13 



Nitrocellulose Sections 



General Principles 



Nature of the Process 



As the name indicates, this chapter is 

 concerned with the preparations of sec- 

 tions of material which has been impreg- 

 nated with a solution of nitrocellulose. 

 This process is not to be regarded as a 

 substitute for the paraffin method de- 

 scribed in the last chapter; it should be 

 used only when paraffin will not give a 

 satisfactory result. This is usually used 

 either for exceedingly minute objects, the 

 orientation of which in paraffin, or the re- 

 tention of which in paraffin sections, is 

 almost impossible, or for very large ob- 

 jects with numerous cavities which cannot 

 well be supported with paraffin. Paraffin 

 in large cavities tends to shrink away, 

 while nitrocellulose solutions do not. 



There are numerous disadvantages in 

 the use of nitrocellulose. The worst for 

 the research worker is the difficulty of 

 preparing serial sections with the sections 

 in their due order. Tliis may be overcome 

 to a certain extent by the process of double 

 embedding (see the next chapter) but tliis 

 itself is less satisfactory than straight em- 

 bedding and should be used only for very 

 small or very difficult objects. 



One of the advantages of nitrocellulose 

 embedding is that the process does not in- 

 volve the use of heat; the materials are 

 impregnated in solutions of increasing 

 strength at room temperature, and these 

 solutions are subsequentl}^ hardened either 

 by evaporation or by chemical means. The 

 size of the nitrocellulose molecules in the 

 dispersions (usually called solutions) em- 

 ploj'ed is so great that the material dif- 

 fuses slowly and the the process is a long 

 one. Various methods have been put for- 



ward for using nitrocellulose at high tem- 

 peratures, but there appears to be fittle 

 justification for them, because, if the ma- 

 terial to be embedded will stand boiUng, 

 it will most certainly stand embedding in 

 paraffin. These processes appear to have 

 been introduced by those who are so ac- 

 customed to celloidin embedding that they 

 do not wish to use anything else. 



Materials Employed 



Cellulose nitrate is not, as its name might 

 indicate, a pure chemical, but is a mixture 

 of a great number of different compounds, 

 the relative proportions of which depend 

 upon the method of manufacture. Few of 

 these mixtures are suitable for cutting sec- 

 tions; and one should alwaj's be used 

 which is specifically prepared for the pur- 

 pose. The best known in the world, and 

 for many years the only one known, was 

 celloidin, supplied by Schering. Its place 

 has been taken in the United States today 

 by Parlodion, marketed by Mallinkrodt. 

 It is unfortunate that the trade names of 

 both of these should be so closely allied to 

 collodion, which is a pharmaceutical solu- 

 tion of pyroxylin unsuitable for section 

 cutting. Cellulose nitrates, other than 

 those marketed under brand names, are 

 broadly classified according to the viscos- 

 ity of the standard solution. This viscosity 

 is expressed in terms of the number of 

 seconds taken by a steel ball of standard 

 size to fall a standard distance through a 

 standard column of the solution. The low- 

 est viscosity normally marketed is that 

 known as 5-second nitrocellulose and is the 

 only one which may be employed in micro- 

 technique. Another point to be watched, 



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