[High-voltage electron microscopic observations on mouse olfactory bulb neurons]

The fine structures of Golgi-impregnated mitral, tufted and granule cells in the mouse olfactory bulb were investigated by a high-voltage electron microscope. Observation was focused exclusively on intraglomerular dendritic trees of mitral and tufted cells as well as granule cell spines. These structures could be precisely observed under high-voltage electron microscopy in contrast to the vague observation available by light microscopy. Intraglomerular dendritic tufts of mitral and tufted cells possess a similar structure. Their most characteristic feature is varicose branches, which are morphologically classified into two parts, relatively thick parts and thinner parts. The latter consist of bead-like varicosities and thin segments less than 0.35 microns, combination of which take on a thread and beads appearance. Another common structure is small, spine-like appendages which can be seen on all parts of the tufts. Granule cell spines can be individually observed with the aid of stereoscopy, even where they are closely clustered. As a result, many more spines could be recognized by high-voltage electron microscopy than by light microscopy. The width of terminal swellings of spines in the granule cell layer are, in most spines, shorter than 0.4 microns. On the other hand, in the mitral body and external plexiform layer, sizes of spines are much more variable; larger ones with a width of between 1.0 and 1.5 microns as well as smaller ones between 0.4 and 0.5 microns are usually found. Consequently, these findings described above proved to be useful for attaining a deeper understanding of both light and thin section electron microscopic findings of these structures.