It' s my understanding that for TEM analysis, tissues must be fixed in glutaraldehyde (or 2% PFA and 2% GTA solution) to be embedded in resin, whereas tissues fixed in PFA is embedded in paraffin.
If you know what to do with that tissue, is much better that you embedded the samples in the resin of interest, that way you can keep it for years! but fix and stored is not the best for TEM, the ultrastructure would be damaged even thought you use 2% of glutaraldehyde
It depends on what you need to see. Formaldehyde fixation is acceptable for TEM, as long as you don't need perfect preservation of membrane structures. The fact that they have been stored for years should not be a problem for TEM. You can embed formaldehyde-fixed tissues in resin, that is not an issue.
It is generally not recommended to store tissue used for electron microscopy in the fixative for longer periods, as aldehydes make the membranes permeable and substances can be washed out of cells, however I have no personal experience in this. The material for embedding is not dependent on the fixative used. Generally, PFA fixation is always sufficient for light microscopy, which is usually done on parafin sections. Sections for electron microscopy are cut from resin embedded samples, and for good electron microscopic results, it is better to use glutaraldehyde or a mixture. If you want to do both from one sample set, you can embed some of your PFA fixed tissue in resin and see if it is sufficiently fixed for your purpose.
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