Fresh skin samples are routinely placed in 10% neutral buffered formalin (NBF) for fixation prior to grossing and processing. Specimens that require immunofluorescence (IF) should be preserved in Michel's or Zeus solution, rather than formalin. Once tissue is fixed in formalin and grossed in properly, it is paraffin processed. Processing tissue is critical to the outcome of the final product, which is the microscopic slide.
In the histology laboratory, the goal of processing formalin-fixed tissue is to prepare it for paraffin embedding and microtomy. Since water-filled tissue cannot easily combine with wax, various processing steps prepare the tissue for paraffin infiltration. The basic steps of standard tissue processing are:
- Fixation
- Dehydration
- Clearing
- Paraffin infiltration
The amount of time this process takes is dependent on tissue size, instrumentation, reagents used, and temperature. Each processing step is critical to subsequent tissue cutting and staining. Problems encountered during cutting and staining are typically a result of suboptimal tissue processing.
NOTE: Skin tissue processing methods vary among laboratories. This course discusses the most widely used methods to date. Although there are other tissue preparation techniques, such as freezing fresh skin tissue for Mohs or embedding tissue in plastic for electron microscopy, the focus of this course is strictly paraffin processing of formalin-fixed skin samples.
