Developmental progression of GAD/EphA4 modular overlap. GAD (green) and EphA4 (red) expression at P0 (a-c), P4 (d-f), P8 (g-i) and P12 (j-l). GAD layer 2 modules are present at all ages (a, d, g, j), becoming more obvious by hearing onset (j). EphA4 modules (b, e, h, k, dashed contours) are easily discernible at all ages and restricted to layer 2. Digital merges (c, f, i, l) show precision of EphA4 and GAD modular overlap (dashed contours). EphA4 labeling was consistent with a dense plexus of fibers and presumptive terminals, surrounding many of the GAD-positive modular neurons. Scale bars = 50µm.