[Morphometrical analysis of projection neurons in reeler mutant mice]

The projection neurons in the cerebral cortex are localized in the specific layers, and present the characteristic shape of cell bodies and dendritic arborization according to their properties. To examine what factors could cause the morphological characteristics of the reeler neurons, of which cortical layers are generally inverted, we morphometrically analyzed three types of projection neurons, callosal (CC-neurons), corticospinal (CS-neurons), and corticothalamic neurons (CT-neurons), by the retrograded labeling method. The results were as follows: 1) Although in the normal mice, the CC-neuron cell bodies in the layer 2 + 3 were significantly smaller than ones in the layer 5, the reeler CC-neurons were uniform regardless of their intracortical positions. 2) The cell bodies of the reeler CS-neurons were normal in size throughout the entire cortex. 3) The cell bodies of reeler CT-neurons were generally larger than normal ones, and they were larger in the location near the pia mater. 4) The apical dendrites of the normal CC-, CS- and CT-neurons were generally directed toward the layer 1 of neocortex, which is the plexiform zone (PZ) in the embryo stage, whereas those of the reeler CC- and CS-neurons were directed toward the upper-middle zone of neocortex, which is the interplexiform zone (IPZ) that corresponds with the normal PZ; however, those of the reeler CT-neurons were not oriented toward dominant direction. These results suggest that the sizes of CS-neurons vary depending upon the target of the projection, whereas CC- and CT-neurons are influenced by the volume of afferent inputs as well as the target of the projection. The afferent factors, especially thalamocortical fibers, may also influence the direction of the apical dendrites of CT-neurons.