Early development of rat ventral root transitional zone: An immunohistochemical and morphometric study

Bundles of ventral motoneuron axons cross the white matter of the spinal co rd, emerge through the cord surface at the CNS-PNS transitional zone (TZ) a nd continue in the PNS as ventral rootlets. This study identifies immunohis tochemical and morphometric changes which characterise the key events in ea rly TZ formation in the rat. E18 is a landmark stage, since it is then that the major events of TZ differentiation are initiated. In the glial process es associated with the TZ, vimentin expression decreases, while that of GFA P increases. In the proximal rootlets the transient expression of CNS marke rs such as GFAP and of neural adhesion molecules such as HNK-1/N-CAM begin to decrease. Their resulting differential expression clearly defines the CN S-PNS interface. These changes coincide with the arrival of glial nuclei at the TZ. Cell clusters which appear on proximal ventral rootlet surfaces sh ortly after their emergence from the cord, have by E18 formed an extensive matrix of processes which segregates the axon bundle. This comprises the ea rliest of two well-defined barriers across the axon bundle. An important fu nction may be to prevent Schwann cell invasion of the cord. Cluster cells d isplay some immunohistochemical features in common with Schwann cells. The second barrier becomes fully established only at P2 and forms the definitiv e CNS-PNS interface. It consists of processes arising from astrocytes surro unding the TZ. Changes in the nuclear density of the cell types correspond closely to their segregating activity. The immunohistochemical and ultrastr uctural changes complement one another to deepen and enhance understanding of TZ development.