Transient lamellipodia predict sites of dendritic branch formation in hippocampal neurons

Extensive branching creates the complex dendritic arbor of mammalian CNS neurons but capturing the complete process of branch formation with time-lapse recordings has been challenging. Here, we report that application of BMP7 to cultured hippocampal neurons accelerated dendritic growth sufficiently to document branches forming in less than 20 h via frequent time-lapse imaging (10-min intervals). In these recordings, most branches emerged as collateral sprouts from the shaft of a parent branch. Analysis of the recordings showed that filopodia were abundant and formed transiently throughout the length of dendrites but among these, only a small subset occurred at sites where branches later emerged. Conversely, formation of lamellipodia was rare and coincided with sites where collateral branches emerged. This pattern suggests that lamellipodial structures act as an important intermediate form of cytoskeletal remodeling related to a cellular commitment to branch, whereas filopodia appear to be related to events prior to such commitment.