Development of the anatomical alteration of the cerebellar fissura prima.

The development of the naturally occurring malformation of the cerebellar fissura prima was monitored in rats starting from 4 days of life to the adulthood. The first sign of the malformation was evident at 10 days of life and consisted of an interruption of the pia mater and the fusion of the external granular layers on the two sides of the fissura. Later, nests of apparently mature granule cells could be seen to be encircled by cells of the external granular layer and to be connected to the granule cell layer by thin bridges of cells. Calretinin immunoreactive fibers followed the bridges of cells to reach the ectopic masses of cells. Towards the end of histogenesis and in adult animals, brush cells and Golgi cells were present in the ectopic masses of granule cells. The latter appeared to contribute to the formation of normal glomeruli, as in the orthotopic granule cell layer. In addition, bundles of parallel fibers crossed the boundary between the molecular layers on the two side of the fissure, thus suggesting that parallel fibers can contact Purkinje cells of the opposite folium. Copyright 2000 Wiley-Liss, Inc.