124 FIXATION 



consent of all who have studied the subject, osmium tetroxide 

 preserves the structure of the living cell better than any other 

 primary or mixed fixative. Fig. 8, A, B (opposite p. 70) gives a 

 good impression of its action. The nuclear sap and the ground 

 cytoplasm in the vicinity of the nucleus become less perfectly 

 homogeneous than they were in life; the nucleus may retract 

 slightly from the cytoplasm; nucleoli become difficult to see; 

 lipid globules gradually darken. Until the latter change has taken 

 place, one might almost suppose that the cell was still alive, except 

 that any Brownian movement will have ceased. Mitochondria are 

 perfectly preserved. 



Methods of washing out. Osmium tetroxide is washed out in 

 running water, because if any were left in the tissues, it might be 

 gradually reduced by ethanol at subsequent stages, with con- 

 sequent darkening. 



Effects on dyeing. Osmium tetroxide leaves cytoplasm readily 

 colourable by basic dyes (after bleaching), but scarcely at all by 

 acid ones. The nuclear sap also tends to be made basiphil, and this 

 interferes with the differential dyeing of the component parts of 

 the nucleus. 



Effects on the histological picture seen in paraffin sections. It is sad 

 to turn from the magnificent view of a cell still lying in osmium 

 tetroxide solution, to look at a paraffin section of a piece of tissue 

 fixed in the same ffuid. The fixation is poor (grade III-IV or IV), 

 even with the addition of 0-7% of sodium chloride to the fixative. 



Cellular aggregates are severely shrunken, so that they are 

 separated by wide artificial spaces; cracks often run at random 

 across the section ; ground cytoplasm, though fairly homogeneous, 

 is strongly contracted, and often condensed round nuclei. The 

 shape of nuclei is w^ell retained. Pischinger ^^^ considered that the 

 nucleus as a w^hole was well fixed. He thought that there was no 

 nuclear membrane in the living cell but only a physical interface, 

 and that while other fixatives thickened the interfacial region to 

 form an artificial nuclear membrane, osmium tetroxide provided 

 an approximation to the living condition. The nuclear sap is rather 

 homogeneously fixed, but the objects contained in it (especially 

 the meiotic chromosomes) are very poorly shown. 



Mammalian testis fixed in osmium tetroxide solution buffered 

 at pH 7*4 is shown in fig. 9, b (opposite p. 74). 



Compatibility with other fixatives. Osmium tetroxide is com- 

 patible with all the fixatives mentioned in this chapter and the 



