Role of Reelin in cell positioning in the cerebellum and the cerebellum-like structure in zebrafish

The cerebellum and the cerebellum-like structure in the mesencephalic tectum in zebrafish contain multiple cell types, including principal cells (i.e., Purkinje cells and type I neurons) and granule cells, that form neural circuits in which the principal cells receive and integrate inputs from granule cells and other neurons. It is largely unknown how these cells are positioned and how neural circuits form. While Reelin signaling is known to play an important role in cell positioning in the mammalian brain, its role in the formation of other vertebrate brains remains elusive. Here we found that zebrafish with mutations in Reelin or in the Reelin-signaling molecules Vldlr or Dab1a exhibited ectopic Purkinje cells, eurydendroid cells (projection neurons), and Bergmann glial cells in the cerebellum, and ectopic type I neurons in the tectum. The ectopic Purkinje cells and type I neurons received aberrant afferent fibers in these mutants. In wild-type zebrafish, reelin transcripts were detected in the internal granule cell layer, while Reelin protein was localized to the superficial layer of the cerebellum and the tectum. Laser ablation of the granule cell axons perturbed the localization of Reelin, and the mutation of both kif5aa and kif5ba, which encode major kinesin I components in the granule cells, disrupted the elongation of granule cell axons and the Reelin distribution. Our findings suggest that in zebrafish, (1) Reelin is transported from the granule cell soma to the superficial layer by axonal transport; (2) Reelin controls the migration of neurons and glial cells from the ventricular zone; and (3) Purkinje cells and type I neurons attract afferent axons during the formation of the cerebellum and the cerebellum-like structure.

Morphological development of the dorsal hindbrain in an elasmobranch fish (Leucoraja erinacea)

The developmental anatomy of the dorsal hindbrain in an elasmobranch fish, Leucoraja erinacea, is described. We focus on the cerebellum, which is a synapomorphy for gnathostomes. Cerebellar development in L. erinacea, a representative of the most basal gnathostome lineage, may be a proxy for the ancestral state of cerebellar development. We also focus on sensory processing regions termed 'cerebellum-like' structures due to common anatomical and physiological features with the cerebellum. These structures may be considered generatively homologous if they share common developmental features. To test this hypothesis, the morphological development of the cerebellum and cerebellum-like structures must first be described. Of particular importance is the development of common features, such as the molecular layer, which is the defining characteristic of these structures. The molecular layers of the cerebellum and cerebellum-like structures are supplied with parallel fiber axons from distinct granule cell populations. These are the lateral granule mass, the dorsal granular ridge, the medial granule mass, and the granular eminences of the cerebellum. Cerebellar and cerebellar-like development in L. erinacea is similar to development in other elasmobranchs. The temporal order in which these granule cell populations develop suggests an evolutionary history of duplication or expansion of an existing developmental event.