566 GROWTH IN TISSUE CULTURE 6 



group of cells may possess the necessary potentialities, may be grown in favourable 

 body fluids or other media, and may be in a state of reproductive quiescence, they 

 are, nevertheless, incapable of differentiation into their specific tissue if this is of 

 a complex type. A fourth factor is required, viz., interaction with cells of the other 

 tissues composing the organ". The thorough investigations of Grobstein (1953a, 

 b, c; 1955) have beautifully demonstrated and more fully substantiated this 

 statement. Normal morphogenesis of the mouse salivary gland depends upon the 

 presence of both mesenchymal and epithelial elements from the rudiment being 

 explanted together. In cultures of submandibular epithelium placed together 

 with mesenchyme from other parts of the embryo, morphogenesis does not occur. 

 Metanephric rudiments, as was first observed by Rienhoffand by Drew, show a 

 similar dependence — i.e. no morphogenesis takes place if either epithelium or 

 mesenchyme is cultivated alone, though in this case several non-metanephric 

 epithelia can induce tubule formation by metanephric mesenchyme. 



That histodifferentiation of explants in tissue culture is not merely the result of 

 development and growth of cells already spatially oriented is shown particularly 

 well by the experiments of Moscona (1952) and Moscona and Moscona (1952). 

 Cells from trypsin-disaggregated tissues of the early chick embryo were found to 

 regroup into structures analogous to the normal organization of these tissues. 

 A suspension of cells from chick mesonephros gave rise to tubules similar to normal 

 urinary tubules. Weiss and James (1955) have shown that the dissociated cells of 

 chick skin, reduced to a suspension of predominantly single cells, will reassociate 

 and by 16 days in vitro develop typical feather germs. 



The study of the interactions between cells growing in tissue cultures, at the 

 morphological and at the chemical levels, is already giving form to the not yet 

 clearly defined concepts of induction and differentiation, and will undoubtedly 

 continue to uncover the basic mechanisms responsible for these processes (see 

 Chapter 4). 



IV. DEFINITIONS OF GROWTH IN TISSUE CULTURE 



As at Other biological levels, so in tissue culture, it is possible to define growth in 

 terms of a number of parameters. To estimate something corresponding to an 

 agreed definition of growth, it is necessary to select one or more of these para- 

 meters for measurement. The nature of the variability and multiple character 

 of the growth process in tissue cultures has been discussed and given due emphasis 

 by Tompkins, Cunningham and Kirk (1947); Weiss (1949a, b) and Waymouth 



(i949> 1952- 1954b). 



For a long time (about 1910-1940) the method of explanting small fragments 

 of tissue into a plasma clot containing embryonic tissue extract, and observing 

 the radial outgrowth of cells which took place approximately in one plane, was 

 the predominant tissue culture technique. "Growth" of such cultures was usually 

 assessed by the methods of Ebeling (1914, 1921) (see below), which consist in 

 making daily measurements of the radius or area of the areola of cells. These 

 methods are very simple to apply. They do not, however, measure a simple 



