FINE-STRUCTURE OF THE MEDULLARY LATERAL-LINE AREA OF CHELON LABROSUS (ORDER PERCIFORMES), A NONELECTRORECEPTIVE TELEOST

The ultrastructure and synaptic organization of the nucleus medialis a nd cerebellar crest of the teleost Chelon labrosus have been investiga ted. The nucleus medialis receives projections from the anterior and p osterior lateral line nerves. This nucleus consists of oval neurons an d large crest cells (''Purkinje-like'' cells) whose apical dendrites b ranch in the overlying molecular layer, the cerebellar crest. In the d orsal region of the nucleus medialis, the perikarya and smooth primary dendrites of the crest cells are interspersed among myelinated fibers and nerve boutons. The ventral layer of the nucleus medialis contains crest cell perikarya and dendrites as well as oval neurons. The cereb ellar crest lacks neuronal bodies, but the apical dendrites of crest c ells receive synapses from unmyelinated and myelinated fibers. In the cerebellar crest, two types of terminals are presynaptic to the crest cell dendrites: boutons with spherical vesicles that form asymmetric s ynapses with dendritic spines and boutons containing pleomorphic vesic les that form symmetric synapses directly on the dendritic shaft. Most axon terminals found on the somata and primary dendrites of crest cel ls in the nucleus medialis have pleomorphic vesicles and form symmetri c contacts, though asymmetric synapses with spherical vesicles and mix ed synapses can be observed; these mixed synapses exhibit gap junction s and contain spherical vesicles. Unlike crest cells, the oval neuron perikarya receive three types of contacts (symmetric, asymmetric, and mixed). The origins and functions of these different bouton types in t he nucleus medialis are discussed. (C) 1995 Wiley-Liss, Inc.