12 Microscopic Histochemistry 



for Cl~ ) or mounted on slides and processed after the re- 

 moval of paraffin. 



Obviously, the freezing-drying method offers great advan- 

 tages. Diffusion artifacts and shrinkage are largely elimi- 

 nated; enzymes are very well preserved. However, it also has 

 several disadvantages. The equipment required is bulky and 

 rather expensive; only small tissue fragments can be used ff 

 distortion by ice crystals is not to occur; sections of the em- 

 bedded material are not easy to handle because they are 

 quite sensitive to water. It should be added that, if frozen- 

 dried sections are run down through xylene and alcohols to 

 water in the same way that regular sections are, most of the 

 proteins will not be fixed and will remain soluble. Unless the 

 slides are coated with collodion, a considerable loss of pro- 

 tein substances and glycogen is liable to occur on hydration. 

 A short (2-3 minutes) bath in 70-80 per cent alcohol be- 

 tween the second 95 per cent alcohol and water may be em- 

 ployed to make proteins insoluble. 



In spite of its disadvantages, the freezing-drying method 

 is a "must" in certain types of histochemical research, and its 

 application has yielded most valuable information regarding 

 the localization of mobile ions in the tissues. 



The second special condition for histochemical localiza- 

 tion is that the chemical reaction or physical means utilized 

 for identification should possess certain features which can 

 be enumerated as follows: 



1. It must be applicable to reactions in situ. Therefore, 

 reactions taking place in solution only (e.g., the Carr-Price^ 

 reaction for vitamin A; the Kober^ reaction for estrogens, 

 etc.) are unsuitable. 



2. It must preserve tissue structure. Methods utilizing con- 

 centrated H2SO4 or KOH can have, at best, a very limited 

 value in localization because tissue structure is badly dam- 

 aged by these reagents. 



4. Carr, F. H., and Price, E. A.: Biochem. J., 20:497, 1926. 



5. Kober, S.: Biochem. Ztschr., 239:209, 1931. 



