Brain extracellular space fixed for electron microscopy.

Adult mammalian brain contains 17--20% extracellular space, but fixatives cause the cellular elements to ingest the extracellular fluid so that the space is reduced to less than 5% with all conventional methods of fixation. This can be prevented by washing out the extracellular fluid with isotonic sucrose, which does not penetrate cells. Subsequent fixation with aldehydes and osmium and conventional processing leads to the preservation of extracellular space in electron micrographs. Extracellular space was found to be unevenly distributed, widely separating some cellular processes while leaving other groups of processes contiguous.