The ventral motoneurone axon bundle in the CNS--a cordone system?

In the developing CNS neighbouring structures are commonly separated by transient barriers termed cordones, some of which coincide with glial elements. Where ventral motoneuron axons cross the spinal white matter as intramedullary bundles to reach the CNS-PNS transitional zone they are surrounded from early development by a glial sleeve resembling a cordone. This becomes better developed with age and, like some cordones, persists into adult life. This could provide a radial conduit which might underlie the capacity of central segments of mature ventral motoneurone axons to regenerate. It may also provide a pathway for glial migration from the central cord to more superficial levels, including the transitional zone, where they help form the CNS-PNS barrier. Axons in the intramedullary bundle and in the surrounding ventral white column mature at different rates. Glial sleeve cells of the intramedullary bundles are apposed to both. Morphometric analysis of the axon-glial relationships of the two populations indicates that glial development proceeds at a different rate in relation to each axon class and that this is influenced by the degree of axonal maturation, which may in turn be related to target contact. Furthermore, early axon glial relationships differ between the two populations. For ventral motoneurone axons these take place in two stages: firstly, glial segregation of axons (resembling that in the PNS) and secondly, oligodendrocytic contact and ensheathment, which leads on to myelination. Axon-glial relationships in the ventral white column begin with the second of these events, as is more typical of early CNS myelination in general.