Myelin Swellings in MS Can Grow, Shrink, and Recover - Nerve Activity Drives the Cycle

Multiple sclerosis lesions - the patches of scarring in the brain and spinal cord that accumulate over the course of the disease - are end-stage damage. But the process that leads to them begins earlier, with something subtler: swellings in the myelin sheath, the fatty insulation wrapped around nerve fibers.

Those swellings have typically been treated in research as static markers - structural signs of early damage awaiting progression to full lesions. A study published in Science by an international team from Amsterdam UMC, VU LaserLab, the Netherlands Institute for Neuroscience (NIN), and the University of Edinburgh shows that characterization is wrong. Myelin swellings are dynamic, they respond in real time to what the underlying nerve fiber is doing, and under the right conditions, they can fully resolve.

Dynamic damage observed directly

The team used two advanced microscopy approaches that allowed them to observe living tissue rather than chemically fixed samples - a critical distinction. Traditional brain research typically preserves tissue with fixatives that lock structures in place, making it impossible to determine whether a swelling was growing, shrinking, or stable at the moment of capture. Third-harmonic generation (THG) microscopy, used in collaboration between the VU LaserLab and Amsterdam UMC, and two-photon microscopy performed at the NIN, enabled observation of myelin changes in three dimensions across time.

The researchers applied these techniques across multiple model systems - zebrafish and mouse models, as well as human brain tissue - allowing them to study the same processes in animal models and human tissue using identical methods. That consistency is methodologically valuable: observations in zebrafish, where the nervous system is more accessible, could be directly compared with findings in mammalian and human tissue.

Nerve activity as the control variable

The central finding was a relationship between nerve fiber activity and swelling dynamics. When researchers increased the activity of underlying nerve fibers, the myelin swellings surrounding them grew larger and became more numerous. When fiber activity was reduced, the same swellings could shrink - and in some cases, the myelin recovered entirely.

This is a departure from the static damage model. It implies that the size and number of myelin swellings in any given moment reflect an ongoing physiological balance between the metabolic demands of active nerve fibers and the capacity of the myelin sheath to maintain its structure. MS pathology, in this view, may be partly a story of that balance being disrupted - and potentially, a story of it being partially restored.

What the team still needs to understand

The researchers are candid that the mechanistic picture remains incomplete. They have established the activity-swelling relationship but not its molecular basis. The next questions are: why does nerve fiber activity cause swellings to form and grow? And what role do surrounding brain cells, including microglia and astrocytes, play in the appearance and disappearance of swellings?

The teams of Maarten Kole at the NIN and Antonio Luchicchi at the MS Center Amsterdam UMC, together with David Lyons's group in Edinburgh, plan to pursue those questions using the model systems established in the current paper. The study does not yet identify specific therapeutic targets, and the path from describing dynamic swelling behavior to developing treatments that modulate it is long. The current work defines the biological phenomenon; translating it into clinical intervention will require understanding the underlying molecular chain.

Why the timing matters for MS treatment

Current MS therapies predominantly target inflammation - they suppress the immune attacks that produce the characteristic lesions. They are more effective at reducing new lesion formation than at reversing existing neurological damage. If myelin swellings genuinely precede lesion formation and are reversible at that early stage, they represent a potential intervention point earlier in the disease cascade than existing drugs reach.

Addressing damage before myelin is irreversibly lost could, in principle, preserve function that current treatments cannot recover. That possibility is what makes the dynamic nature of these swellings clinically significant - though it will take substantially more research before any specific intervention can be proposed.