Ruffed cells in the olfactory bulb of freshwater teleosts. II. A Golgi/EM study of the ruff.

The morphological characteristics of the synaptic contacts in the ruff of the cichlid fish Hemichromis bimaculatus were studies using the combined Golgi-electron microscope technique. Ruffed cells were located in the glomerular and plexiform layers and exhibited a pyriform or round cell body and numerous thin dendritic branches that were highly ramified. Four different segments could be distinguished on the initial portion of the axon (IP) according to the number and density of protrusions. These protrusions or lateral appendages are highly interdigitated, forming a characteristic synaptic field: the ruff. The ruff displayed a very high number of synapses with terminals showing a varied morphology. Protrusions of the ruff were both presynaptic and postsynaptic, taking also part in reciprocal pairs of synapses. Synapses from the ruff to the adjacent prolongation are asymmetrical, the prolongation to protrusion synapses being symmetrical. The axonal shaft participates in fewer synaptic contacts. Boutons contacting with one protrusion can synapt with other one, and can also receive an asymmetric synapse from another terminal, forming a serial synapse. This constitutes the most complex synaptic system observed in the glomerular layer of the olfactory bulb in any vertebrate. The synaptology of the ruffed cell IP is compared with previous reports on other species, with the teleostean mitral cells and with the IP of higher vertebrates neurons, the ruffed cells showing a completely different synaptic pattern.