Myelination of the human auditory nerve: Different time courses for Schwann cell and glial myelin

The goal of this study was to trace the development of myelin in the human auditory nerve. To do this, we used the Woelcke iron-hematoxylin technique to stain myelin sheaths in sections from fetal temporal bones and brain ste ms. In the cochlea, aggregations of Schwann cells are seen in the modiolus and along the spiral lamina by the 15th fetal week. By the 22nd fetal week, dense arrays of stained Schwann cells are present on auditory nerve axone within the temporal bone. By the 24th fetal week, light myelin sheaths exte nd up to, but not beyond, the glial junction. Myelin sheaths are not presen t in the auditory nerve central to the glial junction until the 26th fetal week or later. These results demonstrate a gap of several weeks between the onset of Schwann cell myelination distally and glial myelination proximall y. The period between these two events may represent the time of final matu ration of the organ of Corti.