Ultrastructure of astrocytes in the transitional region of the human eighth cranial nerve.

The nerve root segment where the transition from central to peripheral nervous tissue occurs is referred to as the transitional region (TR). Part of the TR is a continuation of the subpial glial limiting membrane (SGLM) which covers the surface of the brain. To assess the physiological and pathophysiological roles of astrocytes in the TR of the human eighth cranial nerve, electron microscopy was performed on surgically resected specimens of the eight cranial nerve root obtained from patients with cerebellopontine angle tumors. The astrocytic glial dome was sharply bounded by a basement membrane and was mainly composed of the bodies and processes of fibrous astrocytes. Desmosomes and gap junctions were found between these astrocytic processes. Half-desmosomes were found on the edge of the astrocytic glial dome and at the luminal surface of the perivascular glial limiting membrane. These ultrastructural features indicate that the astrocytes of the TR might have some motility capacity that allows adjustment to changes in the pressure exerted by the vessels, the peripheral nervous tissue, and the cerebrospinal fluid. In addition, it is also suggested that lack of structures linking the central and peripheral portions of the nerve and the abrupt change of nerve composition at the TR might predispose this area to injury by tractional forces or pathologic conditions such as tumor or arterial compression.