Formation of excess sublobules in the cerebellum of hypothyroid rats

Cerebellar folia may increase in number in hypothyroid rats (Lauder et al., 1974, Hasebe et al., 2008a). In this study, we aimed to confirm the formation of an excess sublobule and to determine whether excess sublobules are consistently formed in conserved positions in hypothyroid rats. Instead of the foliation pattern partitioned by cerebellar fissures, we employed the bifurcation pattern of the internal granular layer for investigation of complexity of the cerebellar cortex in hypothyroid rats. The basic foliation pattern of the cerebellum was intact in hypothyroid rats, but lobules III to IX frequently showed an increase in the number of sublobules. The excess sublobules were mainly found in the folia and along the shallow region of the fissure. In other words, the excess sublobules were not located in random locations but rather in specific locations. The area in the internal granular layer of lobules V to IX was significantly larger than that in control rats. From the increased area of the internal granular layer it may be inferred that internal granular cells increase in number than those in normal rats. In our study, regions within the cerebellum that show an excess of sublobules correlate with regions that show an intermediate to late-forming internal granular layer (Altman, 1969). Our observations fit with the view that excess sublobules are formed by the external granular layer showing prolonged cell proliferation and hypothyroidism predominantly has an adverse impact on the intermediate to late phases in development of the internal granular layer.