THE NUCLEIC ACID CONTENT OF TISSUES AND CELLS 21 



similar increase in cell number. ^-^ As "hyperplasia" is usually defined as 

 increase in cell number, and "hypertrophy" as increase in cell size,'^^ the 

 more accurate description of the compensatory process would be "kidney 

 h3rperplasia." 



The situation is made more confusing because Kurnick has briefly re- 

 ported^^^ that growth of the remaining kidney is true "hypertrophy without 

 significant hyperplasia." It seems to be the case that cell size, PNA, and 

 protein content increase between 40 to 50 % at 7 days when there is little 

 increase in DNA content, but that cell composition finally returns to its 

 original state. ^-^ When the diet is deficient in protein, the cell number or 

 DNA content of the remaining kidney increases to at least the same ex- 

 tent as in the fully fed animals, but the percentage increases in weight, pro- 

 tein, and PNA per kidney are much lower. ^^e 



Before suitable methods become available for measuring the PNA-P and 

 DNA-P contents of embryonic chick heart explants growing in vitro, in- 

 creases in total NAP during growth were recorded. ^^^ Once net increases in 

 both PNA-P and DNA-P were produced by a suitable culture technique, 

 it became clear that an increase in PNA-P content always preceded the 

 first rise in the DNA-P content of a group of explants,^^* and that the ratio 

 PNA-P/DNA-P, and hence the PNA content per cell, rose appreciably 

 during intensive cell division. ^^s-iso Although the average DNA-P content 

 per nucleus certainly increases during rapid cell division, ^^^ at the end of 

 this phase the values for DNA-P content per individual nucleus show the 

 same distribution in the "fibroblast" cultures as in the 12-day chick embryo 

 liver. 1^- 



The total increments in nucleic acids and protein in growing cultures may 

 depend on the technique employed. Davidson and Leslie'^" found closely 

 similar percentage increases in PN, PNA-P, and DNA-P contents of the 

 chick heart explants over a period of 10 days. On the other hand, Gerarde 

 et aZ.^^^ report that in their lung, heart, and intestine cultures, also prepared 

 from chick embryo, the rise in protein content was three or more times that 



'" p. Mandel, I.. Mandel, and M. Jacob, Co7}ipt. rend. 230, 786 (1950). 



'** A. E. Needham, "Regeneration and Wound Healing." Methuen, London, 1952. 



**^ N. B. Kurnick, Ahstr. 2nd congr. intern, biochim., Paris p. 363 (1952). 



'26 P. Mandel. L. Mandel, and M. Jacob. Com-pt. rend. soc. biol. 144, 1548 (1950). 



>" J. N. Davidson and C. Waymouth, Biochem. J. (a) 37, 271 (1943); (b) 39, 188 (1945) ; 



(c) 40, 568 (1946). 

 128 J. N. Davidson, I. Leslie, and C. Waymouth, Biochem. J. 44, 5 (1949). 

 '" W. Hull and P. L. Kirk, /. Gen. Physiol. 33, 327 (1950). 

 '30 J. N. Davidson and I. Leslie, Exptl. Cell Research 2, 366 (1951). 

 '3' P. M. B. Walker and H. B. Yates, Proc. Roy. Soc. (London) B140, 274 (1952). 

 132 S. C. Frazer and J. N. Davidson, Exptl. Cell Research 4, 316 (1953). 

 1" H. W. Gerarde, M. Jones, and T. Winnick, J. Biol. Chem. 196, 69 (1952). 



