Moniliform deformation of retinal ganglion cells by formaldehyde-based fixatives

ABSTRACT Protocols for characterizing cellular phenotypes commonly use chemical fixatives to preserve anatomical features, mechanically stabilize tissue, and stop physiological responses. Formaldehyde, diluted in either phosphate‐buffered saline or phosphate buffer, has been widely used in studies of neurons, especially in conjunction with dyes and antibodies. However, previous studies have found that these fixatives induce the formation of bead‐like varicosities in the dendrites and axons of brain and spinal cord neurons. We report here that these formaldehyde formulations can induce bead formation in the dendrites and axons of adult rat and rabbit retinal ganglion cells, and that retinal ganglion cells differ from hippocampal, cortical, cerebellar, and spinal cord neurons in that bead formation is not blocked by glutamate receptor antagonists, a voltage‐gated Na+ channel toxin, extracellular Ca2+ ion exclusion, or temperature shifts. Moreover, we describe a modification of formaldehyde‐based fixatives that prevents bead formation in retinal ganglion cells visualized by green fluorescent protein expression and by immunohistochemistry. J. Comp. Neurol. 523:545–564, 2015. © 2014 Wiley Periodicals, Inc. The authors show that formaldehyde fixatives induce the formation of varicosities in rat and rabbit retinal ganglion cell dendrites and axons, and that this is precluded by sucrose, but not (as in hippocampal, cerebellar, cortical, and spinal cord neurons) by glutamate receptor antagonists, tetrodotoxin, Ca2+ exclusion, or temperature shifts.