The olfactory epithelium of mammals is simple in appearance when fixed and stained with conventional techniques and viewed with the light microscope. In it, there are three types of cells, the olfactory receptors, the supporting, and the basal cells, all of which rest upon a thin basement membrane, demonstrable only with difficulty (1-5). In spite of its apparently simple organization and single type of receptor element, the olfactory epithelium is capable of remarkably fine sensory discriminations. Olfactory receptors respond to stimulating substances in extraordinarily small concentration. Moreover, the receptors are sensitive to an enormous range of odoriferous substances.Electrophysiological investigations (6-9) have led several investigators to suggest recently that discrimination depends upon the existence of different types of olfactory receptors with selective sensitivities to basic odors, and although the spatial distribution of receptors may account for coarse discriminations of odors, fine differentiation seems to depend upon specific sensitivity of individual receptors to particular molecular configurations (9).These findings pose the question of whether or not the olfactory receptors exhibit morphological differences corresponding to their functional heterogeneity. By way of answering this, the studies of Le Gros Clark (10-12) have demonstrated several histological differences among receptor cells. For example, the receptors vary in their affinity for silver stains, in the length and diameter of the rod processes, in the size of their terminal swellings, and in the number of olfactory hairs. They also differ markedly in their degree of degeneration after destruction of the olfactory bulb (12).It must be admitted, however, that the limited resolution of the light microscope does not permit a sufficiently detailed analysis of the olfactory receptors and their relationships to the other cells of the olfactory epithelium. For example, the receptor processes are difficult to demonstrate reliably even in the best histological preparations. In addition, many of the central processes of the * This investigation has been aided in part by Grant B-425 and RG 3784 from the United States
An examination of the fine structure of the taste buds in the rabbit was undertaken. Gustatory epithelium was fixed in OsO4 or 1 per cent KMnO~ solution, containing polyvinylpyrrolidone (PVP). Thick sections were examined in the phase microscope and contiguous sections prepared for the dectron microscope.The bud contains two types of cells, gustatory receptors and sustentacular cells. The receptors are characterized by a dark nucleus and densely granular cytoplasm. The apical processes bear numerous microvilli which extend into the taste pore. Imbedded between the microvilli there is a dense substance, which is also present in the apical cytoplasm of the receptors. The sustentacular cells contain a large pale nucleus and less dense cytoplasm. Their basal surfaces rest upon a basement membrane.The subepithelial nerve plexuses comprise the fibers which innervate the gustatory receptors. The nerve fibers vary in diameter from 500 A to 0.3 g, and are ensheathed by Schwaun cells. The intragemmal fibers enter the taste bud between adjacent cells, and are ensheathed by the plasma membranes of the supporting cell until they synapse upon the gustatory cell. The synaptic terminaLs contain synaptic vesicles, which at this junction reside in the postsynaptic element. This observation is discussed with reference to synapses described elsewhere in the nervous system.The problem of the relation of structure to function in the nervous system is one that has occupied neurologists for many years. In recent decades, morphological investigations have failed to keep pace with physiological and chemical advances, primarily because of the limited reliability of silver impregnation and methylene blue staining methods and the limited resolving power of the light microscope. This has been especially true in the general area of sensory receptors, where, morphologically, the neural basis of sensory discrimination is not much better understood today than it was fifty years ago. However, it is now possible with the electron microscope to examine more intimately the relation between structure and function and some progress has been made (22). * This investigation has been aided in part by Grants B-425 and RG 3784 from the United States Public Health Service, Nationai Institutes of Health.A number of primary receptors, e.g., the retina (7, 29-31), the inner ear (10, 32, 34), the Pacinian corpuscle (23), and the olfactory mucosa (6) have already been studied with the electron microscope. The taste buds offer an additional region for study, since the mechanism of taste bud excitation (chemoreception) remains obscure, despite the fact that a good amount of physiological (3,4, 24) and histochemical data (2) is available. Of special interest to the morphologist is the fact that the taste buds undergo degeneration following interruption of their nerve supplies (17, 19, 33). Thus they offer a particularly useful region for correlating structure and function and for observing degeneration.Since the early descriptions of Loven (15), Schwalbe (...
Ciliary ganglia of chick embryos and newly hatched chicks were examined in the light and electron microscopes. Particular attention was given to the fine structure of calyciform synapses, which are characteristically found in ciliary ganglia of birds. The calyciform endings are characterized by large expansions of the presynaptic axons upon ganglion cells, and the terminal processes extend over a considerable area of the cell surface. Often, indeed they appear to envelop the cell.In the electron microscope image, the appositional membranes are separated by a space about 300 to 400 A wide; i.e., the synaptic cleft. At irregularly spaced regions, the appositional membranes show areas of increased density. The presynaptic processes contain clusters of synaptic vesicles, localized at these dense regions. Thus the fine structure complex typical of other synapses is evident. The unique structural features of this synapse are as follows: (a) The calyx or presynaptic terminal derives from a single axon, does not arborize, and terminates upon a single ganglion cell. Thus, unlike the classical bouton terminal, this represents an anatomical device for firing single cells by single axons. (b) The surface area in contiguity, i.e., the area of appositional membranes, is far more extensive than the bouton terminal.
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