6 A. J. D. DE LORENZO 



photomicrographs of olfactory mucosa stained by the Bodian silver tech- 

 nique. Receptor cells, sustentacular cells and basal cells can be identified. 

 However, the apical tips of the receptors and the fine axons comprising the 

 fila olfactoria cannot be resolved in any detail. Large collections of nerve 

 fibers (N)* are clearly evident, but individual fiber detail is not resolvable. 

 Close scrutiny discloses that the bipolar olfactory receptor possesses a 

 long dendritic process in close contiguity with adjacent supporting cells 

 and a terminal process or olfactory rod extends beyond the " limiting 

 membrane " of the epithelium. Since this portion of the neuron is the only 

 portion in direct contact with the external environment, a knowledge of 

 the structural specializations of the surface membranes might be informa- 

 tive to the sensory physiologist. Likewise, the distribution of nerve fiber 

 diameters and the organization of the fila olfactoria are important con- 

 siderations in any attempt to correlate structure and function. 



Receptor Cells 



In the electron microscope the receptor cells are clearly distinguishable 

 from the sustentacular cells (S) by their paucity of organized endoplasmic 

 reticulum (Figs, 3 and 4). Since the fine structure and organization of the 

 epithelium has been described in detail elsewhere (de Lorenzo, 1956, 1957 

 and 1960) this discussion will be limited mainly to the receptors. Figure 3 

 shows the apical process of a single receptor in contiguity with two adjacent 

 sustentacular cells (S). Membrane thickenings or desmosomes, common 

 to epithelial tissues, are shown by the arrows. Note however, that the 

 membrane thickening includes the plasma membrane of the receptor as 

 well. The receptor dendrite then extends as a naked process (Ro), no 

 longer ensheathed by supporting cells, beyond the " limiting membrane " 

 of the epithelium. This extension has been termed the " olfactory rod ". 

 The olfactory rod is cytologically interesting for several reasons. This 

 process contains 6 to 12 cilia in a sepal-like arrangement around the rod. 

 They are identical in fine structure to cilia seen in other tissues. Their 

 number and arrangement on the rod appear to be randomly distributed. 

 The plasma membrane bounding the rod membrane, consists of two dense 

 lines about 20 A thick separated by a light line about 30 A thick and is 

 continuous with the plasma membrane of the receptor (Figs. 3 and 4). 

 Quite often the membrane forms large invaginations (Fig. 4) and is con- 

 tinuous with a large number of vacuoles (V) which are characteristic of 

 cell membranes actively engaged in pinocytosis. A striking concentration 



* The following abbreviations are used throughout this article : Ca, capillary ; 

 G, glands ; N, nerve fibers ; Mi, microvilli ; S, sustentacular cells ; Ro, Receptor terminal 

 (olfactory rod) ; C, cilia ; V, pinocytosis vacuoles ; Vs, " synaptic vesicles " ; M, mito- 

 chondria ; Nu, nucleus ; B, basement membrane ; Sc, Schwann cells ; Es, extracellular 

 space ; K, mesaxons ; P, perikaryon ; A, axons ; Tb, taste buds ; Sy, synaptic junctions ; 

 Tr, taste receptors ; Ct, connective tissue. 



