[PS12-2D-1] STRUCTURE OF U-FIBRE LAYER BENEATH FOCAL CORTICAL DYSPLASIAS: ROLE OF ITS SYNAPTIC PLEXI IN EPILEPTIC NETWORKS
U-fibres are axons that surround cerebral cortical gyri to interconnect regions of the same and adjacent gyri and most of the white matter cores of gyri. They thus differ substantially from deeper subcortical white matter. They form after mid-gestation and normally include a few displaced neurons from layer 6 and sparse synaptic connections.
The great majority of U-fibre neurons are glutamatergic and excitatory. U-fibres normally myelinate later than deep white matter and are selectively spared in leukodystrophies. The U-fibre layer beneath focal cortical dysplasias (FCD) I, II and III, hemimegalencephaly (HME) and cortical tubers of tuberous sclerosis complex (TSC) contains excessive neuronal dispersion and intricate synaptic plexi that are integrated with cortical grey matter circuitry. Deep white matter also contains dispersed neurons, both singly and in aggregates to form irregular nodules large enough to be visualized by MRI as heterotopia and local synaptic plexi interconnect nodules and form local circuits. But the U-fibre layer acts as a chemical barrier to axonal penetration from axons of deep white matter neurons, hence their circuits are not easily integrated into synaptic networks and remain local. An exception is some transmantle dysplasias that are continuous with cortex, but not all transmantle dysplasias reach the cortex and many stop at the U-fibre boundary. U-fibre and deep white matter neurons in FCD II, HME and TSC include dysmorphic, megalocytic forms as well as some normal neurons. Neurosurgeons should include underlying U-fibres in resections of epileptogenic gyri in FCDs. Neuropathologists should describe U-fibre synaptic plexi, which are well demonstrated by synaptophysin immunoreactivity. Neurophysiologists should attempt to record from the U-fibre layer intraoperatively just before resection, to enable neuropathological correlation and better understand epileptic networks in FCD.
The great majority of U-fibre neurons are glutamatergic and excitatory. U-fibres normally myelinate later than deep white matter and are selectively spared in leukodystrophies. The U-fibre layer beneath focal cortical dysplasias (FCD) I, II and III, hemimegalencephaly (HME) and cortical tubers of tuberous sclerosis complex (TSC) contains excessive neuronal dispersion and intricate synaptic plexi that are integrated with cortical grey matter circuitry. Deep white matter also contains dispersed neurons, both singly and in aggregates to form irregular nodules large enough to be visualized by MRI as heterotopia and local synaptic plexi interconnect nodules and form local circuits. But the U-fibre layer acts as a chemical barrier to axonal penetration from axons of deep white matter neurons, hence their circuits are not easily integrated into synaptic networks and remain local. An exception is some transmantle dysplasias that are continuous with cortex, but not all transmantle dysplasias reach the cortex and many stop at the U-fibre boundary. U-fibre and deep white matter neurons in FCD II, HME and TSC include dysmorphic, megalocytic forms as well as some normal neurons. Neurosurgeons should include underlying U-fibres in resections of epileptogenic gyri in FCDs. Neuropathologists should describe U-fibre synaptic plexi, which are well demonstrated by synaptophysin immunoreactivity. Neurophysiologists should attempt to record from the U-fibre layer intraoperatively just before resection, to enable neuropathological correlation and better understand epileptic networks in FCD.