Ill 



CELL GROWTH AND MULTIPLICATION 163 



of varying number or size (as in unequal division, formation of spores, etc.), and 

 the final size will remain constant. The model naturally does not explain the 

 details (such as different size of cells of different tissues, occasional appearance 

 of giant cells, etc.), but seems to elucidate the principle oi^ limitation of cell growth. 

 With respect to the physical prerequisites of division after reaching a critical size, 

 Rashevsky (1948) has indicated a mathematical cell model consisting of a droplet 

 where import and export, anabolism and catabolism of materials take place. 

 Within such system concentration gradients of substances produced and flowing 

 out, and substances flowing in and consumed will appear, and with these 

 gradients forces tending to disrupt the system or to counteract division, respec- 

 tively. It can be shown that the system will grow up to a certain critical value 

 where metabolic forces prevail and lead to spontaneous division. The model 

 admittedly neglects all details of mitotic division, etc. which are obvious from the 

 cytological and cell-physiological viewpoints. It is, however, remarkable that a 

 number of characteristics regarded as specifically "vital" can be derived from 

 this highly simpHfied model: Growth, periodic division, the impossibility of a 

 "spontaneous generation" of such droplet systems, and an order of magnitude 

 (if appropriate physical forces are assumed) that corresponds with the average 

 size of living cells. 



IV. GROWTH OF TISSUES 



(a) Cell renewal 



Similar to the steady state exhibited by the living organism with respect to 

 its chemical components, the organism is in a dynamic state with respect to its 

 component cells. It grows if production of cells prevails, and eventually attains 

 a steady state when wearing-out and renewal of cells balance each other. 



The classification of tissues with respect to cell renewal goes back to Bizzozero 

 (1894). He distinguished three classes of tissues: J. tissues with labile cells which 

 multiply during the whole life span; 2. tissues with stable cells which differentiate 

 in certain directions and divide post-embryonically but not in the adult organism; 

 3. tissues in which mitotic activity ceases at an early period of embryonic growth 

 and which lack physiological as well as restitutive regeneration. 



The classification of tissues with and without cell renewall has been refined by 

 Cowdry (1942, 1957). Two types with two sub-groups each are distinguished: 



J. Intermitotic cells, retaining the capability of division: A. Vegetative inter- 

 mitotic cells, which continually divide in the adult organism and so serve as a 

 cell reservoir [e.g. basal epithelial cells, spermatogonia, hematoblasts) ; B. Differ- 

 entiated mitotic cells which by differentiation and division yield cells of type C 

 or D (e.g. spermatocytes, spermatides, immature leucocytes). 



2. Postmitotic cells, i.e. highly differentiated cells which do not normally divide: 

 C. Reversibly postmitotic cells which normally remain undivided till death, but 

 are capable of division in hyperplasia and regeneration (e.g. liver cells, kidney 

 epithelium, endothelia of blood vessels) ; D. Fixed postmitotic cells showing 

 highest differentiation and lack of division (e.g. ganglion cells, fibers of heart and 

 skeletal muscle, erythrocytes). 



Literature p. 253 



