the day when a vaginal plug was observed is designated as embryonic day ( e ) 0.5 and the day of birth as postnatal ( p ) day 0 . golgi labeling was performed as described ( fd neurotechnologies ) . for in utero electroporation , e13.5 embryos from timed - pregnant wt and npn-2 females from homozygous crosses were used . wt p5 dentate gyrus hippocampi , and wt or npn-2 e14.5 cortices , were dissected and dissociated for primary culture experiments . see supplemental methods for additional experimental procedures , including : tem , serial 3-d reconstruction , immunocytochemistry , physiological recordings , and quantification parameters for spine density , synaptic puncta , sholl analysis , and ap - fluorescence .
the majority of excitatory synapses in the mammalian cns are formed on dendritic spines1 , and spine morphology and distribution are critical for synaptic transmission26 , synaptic integration and plasticity7 . here , we show that a secreted semaphorin , sema3f , is a negative regulator of spine development and synaptic structure . 
 mice with null mutations in genes encoding sema3f , and its holoreceptor components neuropilin-2 ( npn-2 ) and plexina3 ( plexa3 ) , exhibit increased dentate gyrus ( dg ) granule cell ( gc ) and cortical layer v pyramidal neuron spine number and size , and also aberrant spine distribution . 
 moreover , sema3f promotes loss of spines and excitatory synapses in dissociated neurons in vitro , and in npn-2/ brain slices cortical layer v and dg gcs exhibit increased mepsc frequency . 
 in contrast , a distinct sema3a  npn-1/plexa4 signaling cascade controls basal dendritic arborization in layer v cortical neurons but does not influence spine morphogenesis or distribution . 
 these disparate effects of secreted semaphorins are reflected in the restricted dendritic localization of npn-2 to apical dendrites and of npn-1 to all dendrites of cortical pyramidal neurons . 
 therefore , sema3f signaling controls spine distribution along select dendritic processes , and distinct secreted semaphorin signaling events orchestrate cns connectivity through the differential control of spine morphogenesis , synapse formation , and the elaboration of dendritic morphology .