Dynamic imaging of cerebellar Purkinje cells reveals a population of filopodia which cross-link dendrites during early postnatal development

Two-photon microscopy was used to image dye-loaded filopodia of Purkinje cells in acute rat cerebellar slices. In the process of examining filopodia in Purkinje cells from a period of rapid dendritic growth (P10-21), we observed a small subset of filopodia which appeared to form connections between two dendrites of the same cell, usually between the tips of two adjacent dendrites or the tip of a dendrite and the shaft of another. There were fewer of these 'filopodial bridges' present at P18-21 than at an earlier stage in development (P10-12) and they were absent in mature Purkinje cells. Filopodial bridges do not appear to be an artifact of living brain slice preparation as they may also be seen by dye-loading Purkinje cells in slices prepared from perfusion-fixed brain. They have varied morphologies which are mostly similar to conventional, unattached filopodia. However, when measured over tens of minutes, filopodial bridges were observed to be less motile than conventional filopodia as indicated by a reduced index of expansion. While the functions of these novel structures are unknown it is attractive to speculate that they play an instructive role in Purkinje cell dendritic development.