Electron Microscopic Observations of the Olfactory Mucosa and Olfactory Nerve

Abstract

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

Response of olfactory ensheathing cells to the degeneration and regeneration of the peripheral olfactory system and the involvement of the neuregulins

“…This was based on their morphology and position. For example, OECs align alongside the axon and are the only glial cells present in the peripheral olfactory fascicles (DeLorenzo, 1957;Frisch 1967). Within the ONL, the OEC surrounds bundles of ORN axons, whereas astrocytes are confined generally to the interfascicular spaces along the medial, lateral, and dorsal surfaces of the olfactory bulb (Doucette, 1991).…”

Development of olfactory nerve glia defined by a monoclonal antibody specific for schwann cells

“…Instead, each olfactory nerve glial cell ensheaths bundles of many small-diam-0 1992 WILEY-LISS. INC eter (0.2 pm) olfactory nerve axons (Gasser, 1956;De Lorenzo, 1957;Cuschieri and Bannister, 1975;Kreutzberg and Gross, 1977;Farbman and Squinto, 1985;Daston et al, 1990;Burd, 1991). The close contact between axons resulting from this pattern of ensheathment is not observed in mature peripheral nerves containing either myelinating or nonmyelinating Schwann cells (Webster et al, 1973).…”

Primary olfactory axons form ectopic glomeruli in mice lacking p75NTR

“…Unlike Schwann cells, olfactory ensheathing cells arise from the olfactory placode rather than the neural crest (Chuah and Au, 1991). These cells ensheath large bundles of axons, similar to astrocytes in central nervous system tracts (DeLorenzo, 1957), and are believed to facilitate axon growth during development and regeneration (Ramón-Cueto and Valverde, 1995).…”