Abnormal positioning of granule cells alters afferent fiber distribution in the mouse fascia dentata: morphologic evidence from reeler, apolipoprotein E receptor 2-, and very low density lipoprotein receptor knockout mice.

The fascia dentata of the hippocampal formation is characterized by the nonoverlapping and lamina-specific termination of afferent fibers: entorhinal fibers terminate in the outer molecular layer and commissural/associational fibers terminate in the inner molecular layer. It has been proposed that this fiber lamination depends on the presence of the correct postsynaptic partner at the time of fiber ingrowth during development. Pioneer neurons that guide afferent fibers to their correct layers as well as signals located on granule cells have both been implicated. To study the role of granule cells for the lamina-specific ingrowth of afferents, the cyto- and fiberarchitecture of three mouse mutants (very low density lipoprotein receptor knockout mouse, apolipoprotein E receptor 2 knockout mouse, and reeler mouse) that show different degrees of granule cell migration defects were analyzed. Anterograde tracing with Phaseolus vulgaris-leucoagglutinin was used to visualize the afferent fiber systems, and immunohistochemistry was used to determine the position of their putative target cells. In controls, granule cells are packed in a single layer. This laminar organization is mildly altered in very low density lipoprotein receptor knockout mice, moderately disturbed in apolipoprotein E receptor 2 knockout mice, and severely disrupted in reeler mice. These changes in granule cell distribution are mirrored by the distribution of commissural fibers. In contrast, changes in granule cell distribution do not severely affect the laminar termination of entorhinal fibers. These data provide further evidence for a role of granule cells in the laminar termination of commissural/associational afferents to the fascia dentata.