Tissue Processing: Fixation, Dehydration and Clearing

Processing specimens of tissue from post-mortem sampling to finished histological sections is, of course, a critical part of the practice of histology. Often, this aspect of the work is left to technicians, who do it very well indeed. But the research worker might well wish to process his or her own tissue or, at the very least, to understand what has been done to it en route, so to speak, from the post-mortem bench to the stage of the microscope. In this chapter the basics of tissue processing are explained. We begin with fixation, a critical step. Then, we consider preparing the tissue for embedding in a supporting medium ready for sectioning. The medium considered in the next chapter is paraffin wax—still perhaps the most widely used of all the media available—and so the methods discussed here are suitable for preparing tissues for this form of embedding. Many of the techniques described in this chapter and the next are well over one hundred years old. This does not mean that they are outdated; on the contrary, it speaks of their remarkable reliability!