The formalin perfusion technique of Pease has been shown to be a satisfactory fixation method for both light and electron microscopy of the nervous system of rat and goldfish, so allowing correlative studies to be made on tissue from the same region of one brain.
Immediate post-osmication is not necessary for good ultrastructural preservation and even if itis delayed for 1 week, neuronal structure is adequate for all but detailed cytological work. Unosmicated tissue stained with uranyl acetate and lead citrate showed protein-like structures but not lipids, with membranes appearing as unstained bands. ‘Dark’ glial cells occurred near the surface of the cortex and could not be eliminated by prolonged fixation in situ. ‘Dark’ neurons were found rarely.
It is suggested that, with this method of fixation, any poor preservation of tissue in direct light- and electron-microscopical correlative studies is due to the subsequent processing rather than the initial fixation. Preliminary results on material stained by a new Golgi-Kopsch modification and by reduced silver methods and subsequently examined with the electron microscope suggest that such damage is not extensive.