The maturation of the ventral root-spinal cord transitional zone. Part 2. A quantitative ultrastructural study of the dynamics of its early development.

Stereological methods were used to calculate the proportions made up by the various tissue components of the transitional zone between C7 ventral spinal nerve rootlets and the spinal cord in Wistar albino rats at 1, 4 and 12 days after birth. The zone is about 20 micrometer deep at each age studied. Astrocyte processes form its major component, and are most densely packed in the middle and inner thirds of the zone throughout the period examined. The zone projects about 10 micrometer into the ventral rootlet shortly after birth but this projection has disappeared by 12 days. This is likely to be the result of a relative overgrowth of the cord tissue surrounding the attachment of the rootlet. In the outer third of the zone (adjacent to the ventral rootlet), Schwann cell perikarya are prominent features at 1 day, but are virtually absent at 4 days and subsequently. Schwann cell material is absent from the middle and inner thirds of the transitional zone at 4 days. Between 4 and 12 days, there is a relative invagination of Schwann cells into the deepest part of the zone. CNS-PNS transitional nodes are almost all located in the outer third of the zone at 4 days, but by 12 days have become evenly distributed throughout the zone.