The lumbar ventral root-spinal cord transitional zone in the rat. A morphological study during development and at maturity.

In the rat lumbar ventral spinal nerve rootlets, the central-peripheral transition occurs at the surface of a distal projection of central tissue into the rootlet. This changes considerably in appearance during development. In the first week after birth, it grows distally into the rootlet to form an irregular, tapering projection. In the second week after birth, it is retracted and becomes splayed out; its distal surface is irregular and lies just above the surrounding spinal cord. After this, it again grows distally into the rootlet. It forms a tapering projection which generally lies eccentrically in the rootlet, most often towards its dorsal surface. The central ends of the proximal transitional myelinated peripheral internodes generally lie in grooves on the surface of the central tissue projection. However, for a time during the second week after birth, many lie in invaginations into it. Occasional invaginations of the central tissue projection contain large numbers of collagen fibres rather than axons. A ring of collagen fibres surrounds the rootlet immediately distal to its attachment to the cord surface. Though the central tissue projection contains occasional astrocytic perikarya, it consists mostly of closely interwoven astrocyte processes derived from cell bodies situated at the cord surface surrounding the rootlet attachment. Changes in the form and size of the central tissue projection probably result largely from active growth and reorganisation of astrocyte processes. The barrier which these processes constitute probably prevents invasion of the cord by transitional Schwann cells. Before the central tissue projection develops, such invasion is probably prevented by the arrangement of transitional Schwann cells as a close-knit epithelium on the surface of the rootlet. The central tissue projection of ventral rootlets is smaller, more irregular in shape and less highly organised than that of dorsal rootlets. Central-peripheral transitional nodes lie close to the surface of the central tissue projection. They are therefore offset relative to one another and so are less likely to discharge one another. This arrangement may also protect the rootlet against mechanical damage due to traction.