The effect of prolonged formalin fixation on the staining characteristics of archival human brain tissue

Neurodegenerative disorders include wide range of conditions, which affect millions of people worldwide. Unfortunately, they are incurable and irreversibly progressive. Immunohistochemical staining of paraffin-fixed tissues for both diagnostic and research purposes are widely used. However, large amount of brain tissues are fixed but little is known about whether they are suitable for retrospective studies. The study aimed at investigating the effects of prolonged formalin fixation time on immunohistochemical expression of some common neurodegenerative markers in archival brain specimens.

Twenty brain specimens were obtained from hu- man cadavers in the Anatomy Department of King Abdulaziz University that were prefixed in 10% formalin. They were divided into two equal groups according to time of fixation: group 1 - less than 1 year, group 2 - up to 20 years. Histological examination of white and grey matter was done using haematoxylin and eosin, luxol fast blue (LFB) for myelin staining, Con- go red for amyloid plaques, CD 68 for microglial cells, tenascin-C (large ex- tracellular matrix glycoprotein) and caspase 3 antibody for apoptotic cells.

For both groups, corpus callosum sections displayed myelination with LFB staining. The distribution of CD 68 positive microglial cells was evi- dent in frontal and temporal grey matter, but not in corpus callosum sections. Strongly positive masses were seen in Congo red-stained frontal and temporal sections. Anti-caspase 3 immunostaining revealed positively stained neurons.

Histological and immunohistochemical techniques yielded repro- ducible staining results when applied to human brain tissue stored in formalin for long periods; so they can be used in well preserved biobank material which are the most targeting research areas in neuropathology.