Cell clusters on fetal rat ventral roots: prenatal development.

Clusters of cells are a prominent feature on the most proximal part of rat ventral rootlets between 15 and 21 days of fetal life and they increase to a maximum size at 17 days post conception. By means of extensive sheath-like processes the component cells of the clusters encapsulate one another in a very complex manner. Cells also give rise to processes which extend into the underlying axon bundles in the most proximal part of the ventral root and there form a highly complex matrix of interwoven fine cytoplasmic processes which separate individual axons from one another from a very early stage. With maturation, the clusters become smaller and the complex matrix disappears. Cells separate off from the clusters and enter the underlying ventral rootlets where they differentiate into Schwann cells and come to envelop the axons in the manner characteristic of the latter. However, the proximal part of the rootlet remains at a much less advanced state of maturation than more distal parts up to the end of fetal life. It is possible that cell clusters are in part produced by overgrowth of CNS tissue around the axon bundles in the ventral rootlets. This strips the cells distally and causes them to become piled up as collars around the most proximal part of the ventral rootlet.