Ruffed cell: A new type of neuron with a distinctive initial unmyelinated portion of the axon in the olfactory bulb of the goldfish (Carassius auratus). I. Golgi impregnation and serial thin sectioning studies

Abstract

A new type of neuron was recognized in the olfactory bulb of the goldfish (Carassius auratus) by means of light and high voltage electron microscopy of Golgi-impregnated material, combined Golgi-electron microscopy, and electron microscopy of serial thin sections. The neuron is located in the layer between the olfactory nerve layer and the anterior olfactory nucleus. It has a spherical cell body, about 10–20 μm in diameter, and several dendrites which form a spherical dendritic field, about 70–100 μm in diameter, in the vicinity of the cell body. The most remarkable structural feature of this neuron is that its initial unmyelinated portion of the axon (IP) has elaborate protrusions with many synapses. The IP can be divided into three parts, parts 1, 2 and 3, based on its structural features. Part 1 is the initial part of the IP, about 20–40 μm in length. Many elaborate protrusions arise from the shaft and intermingle with one another to constitute a spherical field, about 20–40 μm in diameter, around the shaft. Part 2 is the middle part of the IP, about 10–20 μm in length. There are several collaterallike protrusions, which are scattered along the shaft and extend laterally about 5–15 μm. Part 3 is the last part of the IP, and is cylindrical without protrusions. The length of part 3 varies from 20 to more than 100 μm. The axon acquires a myelin sheath at a distance of 70–250 μm from its origin. Protrusions make synaptic contacts mainly with granule cell dendrites. Some of them are of the reciprocal type. Protrusions are presynaptic in asymmetrical synapses, and postsynaptic in symmetrical synapses with granule cell dendrites. The shaft of the IP also has synapses similar to those on protrusions. The neuron described is a new type of neuron in the vertebrate central nervous system. We propose for it the name “ruffed cell”.