Axons from the trigeminal ganglia are the earliest afferent projections to the mouse cerebellum

The first stage standard model for the development of afferent pathways to the cerebellum is that ingrowing axons target the embryonic Purkinje cells (E13 to E16 in mice). Perinatally and early postnatal (E18 to P15) the climbing fibers translocate to the Purkinje cell dendrites, and as the granular layer develops the mossy fibers translocate from the Purkinje cell somata and synapse with granule cell dendrites. In this report we describe a novel earlier stage in the development. Immunostaining for a neurofilament-associated antigen (NAA) reveals the early axon distributions with remarkable clarity. Axons from the trigeminal system enter the cerebellar primordium as early as embryo age (E9). By using a combination of axon tract tracing, analysis of neurogenin1 null mice, which do not develop trigeminal ganglia, and mouse embryos maintained in vitro, we show that the first axons to innervate the cerebellar primordium are direct projections from the trigeminal ganglia. The data show that the early trigeminal projections are in situ before the Purkinje cells are born, and double immunostaining for NAA and markers of the different domains in the cerebellar primordium reveal that they first target the cerebellar nuclear neurons of the nuclear transitory zone (E9 and E10), and only later (E10 and E11) extend collateral branches to the Purkinje cell plate.