Two Patterns in Apical Dendrite Extensions of Projection Neurons within Cerebral Cortex of Reeler Mutant Mice

Ryoichi Ichikawa^*

• Department of Anatomy, Sapporo Medical University School of Medicine, Sapporo, Japan

Introduction: Pyramidal-like projection neurons in the cerebral cortex exhibit layer-specific positioning of their cell bodies and target specific cortical regions with their apical dendrites. Reeler mutant mice, which lack the gene for the reelin protein gene secreted by Cajal-Retzius cells and have their projection neurons scattered throughout the cortex, display relatively intact global and local neuronal network connections compared with wild-type mice. The irregular morphologies of these cells, which extend their apical dendrites in a neuron-disoriented direction, are thought to compensate for the malposition of the neurons. I aimed to investigate the projection target-specific regulation of this apical dendrite extension pattern in reeler mice. Methods: To this end, three types of projection neurons-corticospinal (CS), corticothalamic (CT), and corticocallosal (CC) neuronswere evaluated using retrograde labeling techniques. Results and Discussion: Reeler CS neurons displayed a congregation pattern of apical dendritic terminal tips in a specific upper cortical zone, whereas reeler CC neurons exhibited a dispersed pattern of scattered tips throughout the cortex. However, reeler CT neurons showed a hybrid pattern, exhibiting characteristics of both congregationand dispersion-type neurons. Moreover, apical dendrite extension of these projection neurons follows either a congregation or dispersion mode from postnatal day 0 (P0), which subsequently defines their terminal tip positioning by P8. Thus, this early patterning of apical dendrite arborization in reeler projection neurons likely contributes to the formation of projection target-specific neuronal connections during the first two postnatal weeks.