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Solitary chemosensory cells (SCCs), which resemble taste bud cells, are present in the epidermis and oropharynx of most primary aquatic vertebrates. Recent studies have led to the description of SCCs also in mammals too. In the airway and digestive apparatus, these elements form a diffuse chemosensory system. SCCs do not aggregate into groups and in SCCs, as in taste bud cells, immunoreactivity forthe G-protein subunit alpha-gustducin and for other molecules of the chemoreceptive cascade was found. Questions remain about the role of the diffuse chemosensory system in control of complex functions (e.g. airway surface liquid secretion) and about the involvement of chemoreceptors in respiratory diseases. Therapeutic actions targeting chemoreceptors could be tested in the treatment of respiratory diseases.

The expression of alpha-gustducin, a G protein alpha subunit involved in bitter and sweet taste transduction, was investigated in chemosensory tissues of adult mice. By immunohistochemistry, alpha gustducin was absent in the olfactory neuroepithelium. Instead, alpha gustducin was expressed in a subset of bipolar cells in the proliferative zone of the vomeronasal neuroepithelium as well as in taste buds. Northern blot analysis confirmed the presence of alpha gustducin in isolated vomeronasal organs. Moreover, immunohisto- chemistry revealed the expression of alpha gustducin in scattered cells of the nasal respiratory epithelium. These results show for the first time that alpha gustducin is expressed in chemosensory tissue outside the alimentary tract, suggesting that common transduction mechanisms could be shared by apparently unrelated chemosensory tissues.

The present work reviews the main applications of nuclear magnetic resonance (NMR)- technology and, in particular, of magnetic resonance imaging (MRI) to tissutal analysis. To date, MRI represents a precise and reliable tool to investigate morphology and functional modification of tissues in vivo, providing information consistent with histology. MRI has numerous advantages over conventional techniques: it is harmless to tissues; volume measurements in vivo could be useful for morphometric studies; the sarne tissue can be examined severa1 times (e.g. at different ages); several organs can be examined at the same time; serial sections of relevant structures can be obtained in al1 planes, thereby allowing detailed reconstruction of the three-dimensional configuration of organs; motion within a tissue can be detected; and subsequent histological and ultrastructural studies of the tissue are possible. The main drawback (besides the cost of the basic instrumentation) is that resolution is relatively low in comparison with light microscopy. Finally, the analysis of the results is difficult, needing interdisciplinary competence, and MRI methods of tissutal analysis are not yet well standardized. Therefore, in our opinion, MRI is an interesting tool, complementary to other histological techniques, and it cannot be ignored by microscopists. However, in vivo MRI data must be evaluated with caution and histological controls are always required.

The frog's inner ear was studied in vivo by high spatial resolution magnetic resonance imaging at 7 Tesla. The vestibule, the internal acoustic meatus, and the auditory tube have been identified. The large otolithic mass contained in the vestibule showed a virtual absence of magnetic resonance signal probably due to its composition of closely packed otoconia.