210 THE MOSAIC STAGE OF DIFFERENTIATION 



and intestinal endothelial cells which had completely dediffer- 

 entiated in vitro were found to possess equally complete powers of 

 redifferentiation. 



The various strains of cells differ not only in their structural 

 characters, but determined physiological differences may also be 

 observed between cells which are morphologically indistinguish- 

 able. Thus, strains of fibroblasts have been found differing from 

 one another in their nutritional requirements, and differing also 

 from epithelial cells and macrophages.^ The differences show them- 

 selves in the rate of proliferation of the cells in any given medium, 

 and by specific reactions, such as cytolysis,^ to certain induced 

 pathological conditions. 



Tissue-culture methods have also thrown certain new light upon 

 the problem of differentiation. It has been found in the first place 

 that fibroblasts, isolated from different organs of the same embryo, 

 exhibit different growth-rates and other physiological characteristics 

 such as resistance to acidity and capacity to digest fibrin: these 

 differences appear to be persistent. For instance, fibroblasts 

 isolated from the skeletal muscle of a 17-day chick embryo have a 

 growth-rate nearly three times as high as that of fibroblasts from 

 the thyroid of the same embryo, and nearly ten times as high as those 

 from the heart. ^ A further and more surprising result is that com- 

 parable physiological differences exist between fibroblasts isolated 

 from the same organ of embryos of different age. For example, 

 fibroblasts from the skeletal muscle of the leg of the 17-day chick 

 embryo have a growth-rate about 60 per cent, higher than those 

 from the same tissue of 8-day embryos.^ These differences con- 

 tinue to be shown even when the strains have been subjected to 

 marked environmental changes, and are returned to standard 

 conditions. 



There seems no escape from the conclusion that the primitive 

 mesenchymatous tissue, from which the fibroblasts of the body are 

 derived, receives some impress affecting its physiological charac- 

 teristics from the regions in which it happens to find itself, and this 

 impress changes with age. As regards the regions, the process is 

 doubtless an aspect of the self-differentiation which we have been 



^ Carrel, 193 1. ^ Horning, 1932. 



^ R. C. Parker, 1932 A. * R. C. Parker, 1932 b. 



