Summary

The granule cells of the olfactory bulb of the rat have been studied in material prepared by the Golgi-Kopsch method for examination with the light microscope, and in material examined with the electron microscope. With the Golgi method, the granule cells are found to have no process which can be identified as a typical axon, but from the superficial aspect of the somata stout peripheral processes arise and pass into the overlying external plexiform layer, while from the opposite side of the cell body several thinner deep dendntes extend towards the deeper parts of the bulb. Both types of processes, as well as the perikarya, have numerous spine-like appendages. On the distal portions of the peripheral processes in the external plexiform layer the appendages are much larger than on the deeper parts of the cell. The deep dendrites have localized swellings along their length which give them a varicose appearance, appendages often arise from these varicosities. The electron-microscopic features of the granule cells correspond well with the appearance of these cells in material impregnated with the Golgi method. The cell somata are small, with very little cytoplasm, and have a relatively large nucleus. The peripheral processes can be identified passing superficially from the perikarya of the granule cells; at their junction with the cell body their appearance is typically dendritic; all the cytoplasmic organelles found in the cytoplasm extend into these processes and none of the features of the initial segments of axons are found. In the external plexiform layer large spine-like appendages, which have been termed ‘gemmules’, arise from the distal portions of the peripheral processes, and participate in reciprocal synapses with the dendrites of mitral and tufted cells. The deep dendrites are much finer than the peripheral processes, and the varicosities which are seen in Golgi material may also be found with the electron microscope. Spines are found on all parts of the granule cells in the granule cell layer, including the peripheral processes, the perikarya and the deep dendrites. In addition to a spine apparatus, these spines commonly have numerous inclusions, including mitochondria, ribosomes, and vesicles which are the same size and shape as the synaptic vesicles present in the gemmules; no synapses oriented away from the spines have ever been found.