The organ of Corti (oC) maintains type II spiral ganglion neurons (SGNs), but not type I SGNs in organotypic culture and brain-derived neurotrophic factor (BDNF) rescues a greater proportion of type II SGNs following explant culture without the oC. (A- H) Single plane confocal images of β-tubulin (green) and peripherin (red) immunofluorescence in the mid-apex-mid-turn region of cochleae from P1 (A-D) and P7 (E-H) mice enabled analysis of type I SGN (immunolabel for β-tubulin only) and type II SGN (immunolabel for β-tubulin and peripherin) density. SGN density in the mid-apex-mid-turn region in vivo was determined in whole mount tissue (whole mount) (A,E). Survival of SGN sub-types was subsequently established following 48 hour culture of cochlear tissue: retaining both SGNs and oC (organotypic) (B,F); just the spiral ganglion, following removal of the oC (explant - control) (C,G) and this spiral ganglion explant cultured with 100 ng/ml BDNF (explant + BDNF) (D,H). Inset shows marked area in corresponding panel at higher magnification and shows detail of type I and type II SGNs. (I,J) Neuron densities for type I (green) and type II (red) SGNs from P1 (I) and P7 (J) mice are plotted for each condition shown in panels (A-H). x-sect, mid-apex-mid-turn ganglion in mid-modiolar cross-sections; wm, whole mount; orgtyp, organotypic; expl BDNF, explant + BDNF; expl contr, explant - control. Mean density and standard error of the mean; one-way ANOVA with post hoc Holm Sidak tests compare SGN survival in the wm, orgtyp and expl contr; unpaired t-tests compare expl contr and expl BDNF; *P < 0.05, ***P < 0.001. (K,L) Percentage survival of type I and type II SGNs in orgtyp, expl BDNF and expl contr relative to mean in vivo whole mount densities are plotted for P1 (K) and P7 (L) mice. Mean density and standard error of the mean; unpaired t-test, *P < 0.05, **P < 0.01, ***P < 0.001. Scale bars: 250 μm.