Anatomy of cultured mouse cerebellum. II. Organotypic migration of granule cells demonstrated by silver impregnation of normal and mutant cultures

The architecture of presumptive cortex in whole mount Holmes impregnations of cerebellar cortex was reassessed in order to accomplish accurate identification of the granule cell neurons now known to be present there and to seek evidence concerning possible granule cell migration in vitro . The granule cells, although their processes failed to stain by Holmes' method, were recognized as a layer of naked nuclei of characteristic size, shape, and close packing. The gross foliar and sulcal architecture of cortex remained very immature, as it was when explanted. However, a substantial minority of cultures developed microscopic organotypic lamination characteristic of mature cortex, with molecular, granular, and Purkinje cell layers. All organotypically laminated areas showed anomalous twinning of the granular layer. In the reeler mutation of the mouse, the granule cell migration fails, producing an abnormal laminar pattern in adult cerebellum. Cultures of reeler cerebellum, although well developed in other respects, never showed any trace of cortical lamination, proving that the lamination seen in normal cultures must be produced by the migration which fails in reeler but succeeds in normal mice. This is reinforced by the twinning of the granular layer: any reasonable explanation requires organotypic migration of some of all granule cells. The preferred explanation involves specific geometrical deformation of the developing explant by angular rotation of presumptive cortex.