CYTOGENETIC ANALYSIS 471 



convenience of procedures for replication to isolate special types of cells from genetically 

 known backgrounds, and minimal numbers of genetic linkage groups. 



One of the more recent surveys on numbers of chromosomes in propagated tissues 

 accrued over the past ten years has been reported by Levan. 781 Although shifts in 

 chromosomal numbers are observed frequently the significance of such changes remains 

 obscure when limited to near-tetraploid derivatives. Likewise, the tendency for ampli- 

 fication of histocompatibility relationships among near-tetraploid sublines derived 

 during the course of propagating near-diploids that display restrictive host specificities 

 remains to be fully clarified,' even though considerations provided by Hauschka, 527 

 Fox, 402 Ising 644 and Sachs and Gallily, 1145 are noteworthy. Since the majority, if not 

 all, of the types of cells continuously propagated undergo numerical alterations soon 

 after displaying rapid proliferation, the cellular population eventually bears little re- 

 semblance to the original parental karyotype and, except for species-specific im- 

 munologic properties, such alterations are difficult to control without the aid of selective 

 and repeated cloning procedures. Klein has circumvented this difficulty during the 

 course of experimentation in vivo by preserving parental immunologic differences and 

 karyotypes of induced malignancies by appropriate methods of freezing. 



The number of normal tissues and viral-induced tumor cells that retain their 

 classic diploid (normal) karyotype when cultured is exceedingly small, and those which 

 are reported as being diploid need further verification. The availability of diploid 

 cells for experiments of various designs would be ideal. Such a development is one of 

 the more challenging aspects of tissue culture today. The numerous altered forms of 

 normal cells presently utilized because of necessity and convenience in maintenance 

 have undergone as much or more mutation as the malignant types currently employed. 

 The recent development of a number of diploid and quasi-diploid cultured derivatives 

 from the Chinese hamster, having 14-hour generation times or less in chemically defined 

 media, has been an encouraging step at this laboratory. Retention of the diploid 

 status in cultures growing rapidly is an essential prerequisite for deriving equally stable 

 monosomic variants by means of extensive cloning procedures. In this way, distinct 

 nutritional variants and mutants induced by X and ultraviolet radiation may be 

 harvested following the use of minimal media. On the other hand, deficient media 

 need not be employed when specific antimetabolites are involved in conjunction with 

 plating trials for the purpose of selecting mutants. Specific mutations may eventually 

 be linked with recognizable chromosomal structures (intact monosomies or structural 

 variants involving heterochromatin) . In the event the normal chromosomal com- 

 plement cannot be identified readily, suitable markers have been utilized by Hsu and 

 Kellogg 604 and Harris and Ruddle. 522 Simplification of cellular types and nutritional 

 requirements must be attempted by all who are concerned with employing, with 

 greater value, the vast genetic storehouse provided by the house mouse, the curious 

 immunologic variegation of the Syrian hamster, differences in viral susceptibility of the 

 primates including man, and the small number of linkage groups and readily identifiable 

 autosomes and sex chromosomes provided by the normal and diabetic Chinese hamster. 



