ABSTRACT
During cerebellar development, cerebellar nuclei (CN) neurons and Purkinje cells are the earliest born among the different neuronal subtypes. Purkinje cells are the sole output of the cerebellar cortex and project to the CN. The CN represents the main output of the cerebellum, which is generated from the rhombic lip and the ventricular zone. We used immunohistochemistry, embryonic cultures, dye tracers and in situ hybridization to examine the origin of a new subset of CN neurons from the mesencephalon during early cerebellar development. Our results show that a subset of CN neurons, which are immunopositive for α-synuclein (SNCA) and Otx2, originate from the mesencephalon and cross the isthmus toward the rostral end of the nuclear transitory zone. Double immunostaining of the SNCA with Otx2 or p75 neurotrophin receptor (p75ntr) indicates that these cells are derived from neural crest cells. We also showed that this population of neurons with nerve fibers terminates at the subpial surface of putative lobules VI/VII. The SNCA+/Otx2+/p75+ cells, which divide the cerebellar primordium into rosterodorsal and caudoventral compartments, show increased cleaved caspase-3 activation, which suggests temporary presence of these cells due to apoptosis. These results strongly suggest that early CN neurons originate from the mesencephalic neural crest population and cross the isthmus to contribute as a subset of the CN. Their temporary presence in the nuclear transitory zone suggests that these neurons/fibers play a regulatory role as a signaling center to attract early afferent pioneer axons and provide neuronal migratory pathway during early cerebellar development.
Significance Statement During cerebellar development two germinal zones are involved in cerebellar neurogenesis: the rhombic lip and the ventricular zone, which are located in the developing cerebellum itself. Our findings indicate that a subset of cerebellar nuclei neurons have an external origin, the mesencephalon, and they are the earliest born neurons that enter to the developing cerebellum. In this study, we focused on the origin of these cells and traced their migratory pathway from the mesencephalon while crossing the isthmus, followed them when they entered to the developing cerebellum. We also demonstrated their potential role on later born cells during cerebellar development.
