Spatial organization of axonal microtubules.

Abstract

Several workers have found that axonal microtubules [MT] have a uniform polarity orientation. It is the "+" end of the polymer that is distal to the cell body. The experiments reported here investigate whether this high degree of organization can be accounted for on the basis of structures or mechanisms within the axon. Substantial depolymerization of axonal microtubules was observed in isolated, postganglionic sympathetic nerve fibers of the cat subjected to cold treatment; generally < 10% of the original number of microtubules/.mu.m2 remained in cross section. The number of cold stable MT that remained was not correlated with axonal area and they were also found within Schwann cells. MT were allowed to repolymerize and the polarity orientation of the reassembled MT was determined. In fibers from 4 cats, a majority of reassembled MT returned with the original polarity orientation. In no case was the polarity orientation as uniform as the original organization. The degree to which the original orientation returned in a fiber was correlated with the number of cold-stable microtubules in the fiber. Stable MT fragments serve as nucleating elements for MT assembly and play a role in the spatial organization of neuronal MT. The extremely rapid reassembly of MT that was observed, returning to near control levels within the first 5 min, supports microtubule elongation from a nucleus. In 3 of 4 fibers examined this initial assembly was followed by an equally rapid, but transient decline in MT number to a value that was significantly different than the initial peak. This observation is difficult to interpret; however, a similar transient peak was reported upon repolymerization of spindle microtubules after pressure induced depolymerization.