NEOPLASTIC DISEASE 



mutations are needed to reach the development of 

 cHnical maUgnancy; 

 (v) in cases where, for hormonal or other reasons, a cell 

 type increases markedly at some particular age, the 

 slope of the age incidence of cancer of that type will 

 increase sharply at this period — the age incidence of 

 prostatic hypertrophy and cancer may be mentioned. 

 All these are, in fact, well-recognized aspects of the malig- 

 nant process — and, of course, served as the stimulus for the 

 development of hypotheses of this type. 



No attempt will be made to give a comprehensive dis- 

 cussion of the clonal selection approach to cancer. Instead, 

 as in the preceding chapter, I shall concentrate on a few 

 themes which seem relevant to the general approach of these 

 lectures. Nothing will be said of the nature of somatic 

 mutation, and the means by which all numbers and tissue 

 structure are maintained in a dynamic equilibrium will only 

 be touched on very lightly. Three topics seem appropriate 

 for somewhat more detailed consideration: 



( 1 ) The turnover of cells in various parts of the body. 



(2) The age incidence of cancer in man in relation to the 

 sequential-somatic-mutation hypothesis. 



(3) The carcinogenic action of some mutagenic agents, 

 notably ionizing radiation. 



I . The turnover of cells in various parts of the body 



Mammalian tissues may be divided into four groups in 

 relation to the average life of their cells as individuals. 



(i) Tissues which in whole or in part are composed of 

 highly expendable cells with a relatively rapid turnover. 



The blood cells are the most obvious examples. All are 

 relatively short-lived, including erythrocytes, granulocytes, 

 lymphocytes and platelets (see Yoffey and Courtice, 1956). 

 We should include here also the other functional types of 

 mesenchymal cell, macrophage tissue lymphocyte and 

 plasma cell. The other important group comprises the epi- 



186 



