The cerebral cortex of the reeler mouse embryo. An electron microscopic analysis.

The cerebral cortex of reeler (rl/rl) and control (rl/+) embryos has been examined at embryonic day 14 (EO = day of the plug). At this early stage, the cells of the cortical plate in reeler are characteristically maloriented when compared to the controls but, despite this malorientation, they assume a normal differentiation; in particular the regions of contact between the cells or the cortical plate are not different from the controls. The marginal zone and the subcortical plate of reeler are less well delimited than in normal fetuses, yet each of these layers contains the same contingent of cells and fibers as the normal embryos. In particular an early afferent fiber system is visualized in reeler and shows the same characteristics as in normal embryos: weak osmiophily, presence of dense core vesicles and of multilocular dense vacuoles, formation of asymmetric synapses in the marginal zone. The nature of these fibers is unknown but it is suggested that they could correspond to the aminergic system described in rats at the same developmental stage. The observations reported, though compatible with the hypothesis that the reeler gene acts upon a cell recognition-adhesion phenomenon between CP cells, do not provide a positive evidence for it and do not exclude other explanations.