Age-related changes in the number of myelinated axons and glial cells in the anterior and posterior limbs of the mouse anterior commissure.

The total number of axons and the number of myelinated axons was estimated in both limbs of the anterior commissure of mouse brains from 5 to 31 months of age. The total number of glial cells and the percentage of each type of glial cell was estimated in both limbs of the anterior commissure at 25, 28 and 31 months since similar estimations had already been carried out in mice aged between 5 and 22 months. The total number of axons and the number of myelinated axons both appeared to fall between 9 and 12 months and to increase again between 22 and 25 months in the anterior limb whereas in the posterior limb the total number of axons remained constant but the number of myelinated axons increased between 22 and 28 months. Two types of abnormality were seen in myelinated fibres at all ages after 12 months. These consisted of degenerating axons enclosed in normal myelin sheaths and apparently normal axons surrounded by vacuolated sheaths. The total number of glial cells in both limbs decreased between 9 and 12 months and increased substantially between 22 and 25 months. There was a statistically significant correlation between the number of oligodendrocytes and the percentage of myelinated axons in both limbs at different ages. There was no change in the very small number of mitotic figures in the anterior commissure to account for the fluctuation in glial number. It is postulated that there is a continuous loss and replacement of myelinated axons in both limbs of the commissure from 12 to 31 months of age and that this is possible since the amount of axon loss is so small that there is no significant phagocytic or astrocytic response. An increased requirement for myelination appears capable of bringing about an increase in the number of oligodendrocytes even in aged animals.