190 Silver Impregnalion II (chap. 15) 



actually are best; the alcohol and xylene embedding processes may 

 remove lipids, which can be an essential part of the tissue. Lipid extrac- 

 tion may result in no impregnation of oligodendroglia and weakened 

 microglia, since their impregnation depends on lipid complexes. Also, 

 periodic and chromic acid oxidation weaken the reaction of oligoden- 

 droglia and microglia. (The reverse is true for connective tissue, when 

 pyridine, periodic, or chromic acid treatment is desirable to suppress 

 nervous tissue elements from confusing the picture.) 



Neurological technics, as perhaps has become evident, so often neces- 

 sitate highly specialized methods that many technicians prefer to avoid 

 them. Precise attention to all details, however, can produce beautiful 

 and exciting slides. Carefully follow directions for fixation — the solu- 

 tion composition and the duration of fixation, and whether fixation is 

 or is not followed by washing. Always wash in distilled water unless tap 

 ^vater is specified. For making silver and other special solutions use 

 double (glass) distilled water if possible and clean glassware — cleaned 

 in cleaning solution (page 412), washed well in running water, and 

 rinsed 4 or 5 times in distilled water. All chemicals should be at least 

 reagent grade. Use no corks in containers and no metal instruments in 

 silver solutions. If in doubt about the age of solutions, make fresh ones. 

 If, during toning with gold chloride, the tissue retains a yellow or 

 brownish hue, the gold chloride is weakened. Prepare a new solution. 



Sometimes artifact precipitates are difficult to avoid, but strict adher- 

 ence to procedure details will reduce them to a minimum. 



Embedding and sectioning will depend on the impregnating or 

 stainino technic to follow. Paraffin and nitrocellulose methods are used 

 at times, but as mentioned above frozen sections usually are more satis- 

 factory. Section thickness frequently is thicker than for other tissues - 

 7 to 20 microns, or even more. 



When fixing an entire brain, do not allow it to rest on the bottom 

 of the container. Carefully insert a cord under the circle of Willis on 

 the underside of the brain and support the two ends of the cord on the 

 sides of the container. The brain, hanging upside down, should be free 

 of the bottom of the vessel but remain completely submerged in fixative. 

 The spinal cord can be supported in a graduated cylinder filled with 

 fixative. Run a thread through one end of the spinal cord and tie the 

 thread around an applicator stick supported on the edges of the cylin- 

 der. For a perfusion method for the brain, see pages 22. 



