CYTOGENETIC ANALYSIS 507 



Whenever employing bakelite caps with cemented rubber linings for any kind of 

 stopper use, limit purchases to the type distributed by Corning Glass Works, only. Specify 

 cemented rubber lining (grey or ochre). It has been our experience that other kinds of 

 bakelite caps with cemented rubber linings are inferior and always troublesome. 



Heavy-duty centrifuge tubes are more serviceable. Pipette-sterilizing boxes should 

 be of monel metal. The Touch-O-Matic bunsen burner (912, Microbiological Associates) 

 is a convenient item wherever high temperature is a comfort problem. An electric heating 

 mantle for the base of the distilled-water flask is most helpful in reducing the heat-load in a 

 room. Whenever possible, the use of rubber policemen (obtainable from any supply house) 

 to scrape cultures, rather than trypsinization, is helpful. Use of policemen should be 

 restricted to flasks that are to be subcultured routinely and not for cytologic use, because of 

 the tendency to form small clumps of cells that interfere with chromosomal preparations. 



DISCUSSION 



Dr. Burdette: Thank you, Dr. Yerganian. Dr. T. C. Hsu, of the M. D. Anderson 

 Hospital and Tumor Institute, will discuss Dr. Yerganian's paper. 



Dr. Hsu: Dr. Yerganian's paper contains in detail the technical advances in 

 mammalian karyology and emphasizes normalcy and uniformity of material. It is 

 indeed of prime importance for genetic studies in somatic cells to maintain populations 

 of cells that are primarily diploid and homogenous. At the present time, however, it 

 is not quite feasible to grow uninterruptedly diploid human cells forever. Some 

 investigators say they could, but extensive investigations conducted by Hayflick and 

 Moorhead 537 show that diploid strains of cells may last as long as 50 transfers before 

 reaching an inevitable stage of degeneration. However, large quantities of such cells 

 can be harvested and frozen during their peak of growth so that they can be thawed 

 when needed. This conclusion was arrived at by many workers. Nevertheless, 

 frozen cells, after thawed and regrown, will finally degenerate as an inevitable outcome. 

 Thus in investigations concerning somatic genetics, one faces a grave problem, namely, 

 inability to procure a constant supply of standard and mutant materials which workers 

 painstakingly establish. 



Most of the perpetuated lines of cells available in laboratories today are aneuploid. 

 In addition, practically all the populations of cells are polymorphic in their chromo- 

 somal constitution. Usually there is a predominating type of cell (the stemline) with 

 its characteristic chromosomal complement, and other types with chromosome number 

 above or below it. In essence, variability, instead of uniformity, is the rule in popula- 

 tions of mammalian cells in vitro. This variability, nevertheless, offers a unique system 

 for genetic studies, a system that has opened a number of new avenues of research, 599 

 and will continue to contribute in various ways to our knowledge of differentiation, 

 metabolism, and heredity. 



How does a diploid tissue, after growth in vitro for a period of time, become 

 aneuploid or heteroploid ? This transformation process has been witnessed in several 

 lines of cells from mouse and Chinese hamster. 389 ' 602 ' 782 In the mouse, the process 

 seems to follow the diploidy-tetraploidy-heteroploidy pattern, and in the Chinese 

 hamster, establishing stemlines around diploidy (hyperdiploidy and hypodiploidy) 



