202 GEORGE H. HOGEBOOM AND WALTER C. SCHNEIDER 



microscope.""--^ This structure, which is referred to as the endoplasmic 

 reticulum^* or the ergastoplasm,^- probably corresponds to the basophilic 

 substance seen in fixed and stained cells under the light microscope. Its 

 detailed morphology is still somewhat controversial; some investigators^^ 

 consider it to be in the form of vesicles and canahculi, whereas others^^ 

 believe it to be lamellar in form. 



II. Cytochemical Methods 



1. General 



Three methods are available for the study of the chemical composition 

 and the biological function of intracellular structures: (1) histochemical 

 methods, (2) submicromethods capable of direct chemical and enzymic 

 determinations on single cells or portions thereof, and (3) the cell fractiona- 

 tion technic. All of these procedures suffer from certain inherent defects 

 that prevent any one of them from providing definitive answers to all 

 cytochemical problems. It will become evident, however, that the third 

 method has provided a much larger amount of information than has either 

 of the other two. Although the results considered in this chapter will there- 

 fore be limited mainly to those of the cell fractionation technic, it is impor- 

 tant to consider the advantages and limitations of the other cytochemical 

 methods as well. 



2. Histochemical Methods 



In the histochemical localization of a chemical compound or of an en- 

 zyme, it is necessary to visualize the compound or the product of the 

 enz^'mic reaction in a tissue section. Physical methods have been developed 

 permitting the quantitative determination of the nucleic acids (by ultra- 

 violet microspectrophotometry-*) and of certain elements (by X-ray spec- 

 trometry") at subcellular levels. These technics, although in some instances 

 capable of providing information that cafi be obtained in no other way, 

 have certain limitations. The former (see Chapter 17) suffers from the 

 disadvantage that it is unable to distinguish between the two types of 

 nucleic acids or between nucleic acids and other related compounds nor- 

 mally found in cells in high concentrations (e.g., the adenosine tri-, di-, and 



2" K. R. Porter and H. P. Thompson, Cancer Research 7, 431 (1947). 



'^ A. J. Dalton, H. Kahler, M. J. Striebich, and B. J. Lloyd, J. Natl. Cancer Inst. 



11, 439 (1950). 

 " W. Bernhard, A. Gautier, and C. Oberling, Compt. rend. soc. biol. 145, 566 (1951). 

 " G. E. Palade and K. R. Porter, Anat. Record 112, 68 (1952). 

 " K. R. Porter, J. Exptl. Med. 97, 727 (1953). 

 " A. J. Dalton, Intern. Rev. Cytol. 2, 403 (1953). 



^^ T. Caspersson, "Cell Growth and Cell Function." Norton, New York, 1950. 

 " A. Engstrom, Acta Radiol. Suppl. 63 (1946). 



