Fig. 3. Immature cochlear afferent innervation pattern at E18 in mice. (A) An overview of TMRD (red; type I SGNs) and peripherin (green; type II SGNs) labeling in a whole-mount preparation displayed as a maximum intensity projection. (B) TMRD- and peripherin-labeled neurites in the organ of Corti exhibited an unorganized innervation pattern under the OHCs. These neurites traveled only a short distance in the outer spiral bundles (OSBs). TMRD labeling was also found under the IHCs. (C,D) Organ of Corti cross section from a single confocal optical section provides an alternative view under the hair cells. (C) TMRD-labeled neurites in the inner spiral plexus region, showing innervation of the IHCs. Type I fibers also traveled along the basilar membrane to innervate the first row of OHCs via the OSBs (arrow). (D) Peripherin-expressing neurites traveled along the basilar membrane, and entered the OSBs to target an OHC. Peripherin-expressing fibers also formed collateral innervation to the IHCs via thin fibers (arrowhead). Scale bars: 50 µm in A; 10 µm in B; 20 µm in C,D. isp, inner spiral plexus; osb, outer spiral bundles.