Changes in synapse morphology during newborn granule cell (GC) maturation. (A) Adult-born GCs of the olfactory bulb (OB) labeled by injection of biocytin. Cell maturation is characterized by progressive growth of the apical dendrite toward the external plexiform layer (EPL). GCL = granule cell layer; MCL = mitral cell layer. Scale bar = 20 μm. (B) Synapses with a single homogeneous postsynaptic density (PSD) were prevalent in GCs at 7 days post-injection (dpi), whereas older cells exhibited a larger percentage of synapses with a more complex morphology including a doughnut-like shape. (C) Representative images of asymmetric synapses between axonal profiles and dendrites of 7 dpi, 21 dpi and pre-existing (green fluorescent protein (GFP)-negative) GCs (three sections of each series are shown with depth). The dendrites of 7 dpi and 21 dpi GCs are identified by GFP immunolabeling. Arrows identify PSD. Scale bar = 200 nm. (D) Three-dimensional reconstruction of synapses based on axonal profiles (PRE = light yellow) with GC dendrites (POST = transparent blue), revealing the variable shape of the synaptic junctions (brown). The synapse of a 7 dpi GC shows a single, compact PSD. Synapses of 21 dpi and GFP- GC have a doughnut-like morphology. Scale bar = 50 nm.