Human brain tissue can be effectively analyzed via electron microscopy at postmortem intervals up to 100 hours if the body is stored at cold temperature after death

I recently saw this interesting quote from Kay et al 2013 in their Nature Protocols article:

For tissue preparation, we have incorporated array tomography and EM preparations into routine brain bank collection. We have managed to conduct very effective EM studies on tissues retrieved from donors with long post mortem intervals, up to 100 hours. In our experience a key element in tissue preservation for ultrastructure analysis is post mortem cold storage of the cadaver, with cold storage in a mortuary of around 4–6°C significantly reducing structural degradation.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3712649/