ULTRASTRUCTURE AND H I S TO P H Y SI O LOG Y OF MEMBRANES 7 



of mitochondria (M) is seen in some olfactory rods, suggesting a high 

 metaboHc activity in these regions. Likewise, small vesicles of the size 

 of " synaptic vesicles " are usually seen in the cytoplasm of the olfactory 

 dendrite. A few filaments and occasional ribosome particles complete 

 the cytological armamentarium of the olfactory rod. U is worth re- 

 emphasizing that the remainder of the olfactory neuron is ensheathed by 

 sustentacular cells and the axon subsequently by Schwann cells and only 

 the " olfactory rod " is bare or unensheathed. 



The apical processes of the supporting cells exhibit another type of cell 

 membrane specialization. The plasma membrane forms numerous micro- 

 villi (Mi) as demonstrated in Figs. 3 and 4. The cilia deriving from the 

 olfactory rod are always enmeshed in the microvilli suggesting a possible 

 functional relationship. Text-fig. I is a schematic representation of the 

 olfactory epithelium incorporating our findings with the electron micro- 

 scope. Although the olfactory receptors are considered to exhibit a high 

 degree of physiological specificity to different odoriferous substances, our 

 studies reveal no significant fine structural differences in the rabbit. A 

 morphological classification for two types of receptors could, however, 

 be made based on the presence or paucity of mitochondria in the olfactory 

 rods (compare Figs. 3 and 4). Whether the number and orientation of the 

 cilia constitute significant fine structural differences in receptor cells 

 remains to be seen. 



The Fila Olfactoria 



The fine unmyelinated fibers which comprise the fila olfactoria of the 

 rabbit were first described by de Lorenzo in 1956 and simultaneously by 

 Gasser in another species. Detailed studies have subsequently appeared 

 (de Lorenzo 1957 and 1960). The most striking features of these nerve 

 fibers are : (1) their mean diameters which are 0.2// and (2) their structural 

 organization with respect to their investing Schwann cells. Figures 5 and 6 

 will help illustrate these features. In Fig. 5 the basal portion of the 

 olfactory epithelium is shown. A thick basement membrane (B) separates 

 the epithelium from the underlying connective tissue. In the upper right 

 corner of the picture, an epithelial cell whose nucleus is labeled (Nu) is 

 seen resting on the basement membrane (B). In the center of the picture 

 is a group of some twenty small axons (N) collectively emerging from 

 above and becoming embraced by a Schwann cell process (Sc). They 

 are already organized as fascicles consisting of nerve fibers in close con- 

 tiguity and remain so organized throughout their peripheral pathway to 

 the olfactory bulb. Figure 6 represents a section through the fila olfactoria 

 and here the organization of the fascicles is more evident. As has been 

 shown in earlier publications (de Lorenzo, 1956 and 1957): (1) the mean 

 fiber diameter is about 0.2/^. (2) The axons are packed closely together, 



