206 GEORGE H. HOGEBOOM AND WALTER C. SCHNEDIER 



account for a relatively small proportion of the total cytoplasm of liver, 

 the finding that they comprise 40 to 50 % of the total cell population is of 

 significance only in investigations in which the composition of the average 

 parenchymal cell is calculated on the basis of the total number of nuclei 

 present.^- '^^ Another possible source of error involved in extrapolating 

 measurements made on large numbers of liver cells to that of the single 

 average liver cell lies in the fact that certain histochemical tests have sug- 

 gested that liver cells appearing to be morphologically identical may have 

 considerably different chemical compositions and enzymic properties.^" 

 If these observations can be accepted as correct, they would represent 

 an important limitation of the cell fractionation technic and an excellent 

 example of the way in which histochemical and cell fractionation technics 

 can supplement each other. 



Finally, it is necessary to consider an argument frequently used in 

 peremptorily dismissing cell fractionation as a cytochemical tool,^"'^'-^^''^ 

 namely, that at the moment of or immediately after cell rupture, so many 

 artifacts occur (such as morphological alterations, adsorption, redistribu- 

 tion autolysis, etc.) that it is utterly useless to isolate the cell structures and 

 study their properties. Since this argument is usually set forth without 

 supporting experimental evidence and without the submission of an ade- 

 quate alternative, it obviously represents neither a realistic nor a construc- 

 tive point of view. As noted on several occasions,"* '^'^ while it is important 

 to recognize that such artifacts can occur, it is at the same time possible 

 to carry out experiments to determine Avhether they do occur. The latter 

 approach has been amply justified. Thus in several investigations to be 

 described later, strong evidence has been obtained for the absence of 

 absorption and redistribution artifacts during cell fractionation. Further- 

 more, certain results obtained with the cell fractionation procedure have 

 been confirmed by entirely independent methods, e.g., by histochemical 

 experiments^^ '^^ and submicromeasurements.^" 



b. Requirements Defining an Adequate Isolation Procedure. 



In employing the cell fractionation technic for the studies of the intracellular dis- 

 tribution of enzymes and other compounds, it must first be borne in mind that the 

 procedures presently available for the isolation of cell components are by no means 

 perfect. Furthermore, as already indicated, in the absence of other productive meth- 

 ods of approach to cytochemical problems, the data obtained by the cell fractionation 



« M. J. Striebich, E. Shelton, and W. C. Schneider, Cancer Research 13, 279 (1953). 

 " W. C. Schneider, G. H. Hogeboom, E. Shelton, and M. J. Striebich, Cancer Re- 

 search 13, 285 (1953). 

 « J. F. Danielli, Nature 157, 755 (1946). 

 " R. Chambers, Cancer Research 10, 210 (1950). 

 « J. F. Danielli, Nature 157, 755 (1946). 



