Compartmentalization of the chick cerebellar cortex based on the link between the striped expression pattern of aldolase C and the topographic olivocerebellar projection

The avian cerebellum is organized into multiple longitudinal stripes defined by expression profiles of aldolase C (zebrin II) in Purkinje cells. The relationship between the aldolase C striped pattern and the olivocerebellar projection pattern is crucial in understanding cerebellar functional compartmentalization. We identified all aldolase C stripes across all lobules with the serial section alignment analysis method and then looked at this relationship by anterograde and retrograde labeling of olivocerebellar axons in the chick cerebellum. Aldolase C stripes were generally consistent and continuous from lobule I through VII and to the medial part of lobules VIII-IXb. The dorsal and ventral lamellas (DL, VL) of the inferior olive projected to the stripes in these areas with a simple mediolateral topographic relation. A few aldolase C stripes appeared at the lateral edge of lobules VI-VIII. Several more stripes were added in the lateral parts of lobules IXa-IXb and IXc-X. The medial column (MC) of the inferior olive projected to the stripes in lobules VIII-X, including the added lateral stripes, with a complex topographic relation. Sharp boundaries between aldolase C-positive and -negative stripes often accompanied a gap in the Purkinje cell layer and bordered topographically distinct groups of axons. Although the compartmental organization of the chick cerebellum is comparable to that of the mammalian cerebellum, several significant differences in the organization suggest partly separate evolutionary lineages of the mammalian and avian cerebella. We propose that rostral lobules may be evolved by rostral extension of medial stripes from caudal lobules in the avian cerebellum.