There is no clear description of the relationship between the REZ and the transitional zone. In the strict sense, REZ means the area nerve comes from the brainstem and transitional zone refers to glia-schwann cell border. The term of REZ was defined by Jannetta as a junctional area between central and peripheral myelin6). Many authors used the terms of REZ and transitional zone interchangeably1,3,11). In a recent study, Tomii et al.13) described the transitional zone as the region where the myelin sheath is composed of both central glial and schwann cell myelin. They have proposed the use of terms root exit point (RExP), RDP (medial detach point) and transitional zone instead of REZ. And they stressed the RDP appears to be a good landmark for the microvascular decompression surgery.

In 1915, Henschen5) has been descried that cranial nerve was composed of two different segments, such as glial and non-glial portion. Skinner12) also reported several characteristics of various cranial nerves as follows. From the brain there is an outgrowth of glia cells, astrocyte and oligodendroglia, along the cranial nerve trunks and the extent of glial outgrowth varies in different nerves. Many investigators named the glial outgrowth segment of cranial nerve as CNS segment of cranial nerve, central myelin, or central glial myelin2,10,13). To measure the exact length of central myelin of facial nerve is difficult because of its irregular distal margin. Skinner12) reported that the glial outspread in the facial nerve extended as far as 2.6 mm from the plane of superficial origin. Adams1) described that the transitional zone is only 1 to 3 mm in length. Jannetta6) also reported the distance to the peripheral boundary of the REZ was 0.5 to 1 cm in the fifth, seventh, and ninth cranial nerves. In 2003, Tomii13) described the root exit zone of facial nerve more detail. He measured the length of the lateral and medial transitional zone were mean 1.9 mm and 0.58 mm, respectively. We measured the distance from medial detach point of brain stem to the most distal glial segment of facial nerve.

Glial portion of the cranial nerve, central myelin, is composed of fibrillary astrocyte and oligodendroglia cells12). And it has a structure similar to that of white matter of the CNS, consisting of parallel traveling nerve fibers, and lacks funicular structure. Nerve fibers are embedded in supporting tissue, but endo-, peri, and epineurium are absent. De Ridder et al.2) stated the layer of pia mater surrounds the central myelin. Other reported central myelin is lined with astrocyte foot plates8). Glial limiting membrane, pia glial membrane, or glial limitans, is a thin barrier of astrocyte foot process associated with the parenchymal basal lamina surrounding the brain10). The main function of glial limiting membrane is to act as a physical barrier of the CNS. We observed that the glial limiting membrane extending from the outermost layer of brain stem, strong stained with GFAP, surrounds the central myelin. We called the layer closing the central myelin as glial sheath like epineural sheath of peripheral nerve. We also measured the thickness of glial sheath of facial nerve and it becomes thin at distal portion than proximal. The glial sheath of central myelin may act as a barrier of the nerve fiber from vascular compression. The cause of hemifacial spams is generally known as that facial nerve compressed by blood vessels located at the REZ. The mechanism of hemifacial spasm is thought that thinning or defect of neural sheath of facial nerve because of vascular compression may result in developing cross talking of neural transmission4,9). Jannetta et al.7) reported that myelin defect exist in the REZ in patient of hemifacial spasm.