Cryostat Article #8: Cryostat (OCT) Embedding Media

This week, we are going to focus on discussing cryostat embedding media. We’ve talked about how to section tissue, how to troubleshoot tissue cutting, but we haven’t focused on the specific embedding media to use with your cryostat. Let’s do that here!

When you have a tissue sample (fixed or fresh), it will need to be embedded in some sort of medium to allow the tissue to be cut cleanly into thinner sections. For tissue samples to be cut in cryostats, the embedding media is usually OCT compound (Figure 1). What does OCT stand for? OCT is short for “optimal cutting temperature.” Most OCT is composed of:

  • Polyvinyl alcohol
  • Polyethylene glycol
  • Non-reactive ingredients
Figure 1. OCT compound comes in many forms, but typically is a squeeze bottle (left). OCT media is squeezed onto cryostat specimen discs (chucks) for sample preparation (middle). Embedded and frozen tissue samples in OCT (right) shows how OCT goes from a transparent media at room temperature to an opaque white compound at freezing temperatures.

There are many different OCT compounds on the market. When choosing the best OCT for you, consider these key OCT compound features:

  • Viscosity: Higher viscosity (i.e., “thicker”) OCT compounds are preferred, because these are not as messy or runny. You won’t get thin OCT running all over your tissue sample while waiting for your embedded sample to freeze. However, a higher viscosity typically equates to a longer freezing time.
  • Freezing time: You want a shorter freezing time because you want your tissues to freeze rapidly before cutting. However, make sure that your freeze temperatures inside the cryostat are not set so low that your OCT embedding process causes cracks in the tissue as it freezes. For Mohs surgery prep and frozen sections from surgery, you’ll want a faster freezing OCT medium. For research purposes, you may be more satisfied with a longer freezing time and higher viscosity OCT compound.

With OCT media, you can prepare tissue samples for your cryostat using little plastic molds (Figure 2), or by directly applying OCT over the tissue sample on specimen discs (chucks).

Figure 2. Left shows examples of square plastic molds for embedding tissue samples with OCT compound. Many plastic molds are disposable. Right shows a tissue sample that has been embedded in OCT media and is already frozen.

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