Distribution and development of peripheral glial cells in the human fetal cochlea

The adult human cochlea contains various types of peripheral glial cells that envelop or myelinate the three different domains of the spiral ganglion neurons: the central processes in the cochlear nerve, the cell bodies in the spiral ganglia, and the peripheral processes in the osseous spiral lamina. Little is known about the distribution, lineage separation and maturation of these peripheral glial cells in the human fetal cochlea. In the current study, we observed peripheral glial cells expressing SOX10, SOX9 and S100B as early as 9 weeks of gestation (W9) in all three neuronal domains. We propose that these cells are the common precursor to both mature Schwann cells and satellite glial cells. Additionally, the peripheral glial cells located along the peripheral processes expressed NGFR, indicating a phenotype distinct from the peripheral glial cells located along the central processes. From W12, the spiral ganglion was gradually populated by satellite glial cells in a spatiotemporal gradient. In the cochlear nerve, radial sorting was accomplished by W22 and myelination started prior to myelination of the peripheral processes. The developmental dynamics of the peripheral glial cells in the human fetal cochlea is in support of a neural crest origin. Our study provides the first overview of the distribution and maturation of peripheral glial cells in the human fetal cochlea from W9 to W22.