Development of the anatomical alteration of the cerebellar fissura prima

The development of the naturally occurring malformation of the cerebellar f issura prima was monitored in rats starting from 4 days of life to the adul thood. The first sign of the malformation was evident at 10 days of life an d consisted of an interruption of the pia mater and the fusion of the exter nal granular layers on the two sides of the fissura. Later, nests of appare ntly mature granule cells could be seen to be encircled by cells of the ext ernal 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 ecto pic masses of granule cells. The latter appeared to contribute to the forma tion of normal glomeruli, as in the orthotopic granule cell layer. In addit ion, 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. Anat Rec 259:150-156, 2 000. (C) 2000 Wiley-Liss, Inc.