120 FIXATION 



Manufacture. Metallic osmium occurs naturally as an alloy with 

 iridium, usually in association with platinum. Deposits are found in 

 Alaska, the Ural Mountains, and in South Africa. There is an 

 ounce of osmium in 1,200 tons of Witwatersrand gold ore. The 

 metal is extremely heavy (density about 22-5). It is also very hard 

 and its alloys are used in making pivots for scientific instruments 

 and tips for nibs of fountain pens. 



Osmium tetroxide is made by heating spongy metallic osmium 

 in a current of air or oxygen. On account of the rarity of the metal, 

 this fixative is extremely expensive. A gram costs ^3. 10. o; thus 

 I ml of a 2% solution costs 1/4 J, and a single drop of it about f^. 



Introduction as fixative. We are indebted to Franz Schulze of 

 Rostock, the inventor of that invaluable histochemical reagent, 

 chlor-zinc-iodide,**^ for the introduction of osmium tetroxide into 

 microtechnique. Unfortunately it is impossible to discover what 

 led him to try it. He noticed that different tissue-constituents 

 differed in their capacity to reduce it to the dark lower oxide. He 

 sent a weak solution (o-i or 0-2%) to his friend and former pupil, 

 Max Schultze, with the request that he should try it in histological 

 investigations. Schultze did so in 1864.^** He plunged the male of 

 the beetle Lampyris splendidula, alive and shining, into Schulze's 

 fluid, and made a microscopical study of the phosphorescent organ. 

 To his surprise, the tracheal end-cells were blackened and thus 

 showed up strongly against the parenchymal cells.***' **^ In 

 collaboration with Rudneff **^ he next tried it on a variety of 

 tissues of plants and animals. He noted especially the reduction of 

 osmium tetroxide by fat, myelin, and tannic acid. It is interesting 

 that his emphasis was at first on the darkening of particular 

 objects, not upon delicacy of fixation. Retaining his interest in 

 phosphorescence, however, he tried it on the marine protozoon 

 Noctiluca, and was now struck by the life-like preservation.**^ 



Reaction with proteins. Osmium tetroxide gives no coagulum 

 with albumin solutions. It renders albumin not coagulable by 

 ethanol or by heat.^^ It sets undiluted egg-white and strong solu- 

 tions of serum albumin, serum globulin, and fibrinogen into gels. 

 It stabilizes gelatine gels against solution by water at 37° C. 



This is an additive fixative. It probably reacts at the double 

 bonds of the side-groups of tryptophane and histidine, linking 

 protein chains together through these. The failure of acid dyes to 

 act after fixation by osmium tetroxide suggests the blocking of 

 amino-groups, but there is no positive evidence of this. 



