Age-related Alterations of the Proximal Axon Segment in Lamina IIIab-pyramidal Cells of the Human Isocortex. A Golgi and Fine Structural Study

In the human isocortex preferably the pyramidal cells of layer IIIab are subjected to a conspicuous age-related alteration in that they develop spindle-shaped enlargements of their proximal axon segment. In Golgi impregnations such IIIab-pyramidal cells with an axon dilatation do not display obvious alterations in the ramification or spine density of their dendrites. Electron microscopically different degrees of this alteration can be recognized; beginning with a cone-shaped and slight dilatation and culminating in a giant spindle-shaped enlargement of the proximal axon segment. The fasciculated microtubules normally characterizing the initial segment are pushed aside or are lacking. Lipofuscin granules, RER cisterns and ribosome rosettes penetrate into the dilatation. The axolemma of those parts of the axon segment in the vicinity of which lipofuscin granules can be encountered lacks a dense axolemmal undercoating. As concerns the adjacent profiles of the dilatation there are no obvious alterations in comparison to a normal proximal axon segment. A slender process arises distally from the dilatation showing the morphological characteristics of the axon initial segment.