Brain-derived neurotrophic factor (BDNF) promotes neuritogenesis of both type I and type II spiral ganglion neurons (SGNs) in explants, but has a greater effect on type II SGNs. (A-D) Analysis of neurite outgrowth in mid-apical-mid-turn region explants from P1 (A,B) and P7 (C,D) mouse cochlea following 48 hour culture in control culture media (A,C) or with 100 ng/ml BDNF added to the media (B,D). Confocal immunofluorescence images show type I SGN neurites (β-tubulin (green) only) and type II SGN neurites (β-tubulin and peripherin (red)). (E,F) BDNF stimulated an increased number of type I (green) and type II (red) neurites per explant (E) and significantly longer type II neurites (F). The neurites extend from the explants in bundles and then separate into individual fibers. This gives the appearance of branching towards the ends in the low magnification images. Data are represented as mean and standard error of the mean; unpaired t-test, *P < 0.05, **P < 0.01, ***P < 0.001. Scale bars: 250 μm.