Old dyes for new scopes: the phagocytosis-dependent long-term fluorescence labelling of microglial cells in vivo.

The nature of the interactions between dying neurons and microglial cells within the developing and injured CNS remains controversial. A new technique for labelling microglial cells is available, which enables further studies of such interactions in a direct way. The value of the method relies on retrograde filling of neurons with vital fluorescent dye, subsequent degeneration of the neurons due to either naturally occurring cell death or as the result of axotomy, and phagocytotic removal of the fluorescent cell debris by microglial cells, which thus become identifiable. The fluorescent dye can be visualized in whole-mounted tissue or after sectioning. Photoconversion of the dye into electron-dense material permits examination of the microglial and dying ganglion-cell interactions at the ultrastructural level. This new principle of the function-dependent, selective fluorescent labelling of phagocytosing microglial cells, which might now be extended to other dyes and to other neurodegenerative models, promises to shed light onto the function of microglial cells within the brain.