164 Applied Biophysics 



young areas were selected in each piece, and their cell population 

 was classified and counted. The average coefficient of variation 

 from the mean in the various pieces for any given tumor was 

 of the order of 10%.^^ Similar observations have been recorded 

 for the histological grading of various biopsies taken from the 

 same tumor.^* ^^' ^^ 



The cellular population of tumors varies with tumor type. 

 In most epithelial growths, 4 classes of cells can be distin- 

 guished according to their viability. There are 2 classes of viable 

 cells : 



A : The resting cells, which are the intermitotic "stock" cells 

 capable both of division and differentiation (depending on the 

 tumor type). They are relatively small, with a large, often 

 hyperchromatic, nucleus and with little and basophilic cytoplasm. 



B : The mitotic cells, i.e., stock cells actually in division. 



There are also two classes of nonviable cells : 



C: The differentiafijig cells, which are cells rendered per- 

 manently incapable of division by the differentiation of their 

 cytoplasmic structures. Most of these cells are large, with a 

 great amount of differentiating cytoplasm and a relatively small 

 vesicular nucleus. 



D : The degeneraiing cells, which are the cells in the process 

 of disintegration. Their structure changes according to the form 

 of degeneration (fatty, mucoid, parakeratotic, etc.), and to the 

 cell type from which they are derived. 



Very immature growths lack the differentiating cells. Figure 

 1 depicts diagrammatically the main characteristics of these four 

 cell categories and their relationship w^ith each other, as indi- 

 cated by the arrows. The cellular composition of the foci is 

 influenced by the tumor bed, i.e., the vessels, stroma, and cells 

 surrounding the tumor strands, which promotes or inhibits 

 mitosis, differentiation, and degeneration. 



Young foci are formed by finger-like projections from tumor 

 strands, and are characterized by the presence of many mitotic 

 cells, the preponderance of resting cells, and the dissolution of 

 the basement membrane at the growing tip of the projection. 

 The comparison of young foci in serial biopsies is best made 



