Synaptic organization in the teleost olfactory bulb.

Some aspects of the synaptic organization in the olfactory bulb of the goldfish were investigated morphologically by light and high voltage electron microscopy of Golgi-impregnated material, combined Golgi-electron microscopy and electron microscopy of the serial thin sections. In this paper we have analyzed two particular synaptic areas in the glomerular layer (and the superficial part of the plexiform layer), that is, the glomerular area and the nest (N), and five types of neurons participating in these two areas. The glomerular area consists mainly of olfactory nerve terminals (ON), mitral cell dendrites (M), and granule cell dendrites (G), which make a fundamental synaptic pattern in this area; ON leads to M in equilibrium G. In addition to these three elements, there are some other types of neuronal processes in the glomerular area. One is the ruffed cell dendrite (r), which frequently gives rise to glial sheet-like appendages coiling around and covering the mitral cell dendrites. Another is a type of thin cylindrical dendritic processes, which occasionally makes mixed synapses with the mitral cell dendrites, thus named here the dendrites of "the mixed-synapse cell". Another particular synaptic area, the nest, is around the initial unmyelinated portion of the axon (IP) of the ruffed cell, which has many protrusions on its IP. The nest consists mainly on the protrusions of the ruffed cell IP (R), the granule cell dendrites (G) and the perinest cell dendrites (PN). R make synapses with G very similar to those between M and G in the glomerular area. The basic synaptic pattern in the nest is R in equilibrium G leads to PN. The correlation between electron microscopically characterized neuronal processes and light microscopically observed cell types was attempted by serial thin sectioning-reconstruction studies and combined Golgi-electron microscopy studies. A part of the neuronal local circuit in the teleost olfactory bulb was described and compared to that in the mammalian olfactory bulb.