CYTOGENETIC ANALYSIS 497 



of tritiated thymidine by heterochromatin as reported by Lima-de-Faria, 788 supports 

 the writer's interpretations for the presence of two forms of the X chromosome in the 

 Chinese hamster. Unpublished findings of Taylor and of Yerganian and Grodzins 

 indicate that one of the small metacentrics of the Chinese hamster also is composed of 

 heterochromatin (differential tritiated-thymidine uptake). The persistence o£ Minutes, 

 particularly viewed among murine ascites tumors of long cultivation (figure 62) may be 

 nothing more than residual chromosomes having a small amount of heterochromatin 

 about the functioning kinetochore. Thus, kinetochores may persist for varying lengths 

 of time in cultures as well as during organic evolution. 



The heteropycnotic sequence noted during meiosis in the male spermatocyte of 

 most mammals is certainly an unique circumstance. The significance of allocycly or 

 differential stainability of heterochromatin and euchromatin remains to be unraveled. 

 Current application of tritiated thymidine to the chromosomes of the Chinese hamster 

 is probably the simplest system one may use to record differences in the rate of DNA 

 (chromatid) duplication along distinct portions of known autosomes and sex chromo- 

 somes derived from both normal and malignant sources. The readily propagated lines 

 of cells having as low as ten hour generation times, classical diploidy, quasi-diploid, and 

 low aneuploid chromosome numbers, including monosomic (XO) sublines and variant 

 mutant X chromosomes, provide an array of exceptional cell types to follow the uptake 

 and/or blockage of essential and substituted purines and pyrimidines. The uptake of 

 tagged synthetics and their distribution, whether they be specific or random, may be 

 readily traced to given chromosomal loci (euchromatic or heterochromatic) by means 

 of autoradiography. Such an approach may well be the basis by which one may 

 influence the hereditary pattern of a given cell line or species. Simplicities for detecting 

 biological and chemical transduction phenomena are currently indicated among the 

 vast number of Chinese hamster type cultures now on hand. 



DESIRABLE PROPERTIES OF SYSTEMS in vitro 



1. Retention of the diploid or monosomic states, in primary and subsequent 

 clonal isolations. 



2. Small number of chromosome or linkage groups. 



3. Recognizable autosomes, sex chromosomes, and known markers. 



4. Comparatively rapid proliferation in chemically defined medium. 



5. Elimination of whole serum requirements or use of dialyzed serum with little 

 or no toxicity. 



6. Relatively stable plating efficiency and general appearance; limited non- 

 disjunction and polyploidy. 



7. Derivation of type from genetically known inbred lines with known histo- 

 compatibilities. 



8. Preferable origin of type from reticuloendothelium or lymphoid tissue following 

 appropriate induction of specific antibodies in the host. 



