Sudan black: a fast, easy and non‐toxic method to assess myelin repair in demyelinating diseases

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Neurological disorders characterized by destruction of the myelin sheath surrounding the axon range from inflammatory CNS diseases, in particular multiple sclerosis (MS) to stroke and spinal cord injury. The limited repair of damaged myelin causes chronic functional deficits 1. Particularly in chronic progressive MS, where chronic demyelination is a defining pathomechanism, it leads to secondary axonal damage and overlying progressive disability of patients 2. Therefore, the search for novel therapies enhancing myelin repair is a top priority in neurological research 1. This challenge has recently attracted much attention as demonstrated by the formation of the progressive MS alliance in 2012 6. This alliance aims at developing new treatments for progressive MS with one dedicated focus being the search for novel myelin repair enhancing strategies 7.
Recently, by using different high‐throughput in vitro methods, a number of drugs which could potentially enhance remyelination were identified 8. However, these candidate drugs need to be tested in vivo for their remyelination potential prior to clinical investigation in human patients 11.
Myelin sheaths that are thinner than expected for the corresponding axon diameter are often taken as the hallmark of regenerated myelin at the sites of myelin injury 12. The gold standard to demonstrate this is either on toluidine blue‐stained semithin sections or by electron microscopy. Both techniques require plastic embedding, small blocks, and highly toxic substances and are labour intensive. Besides that, immunostainings can be very challenging in plastic embedded tissue. Luxol Fast Blue (LFB), the most commonly used myelin stain, on the other hand is used on paraffin sections, but does not allow single axon resolution in a way that reliably demonstrates remyelinated axons. Hence, in order to efficiently screen for remyelinating candidates in vivo, it would be of great interest to have a staining method that combines optimal resolution of myelin structure with the quickness, simplicity and the large block faces of cryosections. Moreover, the potential to combine such an approach with immunostainings would open new possibilities in dissecting underlying disease mechanisms of neuro‐inflammatory disorders or mode of actions of specific remyelinating drugs.
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