EVIDENCE FOR A ROLE OF DENDRITIC FILOPODIA IN SYNAPTOGENESIS AND SPINE FORMATION

Axo-dendritic synaptogenesis was examined in live hippocampal cell cul tures using the fluorescent dyes DiO to label dendrites and FM 4-64 to label functional presynaptic boutons. As the first functional synapti c boutons appeared in these cultures, numerous filopodia (up to 10 mu m long) were observed to extend transiently (mean lifetime 9.5 min) fr om dendritic shafts. With progressively increasing numbers of boutons, there were coincident decreases in numbers of transient filopodia and increases in numbers of stable dendritic spines. Dendritic filopodia were observed to initiate physical contacts with nearby axons. This so metimes resulted in filopodial stabilization and formation of function al presynaptic boutons. These findings suggest that dendritic filopodi a may actively initiate synaptogenic contacts with nearby (5-10 mu m) axons and thereafter evolve into dendritic spines.