Ballooning of myelin sheaths in normally aged macaques.

In aged animal brains, a variety of "holes" are formed in the neuropil. One type of hole, here designated as the myelin balloon, is an abnormality of the myelin sheath and is found in a number of diverse sites in the brain. Profiles of myelin balloons display rather smoothly rounded peripheral contours and typically range up to 10 microm in diameter, although exceptionally large examples may be twice this size. The balloons are bounded by lamellae of myelin, and to accommodate the contents of the balloon, the myelin sheath becomes split at the intraperiod line. Since the intraperiod line is formed by the apposition of the outer faces of the myelin-forming plasma membrane, the contents of the myelin balloons are, in effect, in continuity with the extracellular space, and it is suggested that the contents of the balloons are fluid, with the fluid exerting an outward pressure on the walls of the balloons to produce their spherical shapes. Myelin balloons are not only produced during aging but also occur in a number of genetic strains of mice and in a number of human disease states. They thus represent a non-specific, though distinctive and common, alteration of the myelin sheath and are a reflection of the fact that under a variety of conditions, including normal aging, oligodendrocytes are unable to maintain the integrity of their sheaths.