Cytoarchitecture of Cerebral and Cerebellar Cortices of the Reeler Mouse

Abstract

Homozygous reeler mice have action tremor, dystonic posture, and reeling atxic gait. The mutation at the reeler genetic locus (gene symbol: rl) causes cytoarchitectonic abnormalities to the cerebral and cerebellar cortices. Retrograde transport of horseradish peroxidase (HRP) was used to examine the cells of origin of the corticospinal tract (CST) and the callosal commissural fibers (CC) in the primary motor cortex of normal and reeler mice. In normal controls, CST neurons retrogradely labelled after. HRP injection into the lumbar cord were situated only in layer V. In the reeler, by contrast, the labelled CST neurons were scattered difusely all levels of the corresponding cortical area. In addition to the malpositioning of the somata, the labelled CST neurons in the cortex of the reeler could be divided into two major classes according to their dendritic pattern; typical pyramidal neurons and atypical ones. The typical pyramidal neuron had an atypical dendrite projecting from the superior pole of the soma and ascending toward the pia mater. The atypical pyramidal neurons consisted of six types: inverted, tumbled, bipolar, V-shaped, hook-shaped, and superficial polymorphic. The typical pyramidal neurons in the reeler tend to be situated relatively deep in the cortex and the atypical neurons tend to lie relatively superficially in the cortex.In the normal mouse, CC neurons retrogradely labelled after the injection of HRP into the primary motor cortex are distributed in a bilaminar pattern such that the largest number of cells are located in supragranular layers II and III and infragranular layer V. In the reeler mutant, CC neurons are found in all cortical layers, but two-thirds are situated in the lower half of the cortex. On the basis of the cell shape and orientation of the apical dendrite, CC neurons of the reeler are classified into six morphological types: typical pyramidal, inverted pyramidal, tumbled, hook-shaped, polymorphic, and simple. The apical dendrites of the CC neurons in all layers of the cortex of the reeler mouse are randomly oriented: no direct relationship between the intracortical position of the soma and orientation of the apical dendrite was found. In contrast, CST neurons in the reeler mutant are concentrated in the outer third of the cortex, and there is a relationship between the laminar distribution of these cells and the alignment of their dendrites with respect to the pial surface : the apical dendrites of CST neurons lie in superficial layers tend to be oriented obliquely, whereas those of CST neurons lie in the deeper cortex most often are oriented vertically, i.e., toward the pial surface. Quantitative analysis revealed that the relative intracortical positions of the CST and CC neurons are reversed in the reeler mutant although both populations exhibited greater laminar disposition, and as a consequence, there is more intermingling of the two cell groups in the reeler than in the normal mouse. Thus, the present study suggests that the normal cytoarchitectonics of the primary motor cortex are inverted in the reeler mutant mouse.Cerebella of the normal and reeler mutant mice at two months of age were studied by the Golgi impregnation method to elucidate the detailed configurations of the Golgi epithelial cells. On the basis of configuration of their Bergmann fibers, the Golgi epithelial cells of the reeler were relatively normal, or assumed inverted, swan neck, stellate, bipolar, small, or fan shapes, although their morphological changes were continuous rather than abrupt. A relatively normal type of Golgi epithelial cells which somewhat resemble those of normal controls was present in the reeler, although some differences between them in detailed appearance existed. It is important to note that the radially organized Bergmann fibers ascending from the somata of this type of Golgi epi-thelial cells gave rise to a glial architecture of normal appearance in small areas of