Synapse formation on neurons born in the adult hippocampus

Although new and functional neurons are produced in the adult brain, little is known about how they integrate into mature
networks. Here we explored the mechanisms of synaptogenesis on neurons born in the adult mouse hippocampus using confocal
microscopy, electron microscopy and live imaging. We report that new neurons, similar to mature granule neurons, were contacted
by axosomatic, axodendritic and axospinous synapses. Consistent with their putative role in synaptogenesis, dendritic filopodia
were more abundant during the early stages of maturation and, when analyzed in three dimensions, the tips of all filopodia were
found within 200 nm of preexisting boutons that already synapsed on other neurons. Furthermore, dendritic spines primarily
synapsed on multiple-synapse boutons, suggesting that initial contacts were preferentially made with preexisting boutons already
involved in a synapse. The connectivity of new neurons continued to change until at least 2 months, long after the formation of
the first dendritic protrusions.