GROWTH OF EMBRYONIC TISSUES 



247 



continues to grow at a very constant rate. It continues 

 to advance in the tube without any slowing up at all. 

 In a situation where the volume of the nutrient me- 

 dium is relatively unlimited, growth is also relatively 

 unlimited. So far as one can tell, the mold would 

 grow indefinitely at a constant rate as long as fresh 

 medium were available. Of course, this continual 

 growth applies only to the advancing front of the 

 mold. The rest of the mold soon stops growing. 



Thus there appears to be no real, intrinsic aging 

 process in some kinds of cells. Aging occurs only 

 when the external medium is limited in volume. 

 When it is continually renewed the cells divide in- 

 definitely. 



SUBCULTURE 



STABILE 

 SIZE 



MAXIMUM 

 GROWTH 



SUBCULTURC 



Growth of embryonic tissues 



We find a similar situation in the growth of tis- 

 sues from the embryo when tissues are grown in a 

 culture medium. If we use a limited volume of tissue- 

 culture medium and place in it a piece of tissue such 

 as the heart, we find that this tissue begins to grow 

 at first rapidly and then slows down. Finally growth 

 stops. Now nothing has happened to these cells, be- 

 cause if a group of cells is taken out of this tissue 

 culture and placed in a new, fresh medium, the cells 

 will go through the same cycle (Fig. 163) . And, as in 

 the case of the protozoa and yeast, this process can be 

 carried on indefinitely. As long as the medium is re- 

 newed, these cells continue to divide and grow. In 

 this way cells in tissue culture will grow for years 

 after the organism from which they originated dies. 



Thus aging on a cellular level appears to be a 

 rather complex process, involving the population 

 density of cells, the concentration of the nutrients 

 (including oxygen), and the accumulation of waste 

 products. Growth, in general, continues at a fairly 



STABILE 

 SIZE 



MAXIMUM 

 GROWTH 



ORIGINAL CULTURE 



Fig. 163. Growth in 

 cultures and subcultures 

 of tissues. Bottom to 

 top: Original small 

 piece of tissue grows to 

 a maximum size and 

 then stops. A small 

 piece of this culture is 

 removed to fresh me- 

 dium and growth is re- 

 newed. Subculture of 

 embryonic tissue can 

 probably be carried on 

 indefinitely. 



