Glioanatomy assessed by cell-cell interactions and phagocytotic labelling

In the last three decades of research in neuroscience, fluorescent probes have gone from technical tools in the studies of physicochemical reactions, to being versatile tools in developmental neurobiology, neuroanatomy, angiography, neuromorphology, connectivity, cell death and even photodynamic therapy. Fluorescent dyes belong to heterogeneous groups of substances, but the feature to emit light of a certain wavelength depends on the energy status of the corresponding chemical bond. Therefore, light emission can range from the blue to the infrared spectrum, thus allowing multiple stains of the same cell, or event. The heterogeneity in their structure allows application of some fluorescent dyes for anterograde long-tract labelling, whereas others can be used for retrograde tracing. Lipophilic dyes are suitable for intramembraneous diffusion along cell membranes post-mortem, whereas hydrophilic stains seem more suitable for genealogic cell studies over several cell divisions. In the same time, less attention has been paid by most researchers to the use of fluorescent dyes to monitor neuroglial interactions and glioanatomy in the healthy and diseased brain. Studies of cell-cell-interactions during apoptosis can now be carried out with sequestration and subsequent phagocytosis of intracellular dyes. The present review focuses on recent developments that include the use of fluorescent probes. These probes make it possible to transneuronally assess functions of glial cells during programmed cell death, or induced degeneration. The high variety of available dyes, and their particular accumulation within subcellular compartments, is promising to shed light on some glial cell geometry and functions. The lessons obtained from the vast number of studies in neurons are of increasing importance for cells too, as their functions are not directly accessible. In short, some glial-glial and neuroglial negotiations will be analysed in near future by developing new, or by modifying existing fluorescent probes.