Chapter 2 



Dehydration 



Preparation for Embedding 



Tissues fixed in aqueous solutions will maintain a high water con- 

 tent, a condition that can be a hindrance to later processing. Except in 

 special cases (freezing method, water-soluble waxes, and special cell con- 

 tents), the tissue must be dehydrated (water removed) before certain 

 steps in this processing can be successful. 



Tissues, during fixing and washing, lack an ideal consistency for sec- 

 tioning — cutting thin slices of a few microns in thickness. They may be 

 soft, or may contain a lumen or hollow spaces and are easily deformed 

 by sectioning. If the cells were pierced by a knife, their fluid content 

 could be released and this would allow the cells to collapse. To pre- 

 clude these problems, the fluids in the tissue are replaced by a mediimi 

 which hardens to a firm, easily sectioned material. The cells are filled 

 intracellularly and enclosed extracellularly with the medium and are 

 thereby protected during physical handling. The most universally used 

 media for this purpose are paraffin and nitrocellulose or some variation 

 thereof. Other media, less frequently used, are gelatin and water-soluble 

 waxes. 



Various conditions determine the choice of medium. Paraffin is suit- 

 able for most histological and embryological purposes when thin sec- 

 tions (1-15 yu) are required. Thin sections also can be prepared with 

 nitrocellulose. Serial sections, however, are made more easily with the 

 former than with the latter; also paraflin preparation requires a far 



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