A note on the organization of the amphibian olfactory bulb

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Abstract
After horseradish peroxidase (HRP) injections were made in limited sectors of the main olfactory bulb in the adult frog Rana pipiens, the cellular morphology of mitral cells and granule cells impregnated with HRP were examined in uninjected regions of the bulb. Mitral cells were observed to possess glomerular dendrites and prominent secondary dendrites, both of which have smooth shafts. The glomerular dendrites may be multiple, are often branched, and may arise from secondary dendrites, as well as from the cell body. The axon may also arise from a secondary dendrite. Granule cells have simple or branched peripheral dendrites, and these are spiny, where they intermingle with the mitral cell secondary dendrites. The prominence of the secondary dendrites of frog mitral cells contrasts sharply with their reported insignificance in urodeles, as studied in earlier literature. The layers of the main olfactory bulb are not as fully concentric in the frog, as they are in mammals. The implantation cone and glomerular layer occupy a small part of the surface area of the olfactory bulb on its anteroventral aspect, while the perimeters of the subjacent layers extend farther posteriorly and dorsally in successive steps. The granule cell core extends well beyond the perimeter of the mitral cell layer in a posterior direction. Long secondary dendrites of mitral cells also extend posteriorly beyond the perimeter of the mitral cell‐external plexiform layer and interlace with granule cell peripheral dendrites in a plexiform layer external to the posterior region of the granule cell core. This layer, the superficial plexiform layer, forms an apron around the posterior segment of the olfactory bulb and contributes to the interbulbar adhesion. It appears likely that it is an extension to the microcircuitry typically confined within the external plexiform layer in mammalian species.