Morphological specialisations of rat cranial nerve transitional zones.

Near their CNS-PNS transitional zones (TZs), many rat cranial nerve rootlets are subdivided to a marked degree by a reticulum of fine cytoplasmic processes. Some of the resulting compartments contain only a single myelinated fibre or a bundle of collagen fibrils. The compartments are aligned with the astrocytic tunnels in which the fibres lie as they traverse the CNS-PNS transitional zone. This marked subdivision may help to insulate individual fibres from one another, preventing functional interaction between them. Rootlet sheath cells commonly are closely apposed to, or interdigitate with, astrocyte processes of the TZ. These features may help to strengthen the delicate TZ. The TZ of the trochlear nerve includes a long, generally avascular, central tissue projection (CTP) into the proximal part of the nerve. This is connected to the brainstem and cerebellum by astrocytic bridges. In contrast to the CTP, which is generally avascular, these contain abundant blood vessels which may facilitate metabolic exchange in the trochlear TZ.