Study of the fifth-type cell in the olfactory epithelium

The olfactory epithelium has been shown to contain fifth-type cells and microvillar cells and olfactory receptor cells along with supporting and basal cells. However, the morphological differences among them are unknown, especially between the former two, and most of their functions are still unclear. In this study, the fifth-type cell and the microvillar cell in the olfactory epithelium of mouse were micromorphologically investigated by SEM and TEM. With the aim of elucidation their functions, the morphological change in these cells after resection of olfactory bulb were studied. In addition, the changes in the olfactory epihelium after HRP injections into the postganglionic fiber for the trigeminal nerve and the olfactory bulb were examined. The TEM observation revealed that the fifth-type cell has finger-like microvilli, which are strong and straight. The microvilli were characterized by a specific core structure consisting of microfilament bundles. On the other band, the microvilli of the microvillar cell were meandering and had no core structure, indicating that the two cells are clearly different types. By SEM observation, neither the fifth-type cell nor the microvillar cell was found in the normal olfactory epithelium. When the olfactory cillial mat disappeared as a result of recection of the olfactory bulb, the fifth-type cell became observable. On the 300th-day after the resection, it was found that the olfactory receptor cells had disappeared and the microvilli of the supporting cells were shortened. Thus, the surface structure of the microvillar cell became clearly observable. Then, the microvilli were found to be shorter than those of the fifth-type cell and were distributed radially. Neither the fifth-type cell nor the microvillar cell was not affected by resection of the olfactory bulb. Most of the olfactory vesicles were HRP positive after the HRP injection into the olfactory bulb; however, HRP was not detected in either the fifth-type cells or the microvillar cells. After the HRP injection into the trigeminal postganglionic fiber, it was detectable in part of the nerves in the lamina propria mucosa and the epithelal basement, but not in the fifth-type cell or microvillar cell. These results suggest that the fifth-type cell is a mechanoreceptor for a system other than the olfactory one and the microvillar cell is a kind of supporting cell in an early stage or final stage near death.