Mechanisms of Layer Formation of Cerebral Neocortex and Layer-specific Neural Circuits

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Reeler and yotari mice that are mutant for Reelin or Dab1, respectively, show disorders of cerebral cortical lamination. We injected horseradish peroxidase (HRP)into the upper lumbar enlargement to label corticospinal tract (CST) neurons and wheat germ agglutinin-conjugated HRP (WGA-HRP) into the ventrolateral nucleus of the thalamus to label corticothalamic tract (CTT) neurons in both 19-day-old yotari and reeler mice with the aim of discovering whether or not they show differences in the distribution pattern of layer V or layer VI neurons. Similar injections of tracers were made in normal controls. HRP-labeled CST neurons, which were exclusively distributed in the layer V of the normal cortex, were radially scattered in the cortex of both mutants, but those in reeler were more deeply distributed than in yotari. WGA-labeled CTT neurons, which were mainly located in layer VI in the normal cortex, were superficially distributed just beneath the pia mater in both reeler and yotari cortex. The present quantitative study shows that the distribution pattern of layer V neurons, but not layer VI neurons, differs between reeler and yotari mice, suggesting that the Reelin and Dab1 proteins may play different roles in the migration and cell positioning of layer V neurons.