THE CYTOPLASM 203 



monophosphates, certain coenzymes, and ascorbic acid). The X-ray ab- 

 sorption method is not as yet capable of scanning areas smaller than 10 

 M X 10 M and consequently cannot be applied to cell structures smaller than 

 the nucleus. A number of other histochemical technics depend on the 

 formation of visible products, usually colored, as the result of a chemical 

 reaction or physical combination taking place within the cell. Staining 

 methods have, for example, been used as a means of the qualitative dif- 

 ferentiation in tissue sections between the two types of nucleic acid. Here 

 the mechanism of the reaction is not well understood, and previously 

 accepted claims as to the specificity attained have received a rather serious 

 setback by the recent observations of Alfert,^* who found that methyl green, 

 which has generally been considered a specific stain for DNA, would also 

 stain PNA in certain tissues and, furthermore, would stain PXA in re- 

 fractory tissues if the fixative contained high concentrations of formalde- 

 hyde. The Feulgen reaction^^ is a good example of a histochemical method 

 involving a chemical reaction and is also generally considered specific for 

 DNA. It is discussed in detail in Chapter 17. As in the case of most histo- 

 chemical tests involving chemical reactions, the Feulgen reaction is, in 

 fact, specific only for a certain chemical grouping (the aldehyde group). 

 Although the manner in which the reaction is carried out apparently con- 

 tributes to its specificity for DNA, there is some question as to whether the 

 reaction product and the DNA are localized at the same intracellular site.'" 

 The latter problem represents a source of error that is often shared by 

 histochemical procedures providing the localization of enzymic reactions.^" 

 Here a further complication is introduced by the fact that enzymes are 

 generally labile molecules and are therefore often inactivated to a consider- 

 able extent by the necessary procedures of fixation and embedding. As a 

 result, the possiblity must be considered that the enzyme can be selectively 

 destroyed in certain cells or even in different parts of the same cell. 



Another important consideration relating to histochemical methods 

 arises from the question as to whether they can demonstrate the locahza- 

 tion of enzymes and other compounds at subcellular levels. Danielli^^ 

 apparently feels that the Gomori technics for the detection of alkaline and 

 acid phosphatases,'" for example, are capable, under properly controlled 

 conditions, of localizing the enzymes within cells. Novikoff'- •'* and Palade,^* 

 on the other hand, have made a careful comparison of the results of these 



28 M. Alfert, Biol. Bull. 103, 145 (1952). 



2^ R. Feulgen and H. Rossenbeck, Z. physiol. Chem. 135, 203 (1924). 



'" G. Gomori, "Microscopic Histochemistry." Univ. of Chicago Press, Chicago, 1952. 



31 J. F. Danielli, J. Exptl. Biol. 22, 110 (1946). 



52 A. B. Novikoff, Science 113, 320 (1951). 



33 A. B. Novikoff, Exptl. Cell Research Suppl. 2, 123 (1952). 



34 G. E. Palade, /. Exptl. Med. 94, 535 (1951). 



