Cerebrocortical microdysgenesis with anomalous callosal connections: a case study in the rat

A spontaneously occurring area of cerebrocortical microdysgenesis, resembling microgyria, and having its origin during development was noted in the left auditory cortex of a rat which had received a callosal section in adult life. Adjoining sections were prepared with cresyl violet for visualization of cell bodies, and by the Fink-Heimer method for degenerating axonal terminals. The microdysgenetic region was characterized by fused molecular layers containing ectopic neurons and by abnormal cortical lamination. The architecture of terminal degeneration of callosal axons within this region was abnormal with a notable absence of the usual lamination pattern and the presence instead of dense terminations in what would normally be layers I-IV and in connecting bridges running across layer V. Abnormal developmental processes that might account for the anomalous projection patterns in cortical malformations are discussed in the light of knowledge about normal corticogenesis. The behavioral implications for some childhood learning disorders are also considered.