A quantitative electron microscopic study of myelination in the anterior limb of the anterior commissure of the mouse brain.

An electron microscopic study of myelination was carried out in the anterior limb of the anterior commissure of the mouse brain. The total number of axons increased from 48 700 at 17 days post-conception to 286 500 at 11 days postnatum. The first evidence of myelination was the presence of a few promyelin fibres at 8 days postnatum. Myelinated axons were first found at 11 days postnatum. The most rapid increase in myelinated fibres occurred between 17 and 21 days postnatum, but myelination continued to increase even after 45 days postnatum. There was no change in mean diameter (0-27 mum) of unmyelinated axons after 18 days post-conception. The mean diameter of myelinated axons (0-53 mum) also showed no variation with age. The modal diameter of myelinated axons lay between 0-4 and 0-6 mum. Small fibres (0-2-0-3 mum) myelinated around 32-35 days postnatum. The greatest increase in large myelinated axons (larger than or equal to 0-8 mum) occurred after 25 days postnatum. At all ages sheaths with outer and inner tongues in the same quadrant predominated and by 240 days postnatum 80% of sheaths showed this configuration.