8 ' Fixation (chap. 1) 



formed, however, the pigment can be removed in a solution of 1% po- 

 tassium hydroxide in 80% alcohol or picric acid dissolved in alcohol 

 {Baker, 1958). (See also page 244.) 



Mercuric Chloride [corrosive sublimate), HgCl^ 



Mercuric chloride usually is applied as a saturated aqueous solution 

 (approximately 7%) and is acidic in action, owing to the release of H + 

 and Cl~ ions in a water solution. It is a coagulant of proteins, both 

 cytoplasmic and nucleic. It penetrates reasonably well, but not as rap- 

 idly as acetic acid. It shrinks less than the other protein coagulants, 

 hardens moderately, and distorts the cells less than other fixatives. It 

 is excellent for mucin. 



One disadvantage of mercuric chloride is that it deposits in the tissue 

 a precipitate of uncertain chemical composition. This precipitate is 

 crystalline, perhaps being mercurous chloride (needle shaped) and me- 

 tallic mercury (amorphous, irregular lumps). It may be removed by 

 iodine, the following reaction probably taking place: 2HgCl -j- I2 == 

 HgClo + HgK. Any metallic mercury also ^vill be converted into mer- 

 ctiric iodide. The latter is soluble in alcohol, but the brown color of 

 iodine may remain in the tissue. This can be removed by prolonged 

 soaking in 70% alcohol, or more quickly by treatment with 5% sodium 

 thiosulfate, aqueous. A further disadvantage is that mercuric chloride 

 crystals inhibit adequate freezing, making it difficult to prepare good 

 frozen sections. 



Most stains react brilliantly on tissue fixed in mercuric chloride. 

 Chromatin will stain strongly with basic stains and lakes; cytoplasmic 

 structures will react equally well with acidic or basic stains. 



Osmium Tetroxide, OsO^ 



In solution, usually 1% aqueous, osmium tetroxide takes up a mole- 

 cle of water and becomes HoOsOs, erroneously called osmic acid. In 

 solution ionization is so minute that the pW is almost exactly that of 

 the distilled water used in making the solution. The substance is chem- 

 ically neutral, is not an acid and cannot be isolated. (Osmic acid would 

 be H0OSO4.) Baker (1958) suggests that its name might be hydrogen 

 per-persomate. The solution penetrates poorly and leaves tissue soft, 

 later becoming so friable in paraffin that tissues section badly. Osmic 

 acid preserves the cytoplasm and nuclei, but while it increases the stain- 



