ii8 



FIXATION 



Effects on the histological picture seen in paraffin sections. Form- 

 aldehyde is a poor fixative for tissues that are to be embedded in 

 paraffin (grade IV-V if dissolved in distilled water, III-IV if 

 sodium chloride is added at 0-7%). 



Cellular aggregates tend to be widely separated from one 

 another; cytoplasm shrinks towards nuclei, and cells may lose con- 

 tact with one another; mitochondria are sometimes retained, 

 especially if the fixative be used in strong solution (10% or more).*^ 

 Red blood-corpuscles tend to swell into spheres, but are rather 

 well preserved if an indifferent salt is mixed with the fixative. The 

 interphase nucleus is fixed in a remarkably life-like form, with no 

 coarse network in it; ^^^ the nuclear membrane, heterochromatic 

 segments of the chromosomes, and nucleolus are well shown. The 

 mitotic and meiotic chromosomes, however, are very poorly fixed. 



Formaldehyde gives good results if frozen sections are used, or 

 tissues embedded in collodion. 



Compatibility with other fixatives. Formaldehyde reduces chro- 

 mium trioxide quickly, potassium dichromate much more slowly. 

 The reaction with osmium tetroxide is very slow at room-tem- 

 perature. ^^ Formaldehyde is compatible with ethanol, picric acid, 

 and acetic acid, and is generally regarded as compatible with 

 mercuric chloride. 



OSMIUM TETROXIDE 



Standard concentration for fixation. 1% w/v aqueous solution. 



Formula and formida-weight. OSO4. 254-2. 



Description. Osmium tetroxide occurs as pale yellow crystals. 

 These melt at 41° C. The liquid boils at 131° C, but much vapour 

 comes off before this temperature is reached. The vapour arises 

 also from the crystals and from aqueous solutions ; it is damaging 

 to the epithelium of eyes, nose, and mouth. This vapour has a not 

 unpleasant smell. The name of the metal is derived from the fact 

 that the tetroxide smells. The volatility of osmium tetroxide makes 

 it imperative to keep and use solutions in tightly-stoppered 

 vessels. ^^^ 



The solubility in water at 25° C is 7-24% w/w.^^ (The figure 

 given by Thorpe and Whiteley,^^^ who are usually so reliable, is 

 grossly in error.) Osmium tetroxide is extremely soluble in carbon 

 tetrachloride at 25° C (about 375% w/w ^^) and soluble also in 

 liquid paraffin and certain lipids. 



