Dendrites of mitral cell neurons contain microtubules of opposite polarity.

Axonal extensions of neurons show microtubules (MTs) with a uniform polarity indicating that they originate at or near the perikaryon. To obtain information on the polarity of dendritic MTs, a polarity assay was used to examine MTs in dendrites of interneurons (mitral cells) of the olfactory pathway of the bullfrog. The assay involved incubating pieces of olfactory lobe in a tubulin-containing detergent medium which stabilized pre-existing MTs and provided for assembly of exogenous tubulin onto their surfaces as ribbon structures, which appear as 'hooks' on the MTs when seen in cross-section. The orientation of the ribbons in a clockwise or anticlockwise direction reflects the polarity of the wall lattice of the MT with which they are associated. Mitral cell dendrites show MTs with ribbons oriented in both directions, indicating two populations of MTs with opposite polarity. The two populations are seen in both the proximal and distal regions of dendrites, and the perikaryon, as expected, shows a mixed population of MTs as regards polarity. It is hypothesized that this is true for most neurons, and that dendritic MTs arise from organizing centers in the dendritic process. If this hypothesis is correct, the polarity of dendritic MTs would depend on the orientation of these centers, which remain to be identified.