Definition: Polymicrogyria (PMG) is a malformation of cortical development that arises during the late phase of neuronal migration and into the early phase of cortical organization.
Gross Morphology:
The cortex has many excessively small and shallow gyri, giving a characteristic 'pebbled' appearance.
Macroscopic thickening of the cortex is often due to excessive folding rather than true cortical hypertrophy.
Histological Features:
Loss of normal cortical lamination.
Fused molecular layers within the folds.
Reduction in the number of neurons.
Classification:
Layered (Classic) Polymicrogyria:
Has four distinct cortical layers.
Believed to be post-migrational in origin.
Often accompanied by gliosis and laminar neuronal necrosis, suggesting a post-migration insult (e.g., Toti et al. 1998).
Unlayered Polymicrogyria:
Arises due to disturbances during the terminal stages of migration.
Exhibits poorly laminated cortex with disorganized, heterotopic neurons.
Neurons may be arranged in vertical columns extending into subcortical white matter.
