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.