562 GROWTH IN TISSUE CULTURE 6 



fest those properties which distinguish them from other cells. Appropriate choice 

 of culture conditions can determine whether the cells either multiply rapidly, or 

 survive without proliferation, or manifest or develop their functions and differen- 

 tiated characteristics. 



Survival of cultures, usually with a very low rate of proliferation, results when 

 the total metabolic activity of the cells is lowered, either by creating a partial 

 nutritional deficiency with respect to that necessary to maintain growth, or by 

 reducing the temperature. The pioneers in attempts to create a system in which 

 the cells would survive in dynamic equilibrium were Fischer and Parker (Fischer, 

 1926; Fischer and Parker, 1928-29, 1929). Parker (1936) was able to cultivate 

 chick fibroblasts in a healthy but non-multiplying state for over a year, by using 

 a medium consisting of diluted serum. Addition of embryonic extract to such 

 cultures immediately stimulated proliferation. Brues, Rathbun and Cohn (1944) 

 also restricted growth by use of a nutritionally incomplete medium. Among 

 others whose aim was to maintain cells at a very low rate of proliferation, sufficient 

 only for renewal, and so to set up conditions similar to those obtaining in the adult 

 organism, were Gey, Hanks and Barrett (1948) and Hanks (1948). Their cells 

 were kept in a plasma-embryonic extract medium at a reduced temperature (32°- 

 28°) for several months, and required only occasional replenishment of the medium. 

 This method has practical advantages for maintaining stocks of cells without 

 frequent attention. 



Tissue culture techniques afford possibilities for studying both growth and 

 differentiation of cells and cell populations, and the interdependence of the two 

 processes. The literature on tissue culture contains, in various forms, statements 

 that "cells growing in tissue culture dedifferentiate" and that "growth and differ- 

 entiation are incompatible". In evaluating these assertions, one has first to 

 clarify the meaning attached to the word "differentiation" by individual authors. 

 The review of Bloom (1937) contains a discussion of these problems, preceded by 

 careful definitions of terms. Cellular differentiation is defined as "the process 

 which results in specialization of the cell as measured by its distinctive actual and 

 potential functions", and is "sharply distinguished from those cellular changes, 

 no matter how striking they may be, which are only temporary reflections of the 

 reaction of the cell to new stimuli". Differentiation is really a relative term, for 

 there are all degrees of difference between the totipotent cell and one which can 

 be recognized by various criteria as highly specialized. It is a common experience 

 to see, in cells in tissue culture, a loss of characteristic morphological features, and 

 this has often been loosely termed "dedifferentiation". But, as Strangeways 

 (1924b) said: "It should be clearly understood ... that the term 'dedifferentiation' 

 is here used only to imply the latency, not the loss, of those potentialities which 

 enable cells to assume the specific structure and histological architecture of the 

 organ of which they form part". It is best, unless change of potencies can actually 

 be demonstrated, to assume that cells in tissue culture retain the inherent capacities 

 which their parents had at the time of explantation, and to beware (Weiss, 

 1949a, b) of equating visible changes of form with irreversible changes in character. 

 Very striking changes of morphology can take place in cultivated cells, so that for 

 example Weiss and Fawcett (1953) were able to see, in cultures of chicken leuco- 



