Histology Techniques - Embedding

Paraffin Embedding

Since water and paraffin do not mix, the first step in embedding with paraffin is to replace the water in the tissues with a solvent that is miscible with paraffin. Following are the steps in paraffin embedding:

  1. Dehydration - It is the first part of the process. It is usually accomplished by transferring the block of tissue through a series of alcohol-water solutions beginning with 50 percent and running up to water-free or absolute alcohol.
  2. Clearing - The alcohol is replaced by Histoclear (a non-toxic substitute for xylol) or cedar oil, which is readily soluble in alcohol, and in turn, is replaced by melted paraffin.
  3. Embedding - The actual embedding takes place when the paraffin- infiltrated tissue is placed in fresh paraffin and the latter allowed to cool. It is important to remember that the xylol and other solvents will dissolve the fats of the tissues unless they are fixed by some special chemical such as osmic acid.

Celloidin Embedding

Celloidin is dissolved in equal parts of absolute alcohol and ether. The tissue is dehydrated in alcohol in the same way as for paraffin except that it is transferred from absolute alcohol to a dilute solution of celloidin. As the alcohol and ether evaporate, they are replaced by more concentrated celloidin. It is finally hardened in chloroform and stored in 80 percent alcohol. It is a much longer process than paraffin but causes much less shrinkage and distortion. It is used especially in examination of the eye and brain.

Epoxy Embedding

Introduction of epoxy embedding media has greatly reduced artifacts due to shrinkage and also has allowed thinner sectioning than was possible with paraffin. The thinner sections (approximately 1 u) may be viewed after staining with the light microscope or may be sectioned thinner and examined by electron microscopy.