What fixative is best for traditional histopathology & still allows for molecular testing as well?

Here there is the tried and true method of histology and new PCR technology competing for the same sample to make a diagnosis. Histology allows the veterinary pathologist to examine the organs visually down to the level of the cells with the ability of seeing things in context, rather than in isolation. This means that if there’s a pathogen or damage, there should also be host reaction to it. PCR technology is a good ancillary test to confirm what is seen, down to the DNA/RNA level. It can be used to identify the species of pathogen.

For the purpose of research on AGD in Atlantic salmon gills, they report that seawater Davidson’s and PAXgene®appear to be the best.

Journal of Fish Diseases
Volume 36, Number 10 (October 2013)
	Evaluation of fixation methods for demonstration of Neoparamoeba peruransinfection in Atlantic salmon, Salmo salarL., gills
	Authors:	K Cadoret, A R Bridle, M J Leef, B F Nowak
	Author Affiliations:	no affiliations available
	Source:	Journal of Fish Diseases, Volume 36, Number 10 (October 2013)
	Page Numbers:	831 – 839
	Available Full Text:	Full Text: Subscription Required to view full text Format: PDF Size: unknown Location: Publisher’s Site Authentication: EBSCOhost EJS
	Abstract:	Formaldehyde-based fixatives are generally employed in histopathology despite some significant disadvantages associated with their usage. Formaldehyde fixes tissue by covalently cross-linking proteins, a process known to mask epitopes which in turn can reduce the intensity of immunohistochemical stains widely used in disease diagnostics. Additionally, formaldehyde fixation greatly limits the ability to recover DNAand mRNAfrom fixed specimens to the detriment of further downstream molecular analyses. Amoebic gill disease (AGD) has been reliably diagnosed from histological examination of gills although complementary methods such as in situhybridization (ISH) and polymerase chain reaction (PCR) are required to confirm the presence of Neoparamoeba perurans, the causative agent of AGD. As molecular techniques are becoming more prevalent for pathogen identification, there is a need to adapt specimen collection and preservation so that both histology and molecular biology can be used to diagnose the same sample. This study used a general approach to evaluate five different fixatives for Atlantic salmon, Salmo salar L., gills. Neutral-buffered formalin and seawater Davidson’s, formaldehyde-based fixatives commonly used in fish histopathology, were compared to formalin-free commercial fixatives PAXgene®, HistoChoice™MB* and RNAlater™. Each fixative was assessed by a suite of analyses used to demonstrate AGDincluding routine histochemical stains, immunohistochemical stains, ISHand DNAextraction followed by PCR. All five fixatives were suitable for histological examination of Atlantic salmon gills, with seawater Davidson’s providing the best quality histopathology results. Of the fixatives evaluated seawater Davidson’s and PAXgene®were shown to be the most compatible with molecular biology techniques. They both provided good DNArecovery, quantity and integrity, from fixed and embedded specimens. The capacity to preserve tissue and cellular morphology in addition to allowing molecular analyses of the same specimens makes seawater Davidson’s and PAXgene®appear to be the best fixation methods for diagnosis and research on AGD in Atlantic salmon gills.
	Citation:	K Cadoret, A R Bridle, M J Leef, B F Nowak . Evaluation of fixation methods for demonstration of Neoparamoeba peruransinfection in Atlantic salmon, Salmo salarL., gills. Journal of Fish Diseases, Volume 36, Number 10 (October 2013), pp. 831-839, <http://ejournals.ebsco.com/direct.asp?ArticleID=4A2A81BD0F8FD70E9BF0&gt;
	URL:	http://ejournals.ebsco.com/direct.asp?ArticleID=4A2A81BD0F8FD70E9BF0