Embedding agents

Embedding involves introducing tissues into a medium that is hard enough for sectioning. The agent used must also penetrate into the tissue to give it internal support during the sectioning process. No matter what embedding medium is utilized, the techniques are typically arduous and time-consuming. Some representative embedding agents are:

Gelatin, a protein that often takes up dyes itself, thus obscuring cell structure. It will not section very thin and supports bacterial growth. However, tissues may not need to be dehydrated to be embedded in gelatin since it is water-soluble.

Paraffin, a wax-like material can be used as an embedding material. In many laboratories its use has been replaced by resin embedment. However, specimens embedded in paraffin will last indefinitely, and can be cut as thin as 4-5 micrometers. The process can readily form ribbons of serial sections during microtomy. Paraffin is, unfortunately, a strong lipid solvent and therefore brings about considerable specimen shrinkage.

Resins (plastics) originally started as embedding agents for electron microscopy but, due to their ability to be cut very thin, they have received wide acceptance for light microscopy as well. The plastics need to be polymerized in order to become hardened. In the unpolymerized (liquid) state, many of the ingredients are irritants or even carcinogenic agents. Resin-embedded specimens can be cut as thin as 60 nm for transmission electron microscopy, or often in 1-3 micrometer thick sections for light microscopy.

In this illustration, a vial of chemically fixed specimens is shown on the left, a polyethylene mold for containing a specimen and liquid plastic is shown on the right, and a hardened (polymerized) plastic block containing a small specimen at the tip is shown in the center.