Schematic diagram of neurosensory development in the basal turn of the human fetal cochlea. At W10 (week 10), SOX2 identifies the prosensory domain within the SOX9/SOX10+ cochlear duct epithelium. Neurites from the adjoining TUBB3+/PRPH + SGNs do not yet penetrate into the epithelium. Penetration starts at W11, prior to hair cell differentiation. At W12, the first MYO7A+/SOX9-/SOX10-/SOX2+ (inner) hair cell can be seen, and is contacted by multiple TUBB3+ and PRPH + neurites. Penetrating neurites are also found at the location of the future OHCs. At W14, both the IHCs and OHCs have differentiated, and neurites underneath the OHCs start to run in a spiral direction. At this stage, hair cells still express SOX2. At W20, SOX2 is downregulated in all hair cells, as opposed to the other cells in the organ of Corti. PRPH expression distinguishes between type I (PRPH-) and type II (PRPH+) neurites. Abbreviations: SGN, spiral ganglion neuron; IHC, inner hair cell; O1, first row of outer hair cells; O2, second row of outer hair cells; O3, third row of outer hair cells; OHC, outer hair cell.