Hippocampal pyramidal neurons in culture showed a developmental shift in synapse distribution from dendritic shafts to spines. Using dual wavelength time-lapse fluorescence microscopy, we analyzed the morphogenesis of three synaptic components: dendritic spines, postsynaptic densities (PSDs), and presynaptic vesicles. Local assembly of a major PSD protein, PSD-95, was spatially and temporally correlated with spine morphogenesis. Clustering of postsynaptic PSD-95 and that of a predominant synaptic vesicle protein, synaptophysin, were also correlated. In contrast, pre-existing PSD-95 clusters in dendritic shafts were preferentially eliminated without promoting spine formation. The local and stepwise assembly of synaptic components at the contact sites between dendritic protrusions and axons explains the developmental remodeling of excitatory synapses.
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