Figure 3. Horizontal cell development in WT and PlexA4−/− retinas. A–N, WT (A, C, E, G, I, K, M) and PlexA4−/− (B, D, F, H, J, L, N) retina sections from postnatal P1 (A, B), P3 (C, D), P5 (E, F), P10 (G, H), P14 (I, J), P17 (K, L), and P21 (M, N) mice were immunostained with anti-neurofilament (A–F) or anti-calbindin (G–N). In PlexA4−/− retinas, aberrant horizontal cell neurites directed toward the ONL are observed as early as P14 (yellow arrows) and then more clearly at later time points. White dashed lines indicate the edge of the outermost ONL (J, L, N). Horizontal cells in PlexA4−/− retinas do not show correct neurite stratification within the OPL (yellow arrows) at P3 (D) and P5 (F) compared with WT retinas; however, the ectopic horizontal cell neurites across the INL of PlexA4−/− retinas stratify within the OPL by P10 (H). At P14, horizontal cells in PlexA4−/− retinas begin extending aberrant neurites toward the ONL (J, yellow arrows), and these aberrant neurites extend to the outermost photoreceptor cell body layers by P17 (L). Axon terminal-like structures are observed in the ONL in PlexA4−/− retinas at P21 (N, yellow arrows). O, O′, High magnification of P21 PlexA4−/− retina section immunostained with anti-calbindin (O) and counterstained with TO-PRO3 (O′). Aberrant horizontal cell neurites extend across the ONL visualized by TO-PRO3 (O′, blue) and reach the outermost edge of the ONL (yellow arrows). Scale bars: F (for A–F), N (for G–N), O′ (for O, O′), 50 μm.