Cell clusters on rat ventral roots: postnatal development.

The cell clusters found on the most proximal parts of rat ventral spinal nerve rootlets during prenatal development, persist into the postnatal period. Clusters become smaller as cells leave them to invest axon segments. By 2 weeks after birth, clusters have largely disappeared. Fine processes stemming from clusters interweave at first to envelop axons in a complex matrix. This matrix disappears soon after birth. Axons then become enveloped by cells in clusters in the manner of premyelin Schwann cells. Many of these premyelin cells leave the parent cluster but continue to envelop the unmyelinated axon segment as they do so. Meanwhile, myelination of axons immediately distal to the proximal rootlet segment proceeds at the same rate as in the ventral root generally. Thus, the proximodistal maturation gradient comes to be reversed in relation to many axons in the ventral root, the most proximal peripheral internode being unmyelinated, while more distal internodes are myelinated. This reversal gradually lessens in some cases as the short internodes produce myelin sheaths. It is possible that these short internodes become progressively more heavily myelinated and elongate in subsequent development. All axons in the most proximal part of the ventral rootlet do not necessarily become enveloped by short premyelin Schwann cells, however. When some are released from envelopment by the matrix, the myelinating Schwann cells at more distal root levels extend proximally along each of them, up to the level of the surface of the spinal cord. Some clusters in the early postnatal period contain cells resembling astrocytes.