82 HEWSON SWIFT 



may remain. Because of this the use of 0.1 % gelatin to stabilize deoxyribo- 

 nuclease solutions*^ or of 0.05 M cysteine or hydroxylamine to inhibit 

 proteolytic contaminants"* has been suggested. For quantitative use an 

 assay of proteolytic activity, for example, on hemoglobin,^*' is advisable. 

 The presence of magnesium ions is necessary for pancreatic deoxyribo- 

 nuclease digestion of DNA solutions.^* Several workers, however, have 

 found that magnesium ions are not necessary for action of the enzymes on 

 tissues''^ -'^^ although others disagree.^'" Use of the enzyme at about pH 6 

 has been recommended^*" because of the buffer effects mentioned above. 

 With acetic alcohol fixation, good results are usually obtained with 30- to 

 60-minute treatment in a 0.01% solution, at 37° or room temperature. 

 Deoxyribonuclease acts very slowly on formalin-fixed tissues, and is com- 

 pletely inactive on tissues treated with fixatives containing chromic acid. 

 Vercauteren^^'^** found that certain enzyme preparations were inactive on 

 sections in spite of strong in vitro activity. 



Nucleases can be an extremely important cytochemical tool, both for 

 testing the specificity of nucleic acid reactions and also for adding speci- 

 ficity to otherwise nonselective methods, such as ultraviolet absorption. 

 It is apparent, however, that they are not infallible, and in any quanti- 

 tative work adequate controls must be run. Control slides should be treated 

 with solutions minus the enzyme. Where nucleases are used to fractionate 

 nuclei for ultraviolet studies, the effect of enzymes should be checked with 

 basic staining and the Feulgen reaction, and, as an additional check, frac- 

 tionation may be carried out twice, with the enzymes used in different 

 order. 



VI. Photometric methods for cell studies 



Since the pioneer work of Caspersson,i*® there has been a rapidly growing 

 literature on the photometry of cells, including several recent reviews.i"-^*^ 

 Numerous measuring instruments have been constructed. ^*^-^^* Some have 



1" M. L. Anson, /. Gen. Physiol. 22, 79 (1938). 



'" L. M. Gilbert, W. G. Overend, and M. Webb, Exptl. Cell Research 2, 349 (1951). 



>" R. Vercauteren, Bull. soc. chim. biol. 32, 473 (1950). 



155 T. Caspersson, Skand. Arch. Physiol. 73, Suppl. 1 (1936). 



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



158 H. G. Davies and M. P. B. Walker, Progr. Biophys. 3, 195 (1953). 



159 E. R. Blout, Advances in Biol, and Med. Phys. 3, 285 (1953). 



15" L. Lison, "Histochimie et cytochimie animales." Gauthier-Villars, Paris, 1953. 



i«i A. W. Pollister, in "Biological Effects of Radiation" (A. E. Hollaender, ed.). 

 Vol. 2. McGraw-Hill, New York, in press. 



i«2 A. W. Pollister and L. Ornstein, in "Analytic Cytology" (R. C. Mellors, ed.) Mc- 

 Graw-Hill, New York, in press. 



1" T. Caspersson, /. Roy. Microscop. Soc. 60, 8 (1940). 



1" T. Caspersson, F. Jacobsson, and G. Lomakka, Exptl. Cell Research 2, 301 (1951). 



