Compartmentalization of the Deep Cerebellar Nuclei Based on Afferent Projections and Aldolase C Expression

The distribution of aldolase C (zebrin II)-positive and -negative Purkinje cells (PCs) can be used to define about 20 longitudinally extended compartments in the cerebellar cortex of the rat, which may correspond to certain aspects of cerebellar functional localization. An equivalent compartmental organization may exist in the deep cerebellar nuclei (DCN). This DCN compartmentalization is primarily represented by the afferent projection pattern in the DCN. PC projections and collateral nuclear projections of olivocerebellar climbing fiber axons have a relatively localized terminal arbor in the DCN. Projections of these axons make a closed olivo-cortico-nuclear circuit to connect a longitudinal stripe-shaped cortical compartment to a small subarea in the DCN, which can be defined as a DCN compartment. The actual DCN compartmentalization, which has been revealed by systematically mapping these projections, is quite different from the cortical compartmentalization. The stripe-shaped alternation of aldolase C-positive and -negative narrow longitudinal compartments in the cerebellar cortex is transformed to the separate clustering of positive and negative compartments in the caudoventral and rostrodorsal DCN, respectively. The distinctive projection of aldolase C-positive and -negative PCs to the caudoventral and rostrodorsal DCN underlies this transformation. Accordingly, the medial cerebellar nucleus is divided into the rostrodorsal aldolase C-negative and caudoventral aldolase C-positive parts. The anterior and posterior interposed nuclei generally correspond to the aldolase C-negative and -positive parts, respectively. DCN compartmentalization is important for understanding functional localization in the DCN since it is speculated that aldolase C-positive and -negative compartments are generally associated with somatosensory and other functions, respectively.