194 CELLS, TISSUES, AND ORGANISMS 



connection with various types of mammalian diflFerentiation. These in- 

 clude gene and chromosome alterations, specific changes in cytoplasmic 

 genetic determinants of non-random kinds, induction of non-constitu- 

 tive enzyme activities in response to specific inducers, achievement of 

 new cellular steady states which persist despite restoration of the ex- 

 ternal environment, as demonstrated so elegantly in the experiments 

 of Novick and his co-workers (1959), hereditable changes such as 

 those in the ciliary antigen type of Paramecium ( Sonneborn, 1948 ) , and 

 genetic changes produced by external agents, as in the case of transduc- 

 tion and transformation. Tools by which processes like these can be 

 searched for efiFectivity are now at hand. It seems probable that diflFer- 

 ent types of normal and abnormal differentiation processes in mam- 

 mals will involve different mechanisms. Thus on the one hand, 

 meiosis and the formation of red cells in mammals constitute differ- 

 entiation processes attended by gross but specific changes in the 

 chromosomal constitution; on the other hand, it can be demonstrated 

 with reasonable probability that some cells with similar chromosomal 

 constitution have different antigens. Therefore, whereas cells arising 

 from the skin have a chromosomal constitution similar to that of the 

 white blood cells, skin homografts between persons of the same blood 

 type cause intense immunologic reactions, while repeated blood trans- 

 fusion does not. Other experimental studies have also demonstrated the 

 existence of cell antigens which are tissue-specific (Pressman, 1949). 

 It is evident that study of the biology of the mammalian cell has 

 barely begun. It is easy to foresee several results: the establishment of 

 conditions for control of processes such as meiosis and fertilization in 

 vitro; the setting up of cell banks in which a person's own cells can be 

 used for re-injection in case of traumatic accident, or for treatment of 

 diseases such as leukemia or perhaps even normal aging, thus restoring 

 cells to depleted tissues without the complications of immunologic re- 

 jection; the detection in cultured cells of obscure or recessive gene de- 

 fects; even the use of processes such as specific transformation to 

 change the genetic constitution of germ cells. Both the fundamental 

 and the applied facets of such advances will bring tremendous new 

 possibilities for control of biological processes in man and other mam- 

 mals. 



References 



Bender, M. A., 1959. "X-Ray-Induced Chromosome Aberrations in Mammalian 

 Cells in Vivo and in Vitro," Spec. SuppL, Int. }. Rad. Biol. 104. 



Chu, E. H. Y., 1959. Discussion, in Spec. Suppl. Int. J. Rad. Biol, 13. 



Drew, R. M, and Painter, R. B., 1959. "Action of Tritiated Thymidine on the 

 Clonal Growth of Mammalian Cells," Rad. Res. 11, 535. 



