Sema3B promotes spine retraction on apical dendrites of cortical pyramidal neurons in culture from WT but not CHL1-null mice. A, Histogram depicts quantification of spine density (mean ±SEM) on apical dendrites of WT neurons (DIV14) treated with Fc, Sema3A-Fc, Sema3B-Fc, Sema3C-Fc, Sema3DFc, Sema3E-Fc, Sema3F-Fc, and EphrinA5 (5 nm, 30 min). Treatment with Sema3B-Fc and Sema3F-Fc led to significant decreases in spine density (*p = 8.64E-05 (Sema3B), 1.67E-05 (Sema3F), two-tailed t test; n = 10 neurons for each condition). The total number of spines counted in each condition ranged from 234 to 1149. B, Representative images of apical dendritic branches in Fc- and Sema3B-Fc-treated neuronal cultures from WT and CHL1 KO mice. Scale bar, 10 μm. C, Representative images of apical dendrites in Fc and Sema3F-Fc neuronal cultures from WT and CHL1 KO mice. Scale bar, 10 μm. D, Quantification of spine density (mean ±SEM) in WT and CHL1-null neuronal cultures treated with Fc, Sema3B-Fc, or Sema3F-Fc. Sema3B-Fc led to a significant decrease in mean spine density per neuron (±SEM) for WT but not CHL1 KO neurons (*p = 0.005, two-tailed t test). Sema3F-Fc led to a significant decrease in spine density for WT and CHL1 KO [*p = 1.40E-06 (WT), 6.48E-09 (CHL1 KO), two-tailed t test]. Number of neurons scored: WT, n = 10 (Fc), n = 10 (Sema3B-Fc), n = 14 (Fc), n = 14 (Sema3F-Fc); CHL1-null, n = 18 (Fc), n = 18 (Sema3B-Fc), n = 16 (Fc), n = 17 (Sema3F-Fc). The total number of spines counted in each condition ranged from 234 to 1098.