Elizabeth:
  As you well know, croslinkage by formalin during fixation (as well as xylene as an antemedium) has cause the development of the Heat Induced Epitope Retrieval (HIER) and, although there is not such a thing as "overfixation", many consider that the longer a tissue is in formalin, the more IHC is "affected".
  What I would suggest you to do is to get a fresh specimen, fix it for just 6 hours, process it and use it as a positive control against the specimen kept the longest in formalin.
  Then you will be able to see the difference, if any at all!
  Just a thoght!
René J.

"Stephens, Elizabeth Humes"  wrote:
  Hi everyone! I am wondering whether anyone has looked at how much length of time in formalin affects the ability to do immunohistochemistry? For instance, I want to do a pathology study on heart valves but have hearts that have been in formalin 1 year, 2 years, 3 years and am afraid this will affect
my results. I would be interested in IHC staining of matrix markers including various collagen types, elastin, fibrillin, proteoglycans, glycoaminoglycans, smooth muscle alpha actin, lysyl oxidase, prolyl 4-hydroxylase.

Thank you!!
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