The microscopic techniques which are treated in the present vol- 

 ume are those designed to establish the distribution of elements, 

 groups, substances or activities in microtome sections of tissue by 

 means of examinations under some form of microscope. This re- 

 quires that the factor in question be made apparent by character- 

 istic optical or photographic properties such as a specific color, 

 fluorescence, or radiation. With the exception of absorption histo- 

 spectroscopy, these microscopic techniques are limited to observa- 

 tions which are essentially qualitative in nature. However, the 

 microscopic techniques permit a much greater degree of localization 

 of particular chemical constituents in histologically defined cells, or 

 cytologically defined parts of cells, than is possible by means of the 

 quantitative chemical techniques. Thus, one is often forced to 

 choose between degree of localization and quantitation. Obviously, 

 it would be preferable to establish both the cellular disposition of 

 biologically significant factors and their quantitative relationships. 



/. FREEZING DRYING PREPARATION 



OF TISSUE 



Since the microscopic techniques that will be discussed are almost 

 all based on the use of microtome sections of tissue, it is pertinent 

 that the freezing-drying preparation for sectioning be described in 

 detail. This technique of sudden cooling to low temperatures and 

 rapid dehydration of the frozen material in vacuo has many advan- 

 tages over the usual histological methods employing fixing and 

 dehydrating solutions. The chief of these advantages are a minimum 

 of chemical change in the tissue (there is an almost instantaneous 

 cessation of metabolic activity and no chance for other chemical 

 changes to occur), a mimum of shifting of diffusible constituents 

 (fluid is not used and the fixation is immediate), a greater preserva- 

 tion of cytoplasmic inclusions than is possible with the use of fixing 

 solutions, the possibility of direct paraffin infiltration of dehydrated 



