Radial neuronal assemblies, ectopia and necrosis of developing cortex: a case analysis.

Reduced size of convolutions and midcortical laminar necrosis are approximately co-extensive in the cerebral hemispheres of a child, one of twins, dying at 18 months. Because the underlying laminar arrangement of neurons and the basic gyral pattern are normal, the cortical damage probably occurred not earlier than the third trimester of gestation. Neurons surviving above and below the zone of tissue necrosis, like their homologs in normal cortex, are entrained in multineuronal radial assemblies. Below the zone of necrosis the relative positions of radially adjacent neurons are unaltered. Above, however, in places where the molecular layer is reduced in width, neurons are displaced radially outward toward the pial surface. In places the pia is breached and bridged by a mesenchymal-glial cicatrix. Where this has happened neurons have migrated beyond the cerebral boundary and have established an ectopia in the subarachnoid compartment. These observations suggest that relatively undifferentiated intracortical neurons are held in radial assemblies by bonds which prevent their tangential displacment. The molecular layer appears to serve as a barrier to their radial displacement.