I want to compare the brain sizes of two populations of fish. However, all the samples I have were fixed in 95% ethanol. As far as I know, 4% formalin is the normal fixative for soft tissues. Can I conduct my study, or do I need to obtain samples in formalin? I've scoured the literature, and I can't find any useful recommendations.
I just spoke with my histological colleague about your question. He said that you should be fine for gross examination but not for microscopy. Why? He says, "When working with specimens, the higher resolution microscopy you try to achieve, the more 'potent' the fixative needs to be. A preservative will preserve what tissues are there. However, fixatives (like paraformaldehyde or glutaraldehyde) bind proteins together and, thus, are used to try to 'fix' a tissue posthumous in the manner that it was structured in life. For gross examination, a preserved specimen (without prior fixation) can act as a decent voucher for macroscopic evaluation of its internal organs; however, it will not suffice for any type of detailed histology at the light microscopy or electron microscopy level."
So, the short answer is that it depends on the type of work you need to accomplish.