DRG explants were derived from Swiss Webster, Bax null, and p75 null mouse embryos (E13). For collagen matrix assays, DRGs were dissected out under a stereomicroscope using tungsten needles. Collagen matrix was prepared with 430 μl of collagen (3 mg/ml dissolved in 0.1M acetic acid, Sigma), 50 μl of 10X DMEM medium, and 2.5 μl of 0.8 M NaCa2, and the pH was adjusted to 7.5. Individual ganglion explants were placed in 24-well plates and covered with freshly prepared collagen. Sepharose beads with an average diameter of 150 μm were used. Beads were washed twice with PBS, air dried, and loaded with 10, 20, 50, or 100 ng/μl NT-3 (Collaborative Research, Chemicon) at 4 °C overnight with constant shaking. For negative control, beads were loaded with BSA (10–100 ng/μl) or PBS. Either a single neurotrophin-loaded bead or a single BSA (or PBS)–loaded bead was implanted about 200–1,200 μm away from the ganglion explant. Collagen-embedded cultures were then placed at 33 °C for 15 min for the matrix to harden. Serum-free culture medium was then added to each well. In cultures with WT DRG and control beads, 10% serum was added to ensure viability of the explants. TrkC-Fc (Regeneron Pharmaceuticals, Tarrytown, New York, United States) was added (20 μg/ml) into the culture medium.