l66 GROWTH PRINCIPLES AND THEORY 2 



From the chemical viewpoint, a harmonization between protein synthesis and 

 degeneration is to be expected: In the steady state, cells and tissues with high protein 

 synthesis should present high protein degradation, and in the non-steady state 

 (growth) high protein degradation should be paralleled by a high capacity of 

 growth and regeneration. In a crude approximation, a correlation between 

 content of proteolytic enzymes (expressed, for example, in the autolysis of sur- 

 viving organs), protein turnover and growth capacity can be found (Bertalanffy, 

 1942 and 1951a; Pollister, 1954). At one end of the series are cells undergoing fast 

 renewal, showing strong autolysis and partly playing a predominant role in 

 wound healing and regeneration, such as leucocytes, mucous membranes, gland 

 cells, and the regenerating layers of the epidermis. At the other end are cells 

 with a low content of proteolytic enzymes and little or no capability of division 

 and regeneration, such as muscle and nerve cells. Similarly, newborn animals 

 compared to older ones, and animals treated with thyroid hormone show 

 increased autolysis, corresponding to increased protein metabolism. 



Correlations between cell renewal and susceptibility to malignant growth as 

 expressed by the frequency of carcinoma in the various organs can also be found 

 (Leblond, Storey and F. D. Bertalanffy, 1951). 



{b) Cell constancy 



The extreme case of cells incapable of division is found in cell-constant animals 

 (rotatoria, nematodes, tardigrades, appendicularias) where after embryonic 

 development no cell renewal, physiological or restitutive regeneration takes 

 place. For example, the rotator Hydatina senta consists of 950 somatic cells distrib- 

 uted in exact numbers among the various organs. Cell constancy does not, 

 however, exclude cell growth. For example, the cells of the intestinal epithelium 

 of Ascaris grow to a size that corresponds to the total size of the freshly hatched 

 larva. In the mammalian organism, heart muscle fibers and ganglion cells are 

 incapable of division post-embryonically. These organs are probably cell-constant 

 (heart: Linzbach, 1950). Nevertheless, there is a continuous renewal of cytoplasm 

 in the nerve fiber (Weiss and Hiscoe, 1948); nerve cells still grow, their nuclei 

 belonging to the highest class of the nuclear series; and the nerve fiber is highly 

 capable of regeneration. Similarly, considerable cell growth takes place in the 

 fibers of the heart muscle (Linzbach, 1955; Fig. 4, p. 160). 



[c) Mathematical theory of growth of tissue cultures 



The quantitative aspects of the growth of organs within the organism will be 

 considered later (p. 234fr.). Here only the growth of tissues isolated from the 

 organism and grown in vitro will be briefly reviewed. Several formulas for the 

 growth of tissue cultures have been proposed of which the following appear to 

 have the best theoretical background, and are special cases of a general model 

 and equations of growth to be discussed in the following (p. lyyflT.). 



Buch Andersen and Fischer (1929) empirically found that for the growth of 

 the radius of a tissue culture an equation of the type of equation (5.27) and (5.29) 

 applies. It can be assumed that synthesis and consequent cell multiplication 

 depend on the surface, i.e. diffusion of nutrients into the culture, while growth 



