Morphological relations between the receptor neurons, sustentacular cells and Schwann cells in the olfactory mucosa of the salamander.

The morphological characteristics of olfactory receptor neurons and their relations to the sustentacular cells and to the sheath cells of Schwann in the olfactory mucosa of the salamander (Ambystoma maculatum et tigrinum) were studied using a modification of the rapid Golgi technique. The bipolar receptor neurons had a fusiform-shaped cell body whose apical pole gave rise to a surface-reaching dendrite and a basal pole which gave rise to an axon. The length and width of the dendrite, although variable, were positively correlated with the relative depth at which the cell body was located in the sensory epithelium. Beyond the initial segment, the axon had a sinuous course prior to its entrance into the lamina propria. Within the lamina propria, the axons were associated with the sheath cells of Schwann to form the olfactory nerve fascicles. The processes of adjacent sheath cells formed an elaborate network of continuous cavities through which the axons coursed. Two morphologically distinct varieties of sustentacular cells, designated types I and II, were found in the sensory epithelium. Both types had a columnar profile consisting of an elongated cell body, a central stalk, and a basilar expansion of the stalk found at the junction of the epithelium with the lamina propria. The central stalk of type I sustentacular cells was unbranched, whereas that of type II cells gave rise to riblike processes from which cytoplasmic veils extended to envelop the cell bodies of receptor neurons. The basilar expansions were found in close apposition to the wall of capillaries or to acinar cells of Bowman's glands located in the most superficial region of the lamina propria. The morphological relationships and possible interdependencies among receptor neurons, the types of sustentacular cells, and the sheath cells are discussed.