Ultrastructure of the olfactory neuron of the bullfrog: the dendrite and its microtubules.

The ciliated dendritic bulb of the olfactory neuron of the bullfrog was studied with the electron microscope, with emphasis on microtubular elements. Methods used included various fixation procedures with and without detergent extraction, serial sectioning, microtubule polarity assays, and an assay to demonstrate F-actin. Structural continuity exists, via microtubules, between the ciliary membrane and the perikaryon of the neuron. One type of structural link connects the distal end of the basal body to the plasma membrane and, in slightly oblique cross sections of the basal body, the link shows a highly characteristic tripartite profile resembling a claw hammer. The six to ten basal bodies of a dendritic bulb have a lateral foot that serves as an organizing center for microtubules, and these microtubules (totaling about 150) extend toward the perikaryon in the basal half of the epithelium. Polarity assays indicate that the attached or minus ends of dendritic microtubules are in the dendritic bulb, with their plus or fast-growing ends near or within the perikaryon of the neuron. It is shown that dendritic microtubules are depolymerized by direct osmium tetroxide fixation, in contrast to olfactory axonal microtubules, which persist after such fixation. F-actin appears to be abundantly present in the dendritic bulb of the neuron, and it is possible that this actin could play a role in shape changes of the dendrite. The various findings provide new information about the olfactory dendrite, its microtubule organizing centers, and the nature and relationships of its microtubules.