Springer Semin Immunopathol 2002;24(2):105-25
Tsunoda I, Fujinami RS.
Department of Neurology, 3R330 School of Medicine, University of Utah, 30 North 1900 East, Salt Lake City, Utah 84132-2305, USA.
The primary target in multiple sclerosis (MS) is believed to be either myelin itself (myelinopathy) or the myelin-forming cell, the oligodendrocyte (oligodendrogliopathy).
Although axonal injury occurs in MS, it is regarded as a secondary event to the myelin damage.
Here, the lesion develops from myelin (outside) to the axon (inside) (Outside-In model).
Recently, gray matter lesions and axonal injury in normal-appearing white matter have also been reported in MS.
This raises two questions.
1) Is axonal injury exclusively secondary to myelin damage or from a direct insult to the axon or neurons (axonopathy)?
(2) Is the injured axon regarded as only an end result of pathology or disease, or can axonal injury contribute to the spread of secondary damage, including demyelination?
The former is raised from the fact that axonal damage has been reported in several virus infections, including human immunodeficiency virus, human T-lymphotropic virus 1, herpes simplex virus and coronavirus, which also cause demyelination.
The latter possibility where axonal injury leads to other changes is raised from the rather unexpected similarity between spinal cord injury (SCI) and MS where axonal injury, oligodendrocyte apoptosis and demyelination are all present.
In SCI, transection of axons leads to delayed oligodendrocyte apoptosis with secondary demyelination.
Neurofilament immunostaining of spinal cord sections demonstrates that axonal injury with oligodendrocyte apoptosis also precedes demyelination in an animal model for MS, Theiler's murine encephalomyelitis virus infection.
This implies that axonal injury could trigger demyelination.
In this instance, lesions develop from the axon (inside) to the myelin (outside) (Inside-Out model).