The development of the spiral ganglion in the human foetus.

The development of the spiral ganglion was studied in steps sections of 81 human temporal bones. By the 8th week, the spiral ganglion has already separated from the vestibular ganglion. At 13 weeks two distinct populations are observed that correspond to neuron and Schwann cells. At 15 weeks the spiral ganglion has increased its distance from the cochlear duct and is surrounded by mesenchyme near the scala tympani. At 14 weeks a gradual decrease in the nucleus-to-cell area ratio was observed in spiral ganglion neurons that may reflect a morphological adaptation to function. By the 23rd week the modiolus begins to ossify and the spiral ganglion is surrounded by bony trabeculae. The time course of spiral ganglion development follows that of the stria vascularis and organ of Corti, although maturation changes are still observed in the neuronal population even beyond 20 weeks.