Topographic organization in the cerebellar nuclei and inferior olive in relation to cerebellar hemispheric lobules in the mouse: Distinction between crus I and neighboring lobules.

The parallel closed-loop topographic connections between subareas of the inferior olive (IO), cerebellar cortex, and cerebellar nuclei (CN) define the fundamental modular organization of the cerebellum. The cortical modules or zones are organized into longitudinal zebrin stripes which are extended across transverse cerebellar lobules. However, how cerebellar lobules, which are related to the cerebellar functional localization, are incorporated into the olivo-cortico-nuclear topographic organization has not been fully clarified. In the present study, we analyzed the lobular topography in the CN and IO by making 57 small bidirectional tracer injections in the lateral zebrin-positive stripes equivalent with C2, D1, and D2 zones in every hemispheric lobule in zebrin stripe-visualized mice. C2, D1, and D2 zones were connected to the lateral part of the posterior interpositus nucleus (lPIN), and caudal and rostral parts of the lateral nucleus (cLN, rLN), respectively, and from the rostral part of the medial accessory olive (rMAO), and ventral and dorsal lamellas of the PO (vPO, dPO), respectively, as reported. Within these areas, crus I was specifically connected to the ventral parts of the lPIN, cLN, and rLN, and from the rostrolateral part of the rMAO and the lateral parts of the vPO and dPO. The results indicated that the cerebellar modules have lobule-related subdivisions and that crus I is topographically distinct from other lobules. We speculate that crus I and crus I-connected subdivisions in the CN and IO are involved more in nonmotor functions than other neighboring areas in the mouse.

Divergent projections of single pontocerebellar axons to multiple cerebellar lobules in the mouse.

The basilar pontine nucleus (PN) is the key relay point for the cerebrocerebellar link. However, the projection pattern of pontocerebellar mossy fiber axons, which is essential in determining the functional organization of the cerebellar cortex, has not been fully clarified. We reconstructed the entire trajectory of 25 single pontocerebellar mossy fiber axons labeled by localized injection of biotinylated dextran amine into various locations in the PN and mapped all their terminals in an unfolded scheme of the cerebellum in 10 mice. The majority of axons (20/25 axons) entered the cerebellum through the middle cerebellar peduncle contralateral to the origin, while others entered through the ipsilateral pathway. A small number of axons (1/25 axons) had collaterals terminating in the cerebellar nuclei. Axons projected mostly to a combination of lobules, often bilaterally, and terminated in multiple zebrin (aldolase C) stripes, more frequently in zebrin-positive stripes (83.9%) than in zebrin-negative stripes, with 66.5 mossy fiber terminals on the average. Axons originating from the rostral (plus medial and lateral), central and caudal PN mainly terminated in the paraflocculus, crus I and lobule VIb-c, in the simplex lobule, crus II and paramedian lobule, and in lobules II-VIa, VIII and copula pyramidis, respectively. The results suggest that the interlobular branching pattern of pontocerebellar axons determines the group of cerebellar lobules that are involved in a related functional localization of the cerebellum. In the hemisphere, crus I may be functionally distinct from neighboring lobules (simple lobule and crus II) in the mouse cerebellum based on the pontocerebellar axonal projection pattern.

Single axonal morphology and termination to cerebellar aldolase C stripes characterize distinct spinocerebellar projection systems originating from the thoracic spinal cord in the mouse.

The spinocerebellar projection has an essential role in sensorimotor coordination of limbs and the trunk. Multiple groups of spinocerebellar projections have been identified in retrograde labeling studies. In this study, we aimed at characterizing projection patterns of these groups using a combination of anterograde labeling of the thoracic spinal cord and aldolase C immunostaining of longitudinal stripes of the cerebellar cortex in the mouse. We reconstructed 22 single spinocerebellar axons, wholly in the cerebellum and brain stem and partly, in the spinal cord. They were classified into three groups, (a) non-crossed axons of Clarke's column neurons (NCC, 8 axons), (b) non-crossed axons of marginal Clarke's column neurons (NMCC, 7 axons), and (c) crossed axons of neurons in the medial ventral horn (CMVH, 7 axons), based on previous retrograde labeling studies. While NCC axons projected mainly to multiple bilateral stripes in vermal lobules II-IV and VIII-IX, and the ipsilateral medial cerebellar nucleus, NMCC axons projected mainly to ipsilateral stripes in paravermal lobules II-V and copula pyramidis, and the anterior interposed nucleus. CMVH axons projected bilaterally to multiple stripes in lobules II-V with a small number of terminals but had abundant collaterals in the spinal cord and medullary reticular nuclei as well as in the vestibular and cerebellar nuclei. The results indicate that, while CMVH axons overlap with propriospinal and spinoreticular projections, NCC and NMCC axons are primarily spinocerebellar axons, which seem to be involved in relatively more proximal and distal sensorimotor controls, respectively.