato, 1959 ) . 



These new methods have made possible many kinds of studies 

 which focus on the potentialities of the mammalian cell as an inde- 

 pendent microorganism. Thus, on the genetic side, mutant clones of 

 cells have been established, and at least a beginning has been made in 



