Meningeal cells stimulate and direct the migration of cerebellar external granule cells in vitro

The external granular layer is a secondary proliferative zone that arises from the caudolateral margin of the cerebellar ventricular zone and then spreads beneath the pial surface, eventually covering the entire cerebellar anlage. Here, both a part of the Bergmann glia and granule cells are generated. Selective destruction of the leptomeningeal cell layer during development in vivo disrupts the subpial extension of the external granular layer and the laminar deposition of its descendant cells. The mechanisms by which meningeal fibroblasts exert their controlling influence on cortical development have remained unclear but could involve diffusible factors and/or interactions mediated by direct cellular contacts. In order to test these assumptions, we have co-cultivated cerebellar slice explants with meningeal cells with and without interposition of a microfilter barrier. In this setup, meningeal cells by a diffusible factor stimulated the emigration of immature neurons exclusively from the external granular layer. This effect could also be elicited by fibroblasts from other tissues but not by nonfibroblastic cells such as, e.g., astroglia. In the Boyden chamber assay, the migration of undifferentiated neurons isolated from the external granular layer was chemotactically oriented towards the source of meningeal cell-conditioned media. In comparison, neurons from the internal granular layer did not respond to this stimulus. The attraction of immature neurons towards the pial surface could (1) represent a mechanism for the establishment of (subpial) secondary proliferative zones and (2) hypothetically also play a role in the outward-directed migration of postmitotic cells, e.g., in the isocortical anlage.