MICROELECTRODES 



199 



MICROINCINERATION 



duced and which was in fact the inspira- 

 tion of G. L. Kite's first microdissection 

 apparatus. Today this has been very 

 greatly improved chiefly by Chambers 

 and Peterfi. See detailed account by 

 Chambers under Micromanipulation. 



Microelectrodes, see full discussion, Mc- 

 Clung, Microscopical Technique, 1950, 

 p. 532. 



Microglia. Method for impregnating with 

 silver in pyroxylin (celloidin) sections 

 (Weil, H. and Davenport, 11. A., Trans. 

 Chicago Path. Soc, 1933, 14, 95-96). 

 Wash 15yu sections in aq. dest. Treat 

 for 15-20 sec. with silver solution (made 

 by adding 10% aq. silver nitrate drop by 

 drop from a burette to 2 cc. cone, am- 

 monia (28%) shaking to prevent ppt. 

 formation until about 18 cc. have been 

 added and the solution has become 

 slightlj' opalescent). Transfer to 15% 

 formalin, moving section rapidly until 

 coffee-brown in color. Pass through 

 3 changes aq. dest. Dehydrate in 

 alcohol, clear in xylol and mount in 

 balsam. 



Microglia and Oligodendroglia. In frozen 

 sections 20-40m of formalin fixed mate- 

 rial. Immediately place them in aq. 

 dest. + 20 drops ammonia per lOO cc. 

 Thence pass directly to 5% aq. am- 

 monium bromide 4O^50°C. 10-15 min. 

 Equal parts ammonia, pyridine and aq. 

 dest. 2 min. Then 3-5% aq. sodium 

 sulfite, 2-3 min. Pass through and 

 shake in 3 changes 1 min. each of follow- 

 ing: 8 parts 5% aq. sodium carbonate, 

 2 parts, 10% aq. silver nitrate + am- 

 monia till ppt. Reduce in 1% formalin 

 less than 1 min. Wash, dehydrate clear 

 and mount (King, L. S., Arch. Neurol, 

 and Psychiat., 1937, 38, 362-364). 



Microincineration — Written by Gordon H. 

 Scott, July 26, 1946 and revised by him 

 January 16, 1951 — This method is one 

 which has been used by plant and ani- 

 mal histologists intermittently for over 

 a hundred years. In concept it is 

 simple in that it consists primarily of 

 ashing tissue sections carefully so as 

 to retain the minerals in their position 

 in the fixed tissue. The ashing can be 

 done on glass or quartz slides by a vari- 

 ety of heating processes. Most tissues 

 in the body can be treated by the ash- 

 ing process with some success. Those 

 which contain large quantities of phos- 

 pholipids ordinarily do not give as good 

 results as tissues lacking them. 



The method is one which requires 

 some care and the observance of cer- 



tain very definite precautions if good 

 results are to be had. 



Fixation: There are two methods of 

 fixation which can be used. These are 

 the chemical and the frozen-dehydra- 

 tion. If the cryostat or other suitable 

 devices for frozen-dehydration are not 

 available, fixation by absolute alcohol 

 plus 10 per cent formalin yields reason- 

 ably good pictures. This particular 

 fixative is one of the few chemical mix- 

 tures which dissolves the minimum of 

 mineral from fresh tissue and adds none 

 to it. Tissues are passed through re- 

 peated changes of absolute alcohol to 

 dry them and are then infiltrated wdth 

 paraffin in the usual manner. 



The alternative method, that of 

 frozen-dehydration, is the most suitable 

 for preparation of tissues for micro- 

 incineration. (See Altmann-Gersh and 

 Cryostat.) This technique yields tissues 

 which, except for the ice crystal forma- 

 tion, have not been altered, to any 

 perceptible degree, either ph3^sically or 

 chemicallJ^ Dehydration at suffi- 

 ciently low temperatures maintains an 

 ice-salt equilibrium and no shifting of 

 minerals in the cell results. If the 

 paraffin infiltration is done with care, 

 shrinkage and consequent cellular dis- 

 tortion is avoided. 



Methods of examination of the in- 

 cinerated preparations are several. 

 One of the simplest and best for study 

 and for photograph}' is the dark-field. 

 Of the several types of dark-field, the 

 cardioid condenser probably gives the 

 most uniform results. Illumination 

 from above with the incident light fall- 

 ing on the slide at an angle of 30° is 

 advised by Policard. This has some 

 advantages over the dark-field but 

 makes the use of higher magnifications 

 difficult if not actually impossible. Cel- 

 lular details are, therefore, to be ob- 

 served best by using the cardioid dark- 

 field. 



Identification of minerals. Some 

 good results can be achieved by the use 

 of ultraviolet light and with the sub- 

 sequent fluorescence of minerals. Stu- 

 dents should consult reference and text- 

 books on mineralogy for details of 

 identification. 



Calcium and magnesium are charac- 

 terized in the dark-field by their dense 

 white ash residues. Iron is oxidized 

 during the incineration process and 

 appears as varying tints of red. The 

 amount of this element present can be 



