Afferent and efferent connections of the cerebellum of the chondrostean Acipenser baeri: a carbocyanine dye (DiI) tracing study.

The afferent and efferent connections of the cerebellum of the primitive bony fish Acipenser baeri were studied in fixed brains with a fluorescent lipophylic carbocyanine (DiI). The three regions of the cerebellum (the auricles, valvula, and corpus) showed similar afferents, mostly originated from extensive precerebellar populations of the midbrain tegmentum and from the inferior olive. A pretectal nucleus was also labeled after DiI application to the three regions of the cerebellum. However, DiI application to the pretectal region revealed that the pretectocerebellar projection mainly targeted to the caudal region of the corpus cerebelli. Some precerebellar cells were observed in the torus semicircularis, isthmic central gray, and rhombencephalic reticular formation. Primary fibers of the anterior lateral line nerve and neurons of the octavolateral area also project to the auricle. After DiI application to the auricles, most ascending efferents coursed to the region of the nucleus of the medial longitudinal fascicle and thalamus, mostly contralaterally. Ipsilateral descending fibers were also labeled in the medullary octavolateral area. Application of DiI to the nucleus of the medial longitudinal fascicle revealed three clusters of cerebellar projection neurons located in the granular layers of the auricles, valvula, and corpus cerebelli, mostly contralateral to the application site. These cerebellar projection neurons did not exhibit a number of characteristics of teleost eurydendroid cells (i.e., the cerebellar efferent cells of teleosts), such as the presence of spiny dendrites ascending to the molecular layer. Comparison of the afferent and efferent projections of the sturgeon cerebellum with those reported in teleosts supports the hypothesis that some traits observed in the teleost cerebellar system represent recent evolutionary developments. Copyright 2003 Wiley-Liss, Inc.