Cerebellar foliation in rats. 5. Structural relations between Purkinje cells and heterotopic external granular layer in normal and protein deprived foetal rats.

Different types of heterotopia of granule cells and of the external granular layer (EGL) occur naturally in the cerebellum of rats. In the present study sagittal plastic sections of the cerebellum of 75 normal and protein deprived rat foetuses from the four last days of gestation were examined. Perivascular nests of EGL cells were present in all of the rat foetuses examined without obvious differences between normal and protein deprived foetuses. They were most frequent in regions adjacent to the fissures. It was found that the EGL nests affected the orientation and position of Purkinje cells when passing through this layer. In 2 out of 37 protein deprived foetuses, there were continuous layers of Purkinje cells surrounding some EGL nests in the sub-cortical region associated with a malformation of the posterior lobe. These heterotopic Purkinje cells had an apical process mostly pointing towards the EGL nests and there was a primitive molecular layer lying between the heterotopic Purkinje cells and the EGL cells. These findings indicate that migration of Purkinje cells towards the perivascular EGL had occurred possibly as a consequence of transfer of the endfeet of radial glial cells from the pial to the vascular mesodermal surface. Similar mechanisms may be involved in nervous system malformations such as cerebellar polymicrogyria in man. Experimental protein deprivation has not previously been reported to result in major malformations of the central nervous system. Although two protein deprived foetuses had cerebellar malformation which were not seen in normal foetuses, the present material does not allow any conclusions with regard to a causal relation between the protein deprivation and the malformations.