Reorganization of longitudinal compartments in the laterally‐protruding paraflocculus of the postnatal mouse cerebellum

The paraflocculus and the neighboring smaller flocculus form a remarkable protrusion in the ventrolateral aspect of the mouse cerebellum, in which the longitudinal compartments are conspicuously oriented perpendicularly to the sagittal plane. The developmental process of such anatomical arrangements in these lobules has not been fully clarified. Here, we used the genetic tractability of pcdh10-lacZ knock-in (OL-KO), IP 3 R1-nls-lacZ transgenic (1NM13) and Gpr26cre-Ai9-AldocV mice to track the development of compartments and examined local longitudinal orientation of Purkinje cells within the paraflocculus and flocculus. We observed a distinct pcdh10-positive (pcdh10+) compartment in the flocculus, whereas the paraflocculus and other lobules had a continuous paravermal pcdh10+ compartment, in the embryonic OL-KO cerebellum. During the first postnatal week, the parafloccular pcdh10+ compartment shifted laterally to the most lateral edge in the caudal part of the protruding paraflocculus. Although the most medial edge of the parafloccular pcdh10+ compartment remained in the nonprotruding part of the paraflocculus, it was disrupted from the originally continuous pcdh10+ compartment in the copula pyramidis. The local longitudinal orientation changed gradually along with the mediolateral extent of the copula pyramidis, almost becoming perpendicular to the sagittal plane in the laterally connected paraflocculus in the adult cerebellum. This rotational change in orientation was derived from the short U-shaped embryonic cerebellum, in which the surfaces of the flocculus and paraflocculus were oriented laterally. These results indicated that the peculiar compartmental organization of the paraflocculus originates from the embryonic common hemispheric compartmental organization and shaped by the significant reorganization process in the first postnatal week.