Paraffin wax is used in the tissue processing sequence where, after fixation, dehydration, and clearing, the tissues are infiltrated with wax to give it structure and support during microtomy. There are a multitude of options for the user to consider, and oftentimes one person’s individual preference has much more to do with personal choice than science. Many laboratorians will use a paraffin wax that they were trained on, or one that they got used to because it is what their respective workplace uses. But there ‘is’ an original science that comes with the various options, and within this article we will discuss the most prominent application ideas behind product design.
Discussions of one wax being different from another involve terms such as polymers and additives, without really having an understanding of what each means and how it alters performance of the wax in infiltration (tissue processing) and microtomy. Many commercial vendors offer a ‘high’ polymer wax versus a ‘low’ polymer wax. Or even more confusing, they might offer one having a specific additive (resin, plasticizers, etc.) over another. The paraffin molecule is a simple straight chain molecule of carbon and hydrogen content.
The paraffin molecule is a simple straight chain molecule of carbon and hydrogen content. The image to the right illustrates that there is no challenging configuration to the molecule. Once we move past the original molecular construction of a paraffin wax molecule, we begin to see options such as polymers and various additives.
By definition, polymer is a mixture of various molecules formed in a repeating pattern. Paraffin wax used in histology infiltration has polymer blends added to the simple molecule that give it a self-repeating quality. This gives the wax strength enough to peel off the blade to form a smooth section instead of crumbling on the knife, and it enhances the ability to produce repeated sectioning into a ribbon. Within this brief description, the most important thing to note is that polymers and additives will increase the size of the paraffin molecule.
Understanding this now takes us to the original science behind the various paraffin wax types, (e.g., high polymer v/s low polymer). To understand the science of design in wax infiltrates, we must consider what is known as the ‘Sieve Concept’. When tissues are being infiltrated during tissue processing, we must remember they are being infiltrated down to the molecular level of the tissue molecules. The Sieve Concept says that simple, singular elements (such as a simple straight-chained molecule), can pass through the small porous screen (or interstitially in tissues) very readily without long filtration delays. Water passing through a sieve would be a perfect example. On the other hand, a thicker, more viscous solution moves through the small pores of a sieve slower. The same with tissue interstitially.
The amount of polymer in an embedding medium will progressively affect its rate of infiltration. High polymer waxes have either a longer polymer chain paraffin molecule, or a high number of short chain polymers in the wax. This would give it the characteristic of a slower infiltrating medium (tissue processing function), but one that gives better performance in sectioning because of the polymers. Conversely, a low polymer wax has fewer polymers and because of the smaller size of the polymer-paraffin molecule, it infiltrates faster than a high polymer wax. This would be more advantageous during tissue processing when dealing with animal tissue which is traditionally more dense and fibrous, than with bone.
The science suggest that low polymer waxes are more advantageous for tissue processing, and high polymer waxes are more advantageous for microtomy. Thus, you will see some labs choose to use two different paraffins in their labs: one for one application and a different one for the other. Oftentimes personal preference drives this decision, and it has nothing to do with the science. The reality of today’s paraffin products on the market is that most paraffins are designed for universal application, meaning they can be used for both tissue processing and microtomy application. Except in special research applications, you will rarely see a major difference in tissue processing infiltration in the major product in the market today. However, you can traditionally see more advantages in microtomy when using a high polymer wax. Increased polymers give the wax a heightened ability for thinner sectioning and ribbon strength. This is the nature of polymers in general.
In this article we have given a general understanding of paraffin wax content and the causes of variability in infiltrations rate. We also discussed the applications and advantages of polymers in wax. In Part 2 of this series we will discuss the various additives found in embedding mediums, and there characteristics in infiltration and embedding.
- Brown, S., “The Art & Science of Histotechnology”, Lab Management Consultants, 2011.
- Brown, S., “The Basic Dynamics of Fixation & Processing”, Lab Management Consultants, 2008.
- Brown, S., “The Wax Museum”, Workshop Symposium-National Society for Histotechnology, Lab Management Consultants, 2008.
- Visser, G., “Waxes Used In Hot Melt Adhesives”, Sasol Wax Americas, Inc., 2007.