Crus I in the Rodent Cerebellum: Its Homology to Crus I and II in the Primate Cerebellum and Its Anatomical Uniqueness Among Neighboring Lobules.

In the human cerebellum, the crus I and crus II lobules (or the ansiform lobule), which are implicated in cognitive and visuomotor functions, are significantly expanded compared to other anterior and posterior lobules, which are involved mainly in somatosensorimotor function. In applying rodent models, it is essential to identify the lobules that are homologous to human crus I and crus II. Observation of the lobular structure in human, macaque, marmoset, rat, and mouse has indicated that human crus I and II are homologous to crus I in rodents (referred to as "ansiform area, AA"). This new lobular definition is supported by lobule-based mapping of the olivocerebellar climbing fiber and Purkinje cell (PC) projection patterns in rodents; crus II and simple lobules are innervated by the mediocaudal part of each inferior olive subnucleus and project to the dorsal part of the cerebellar nuclei, while crus I (or the AA) is innervated by the rostrolateral part of each inferior olive subnucleus and projects to the ventral part of the cerebellar nuclei. Concerning zebrin stripes, the central lobules (lobules VI-VII and AA or crus I in rodents) show a laterally expanded arrangement solely of positive stripes. Our recent analysis has shown that this arrangement of zebrin-positive stripes in the AA originates from their developmental process. Between E14.5 and E17.5, lateral protrusion and shift has been observed in the domains of protocadherin 10-positive PC subsets (which would become zebrin-positive later) in the central area of the immature cerebellum that eventually becomes lobules VI-VII and AA or crus I. These data suggest that the AA (or crus I in rodents) is a unique lobule in the mammalian cerebellum which is characterized by distinct connectivity from neighboring lobules, a massive expansion in skillful primates, and the formation of longitudinal stripes different from that in neighboring anterior and posterior lobules.