Neue morphologische Grundlagen zur Physiologie des Riechens und Schmeckens

Summary

New results as revealed by scanning and transmission electron microscopy have given us further knowledge about the structure of the olfactory region of vertebrates. With comparative studies we are now able to discuss the functional relationship of this region. In all vertebrates the olfactory cell is a primary sensory cell. The apical segment of the olfactory cell with its olfactory vesicle is involved in the formation of the olfactory border. As a rule the receptor possesses cilia or cilia-like processes. These are absent in the olfactory receptor of the shark, the microvillus receptor of the fish and the olfactory cell of Jabonsons organ of amphibians, reptiles and mammals. The odorous substances in the fish are brought to the receptor membrane by the water flow. In air breathing vertebrates a terminal film is present. This film is a product of secretion from the Bowmans glands. Gasous odorous substances must first be dissolved in the terminal film and penetrate it before reaching the receptor membrane.

The cilia-like olfactory process of the fish in the proximal segment is not essentially different from the kinocilia of the supporting cell, except that they are shorter. In contrast the olfactory cell of air-breathing vertebrates form cilia-like processes with a short cilia-like proximal segment and a long and very thin distal end piece. In the amphibians and sauropsidians the end pieces can have a lenght of up to 150 μ and up to 80 μ in mammals. The olfactory vesicles with its processes undergo continuous regeneration.

The olfactory epithelium of man show the same structural formation as observed in other mammals. Regressive changes in the adult can lead to a reduction in the number of sensory cells and also to a flattening of the epithelium. Morphological criteria for regenerative processes in the sensory cell structures are present.

A specialized olfactory cell type has been found in some teleosts. This cell is characterized by a small pit below the olfactory border in which the cilia of the olfactory cell are redrawn. There is some evidence that this olfactory cell type may be compared with the olfactory cells in the parafollicular tubes of lamprey.

The so called rod-shaped receptor in the olfactory mucosa of fishes has no axon and is therefore no olfactory cell. The same kind of cell is also present in the olfactory mucosa of airbreathing animals. We classify this cell as brush cell.

Comparative electron microscopic studies reveal identical ultrastructural organization of the olfactory bulb in all classes of vertebrates, including cyclostomes and man. The size and structure of synapses in the olfactory bulb are specific for each connection type. The dark endings of the olfactory receptor cells have small axo-dendritic contacts to the bright mitral or tufted cell processes within the glomeruli. Granule cells, periglomerular cells and mitral cells interact by dendro-dendritic, dendro-axonic and somato-dendritic synaptic complexes which often have “reciprocal” arrangements. Presynaptic endings on the granule cell dendrites and somata contain a large number of small synaptic vesicles and have membrane complexes more than 0.5 μm in diameter. In the periventricular or central zone of the olfactory bulb excitatory synapses with interdigitation between the pre- and postsynaptic processes are present. We are able to give schematic representations of postulated nerve circuits with the aid of the different morphological appearances of the different synapses.

The cellular composition of the taste buds of different mammals can be described from electron microscopical studies. As a rule 5 cell types which regenerate through mitosis from the basal or marginal cells can be differentiated. Only the active sensory cell forms synaptic membrane complexes. It sends rod-shaped processes into the taste pores. Growing and dying sensory cells do not possess these processes. The supporting cells surround the single sensory cell. The apical pole of the supporting cell enters the taste pore by a bundle of microvilli. Secretory granules accumulate in the apical part of the supporting cell and empty their contents into the taste pore. The terminal processes of the myelinated afferent nerve fibres form a plexus in the lamina propria and penetrate the taste bud with numerous itraepithelial branchings.