Quantitative studies on the maturation of central and peripheral parts of individual ventral motoneuron axons. I. Myelin sheath and axon calibre.

The entire lengths of central and peripheral myelin sheaths on the same ventral motoneuron axons of young rats were studied during the rapid early phase of myelination which occurs during the first 2 weeks after birth. Sheath thickness varied along central internodes at all ages, though this variation was less in the more mature sheaths. Usually there was a single level of maximum thickness, most often near the middle of the internode, but frequently close to one 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. Comparison of 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, however, peripheral sheaths were less variable along more than twice that of central internodes belonging to the same fibre. At first, the unrolled glial unit was oval in outline while the unrolled Schwann cell was trapezoidal. Subsequently, both eventually became trapezoidal. The calibre of central and peripheral stretches of the same ventral motoneuron axons were also compared with one another in young rats during the first 2 weeks after birth. At each age, the great majority of axons were thicker peripherally than centrally. The mean circumference of the peripheral segment exceeded that of the central by a factor which averaged 1.3 over the period studied. Circumference varied from one level to another along both central and perpheral stretches of the same axons. The degree of this variation differed considerably from one axon to another. In many central fibres, and in all peripheral stretches of fibres, calibre variation followed no particular pattern. However, the calibre of some central axon segments gradually increased as the fibre was traced distally towards the cord surface. A proportion of axons was dilated just deep to the cord surface. These dilatations were frequently myelinated. No association was found between patterns of longitudinal variation in axon calibre and in sheath thickness along the same fibre.