Fine structure of the olfactory bulb in the goldfish, Carassius auratus

Ultrastructural studies have aided in identifying the principal cell types and the layers of the olfactory bulb in goldfish. The concentrically arranged four layers are: the primary olfactory nerve layer (most peripheral), the glomerular layer, the mitral cell layer, and the granule cell layer (deepest). Three types of cell bodies were differentiated by their size and cytoplasmic organelles: mitral cells, granule cells and short axon cells. The axons of the primary olfactory nerve formed synaptic contacts with mitral cell dendrites in the glomeruli. Reciprocal synapses were observed between the granule cell dendrites and the mitral cell dendrites in the mitral cell layer. Terminals of the centrifugal fibers forming synaptic contacts with the granule cell dendrites were identified in the granule cell layer. The relationships between the cellular and synaptic organizations and the neural mechanism of olfaction are discussed.