Histological Study of Olfactory Bulb in Man

Abstract

The olfactory bulb in man is divisible into the layer of olactory fibres, the glomerular layer, the gray matter layer and the layer of granule cells and nerve fibres, counting upward from the inferior side. The layer of olfactory fibres is composed of many small bundles of olfactory fibres running irregularly. The olfactory fibres are not independently running fibres as are the other sensory fibres, but are represented by a peculiar mesh formation of weakly stainable, minute neurofibrils of various sizes. However, a small number of strongly stainable, independently running, minute fibres is also found among the above. There are also numerous glial cells in the olfactory bundles. The glomeruli in the glomerular layer are represented by the regular mesh formation of neuro-fibrils finer than those in the olfactory bundles and go over into the small bundles of olfactory nerve, only rarely containing glial cells. The gray matter layer is composed of minute weakly stainable neurofibril nets perhaps of vegetative nature, containing a large number of filial cells as well as scattered weakly stainable nerve cells. The nerve cells in this layer are classifiable into those corresponding to the so-called mitral cells and vegetative nerve cell. However, as the former hardly appear as rounded, mitre or octopus-shaped cells, I wish to propose the name of bulbus cells for them. These are shaped like elongated spindle or triangle, or are stellate, are multipolar with 3-6 nerve processes, having a rather small cell nucleus respectively, and their cell bodies contain weakly stainable neuro-fibril nets. Their processes, both the dendrites and the axons, are made of stout fibres, the former, sometimes after sending out 2-3 rami, lose in size as they run peripherally and finally run into the above mentioned glomeruli, where they play a part in the formation of the latter. This last observation has cost me some overwhelming difficulties. The afferent axons finally become strongly stainable conspicuous fibres and merge into the layer of nerve fibres. Vegetative nerve cells are found not only in this layer but also in the vegetative nucleus described below in large number. These are rounded, spindle-shaped or conical cells, as those.found in the brain and the spinal cord, contain weakly stainable delicate neuro-fibril nets and send out 2-4 nerve processes. It must be also mentioned that at they contain a large cell nucleus in the midst of their cell body. The layer of granule cells and nerve fibres is composed of numerous bundles of nerve fibres derived from the axons of the bulbus cells running parallel to the long axis of the olfactory bulb, and many round cells arranged alternatively with these fibre bundles. The granule cells are not multipolar nerve cells as accepted hitherto, but are glial cells as those found in the granular layer of the cerebellum (Yamamoto). Cajal's so-called pyramidal cells are not sensory in nature, but actually correspond to the vegetative cells in the spindle-shaped vegetative nucleus stretched at the transitional part between the olfactory bulb and tract. Minute vegetative fibres are seen entering and leaving both the ends of this nucleus. In short, the dendrites from the bulbus cells go out to play a part in forming the glomeruli, the latter go over into the olfactory fibres, which finally reach the epithelium of the olfactory mucosa, while their axons transmit the received stimuli to the highest olfactory center in the brain. consequently, the bulbus cells are interpreted as the olfactory neurons of first order corresponding to the sensory nerve cells in the cerebrospinal ganglia. No other sensory cells related with the bulbus cells are found in the olfactory bulb.