The anterior medullary velum and its involvement during autoimmune demyelination

The anterior medullary velum (AMV), a thin lamina of central white matter forming the roof of the IVth ventricle, has been analysed ultrastructurally in the normal guinea pig and in guinea pigs with chronic relapsing experimental allergic encephalomyelitis. In the latter condition, the AMV appeared to provide a route of access for haematogenous elements from the circulation into the ventricular space. The normal AMV consisted of fascicles of myelinated nerve fibres embedded in a layer of highly attenuated ependymal cells. Between the fascicles, the AMV was comprised merely of a layer of ependymal cytoplasm, in places about 0.5 micron thick. In contrast to ependymal cells from other levels of the neuraxis, in ependymal cells in the AMV, ciliary rootlets of the basal body apparatus were extraordinarily long, numerous and prominent. Their prominence might be related to a need for increased flexibility in this region of the ventricular system. Despite previous claims to the contrary, nerve cell bodies were present within the AMV as well as many synaptic complexes, unmyelinated axons, and supra- and subependymal axons believed to belong to the serotoninergic plexuses. During autoimmune demyelination, the meningeal space over the AMV became heavily infiltrated, inflammatory cells entered the nerve fibre bundles, myelin was destroyed and, perhaps related to disruption of the ependymal layer in places, haematogenous macrophages gained access to the ventricular surface of the AMV. Clinical relapses were accompanied by renewed inflammatory and demyelinative activity and further attenuation of the AMV with concomitant fibrous astrogliosis. Thus the AMV is described in detail for the first time at the ultrastructural level and is presented as a region vulnerable during periods of meningeal infiltration. The cytoarchitecture of the AMV might contribute to the genesis of demyelinated plaques around the IVth ventricle.