QUANTITATIVE STUDIES ON MATURATION OF CENTRAL AND PERIPHERAL PARTS OF INDIVIDUAL VENTRAL MOTONEURON AXONS .1. MYELIN SHEATH AND AXON CALIBER

Abstract

The entire lengths of central and peripheral myelin sheaths on the same ventral motoneuron axons of young rats were studied during early myelination. Sheath thickness varied along central internodes at all ages, being less in mature sheaths. Usually there was a single level of maxium thickness, most often near the middle of the internode, but frequently close to 1 end. Some sheaths varied little in thickness along their lengths; a few were bimodal. The number of turns decreased on either side of the maximum as the sheath was traced towards the bounding nodes. Serially adjacent sheaths along the same axon showed no relationship between their mean thickness or patterns of longitudinal thickness variation. Patterns of thickness variation in peripheral sheaths were very similar to those found centrally. At a given age, peripheral sheaths were less variable along their length than central ones. The mean thickness of peripheral internodes averaged more than twice that of ventral internodes belonging to the same fiber. At first, the unrolled glial unit was oval in outline while the unrolled Schwann cell was trapezoidal. Both eventually became trapezoidal. At each age the great majority of axons had a greater caliber peripherally than centrally; the average ratio between the mean circumference of the 2 segments was 1.3. Circumference varied from 1 level to another centrally and peripherally. The degree of this variation differed considerably from 1 axon to another. In many central fibers and in all peripheral stretches of fibers, caliber variation followed no particular pattern. Some central axon segments gradually became thicker when traced distally towards the cord surface. A proportion of axons were dilated just deep to the cord surface. These dilatations were frequently myelinated.