THE RELATION OF CELLS TO ONE ANOTHER 21 3 



moting the multiplication of fibroblasts and of epithelial 

 cells. This phenomenon must be attributed to the production 

 of either embryonic proteins, or of proteoses and peptones. 



4. Specific Growth-Inhibiting Factors for Various Cell 

 Types. When fibroblasts and epithelial cells are placed in a 

 medium composed of diluted plasma, they go on multiplying 

 for a few days, but their rate of multiphcation is less active 

 than in Tyrode solution. The significant fact is thus brought 

 to hght that not only is plasma not a nutrient substance for 

 these cells, but that it inhibits their multiphcation. This 

 effect increases progressively with the age of the animal that 

 supplies the blood. It is not due to a special condition of the 

 proteins during adult and old age. Once isolated, these 

 proteins have no inhibiting or activating effect on cell 

 proliferation. But quite the reverse, the lipoids that can be 

 extracted from plasma possess a very marked inhibiting 

 effect on the growth of fibroblasts. The plasma of an old 

 animal contains a large amount of lipoids, and they are 

 more toxic for tissue cells. However, the substances exerting 

 such a marked effect on fibroblasts and epithelial cells do 

 not prevent the multiplication of macrophages. Macrophages 

 proliferate in the blood of an old animal, although the 

 multiplication is slower than when they are cultivated in the 

 plasma of a young animal. 



5. Morphological Effect oj ■ Nutritional Changes. When 

 cells endowed with definite morphological characteristics 

 are placed in a medium where their nutrition becomes 

 modified, marked changes occur in their appearance. Blood 

 monocytes cultivated in a medium containing some red 

 blood corpuscles, or particles of protein precipitate, or 

 fragments of muscle killed by heat, increase rapidly in size. 

 After a few days, they may be ten times longer than they 

 were originally, and closely resemble tissue macrophages. 

 In fact, they cannot be distinguished from them by any 

 known morphological criteria. Inversely, tissue macrophages 

 cultivated in a medium containing only a minute amount of 

 nutrient substances decrease progressively in size and lose 

 their large neutral red vesicles. The mitochondria shrink 

 and the nucleus itself becomes smaller. The cells assume an 

 appearance analogous to that of blood monocytes. A similar 



