Chapter 21 



The Individuality Differentials and Potential 

 Immortality of Tissues 



In the preceding chapter we have analyzed the significance of individual- 

 ity differentials in the life of tissues growing in vitro and have tried 

 to explain the relative independence of the cells living under these con- 

 ditions from the nature of the individuality differentials and the diminution 

 in the significance of the species differentials of the surrounding media. The 

 same factors which are active under these conditions enable the cells to live 

 and propagate indefinitely, provided definite experimental requirements are 

 fulfilled. It could be shown that some cells and tissues of mammalian 

 organisms are potentially immortal. This holds good, with the reservation 

 that the term "immortality" is applied here in a relative, not in an absolute 

 sense, the immortality being limited by the need of the existence of certain en- 

 vironmental factors, which in all probability will come to an end in some dis- 

 tant future. 



However, the potential immortality of various mammalian tissues was first 

 recognized in the case of tumors. In 1901, we showed that it is possible to 

 transplant tumors through many consecutive generations of animals of the 

 species or strain in which the tumor originated. There seemed to be no 

 limit to the continuous life inherent in the propagated cells, inasmuch as the 

 termination of these long continued serial transplantations depended solely 

 upon accidental, unfavorable factors which could be avoided. Furthermore, 

 since it was evident that tumor cells are merely ordinary tissue cells which 

 could be transformed into tumor cells at will under well-defined experi- 

 mental conditions, the conclusion was justified that also the normal cells 

 from which the tumor cells were derived, have the potentiality to immortal 

 life. 



Subsequently, a second method, already mentioned, was used by Carrel 

 and Ebeling, who transferred embryonic connective tissue cells serially from 

 generation to generation in tissue culture. Here the embryonic cells are 

 stimulated to multiply indefinitely by the conditions which have been pre- 

 pared for them experimentally; when transferred serially to fresh culture 

 media, they may be kept alive indefinitely. But while it is mainly embryonic 

 fibroblasts which have been propagated in this way from generation to 

 generation, there are a considerable number of types of normal cells which, 

 after transformation into tumor cells, have acquired the ability to propagate 

 indefinitely. This is true not only of different types of connective tissue cells, 

 but of mammary gland tissue and various other epithelial cells; also of 

 endothelial and cartilage cells ; indeed, it is in principle true probably of all 

 cells which constitute transplanted malignant as well as some benign tumors. 



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