ASTROCYTES AND MICROGLIAL CELLS INCORPORATE DEGENERATING FIBERS FOLLOWING ENTORHINAL LESION - A LIGHT, CONFOCAL, AND ELECTRON-MICROSCOPIC STUDY USING A PHAGOCYTOSIS-DEPENDENT LABELING TECHNIQUE
I. Bechmann et R. Nitsch, ASTROCYTES AND MICROGLIAL CELLS INCORPORATE DEGENERATING FIBERS FOLLOWING ENTORHINAL LESION - A LIGHT, CONFOCAL, AND ELECTRON-MICROSCOPIC STUDY USING A PHAGOCYTOSIS-DEPENDENT LABELING TECHNIQUE, Glia, 20(2), 1997, pp. 145-154
Entorhinal lesion leads to anterograde degeneration of perforant path
fibers in their main termination zone in the outer molecular layers of
the dentate gyrus. Concomitantly, astrocytes become hypertrophic, and
microglial cells alter their phenotype, suggesting participation in a
nterograde degeneration. This study analyzes the involvement of these
lesion-induced activated glial cells in the process of phagocytosis of
degenerated axonal debris. We established a phagocytosis-dependent la
beling technique that allows for direct and simultaneous visualization
of both labeled incorporated axonal debris and incorporating glial ce
lls. Stereotaxic application of small crystals of the biotin-and rhoda
mine-conjugated dextran amine Mini Ruby (MR) into the entorhinal corte
x led to strong and stable axonal staining of perforant path axons. Fo
llowing entorhinal lesion, labeled terminals and fibers condensed and
formed small granules. Incorporation of these rhodamine-fluorescent gr
anules resulted in a phagocytosis-dependent cell labeling. During the
first 3 days, we were able to identify these cells as microglia by usi
ng double-fluorescence and confocal microscopy. The first unequivocall
y double-labeled astrocytes were found 6 days post lesion (dpl). Where
as in all stages a subpopulation of microglial cells remained devoid o
f MR-labeled granules, all astrocytes in the middle molecular layer we
re double-labeled after long survival times (20 dpl). On the ultrastru
ctural level, labeled granules appeared to be perforant path axons con
taining the tracer. Both terminals and myelinated fibers could be seen
inside the cytoplasm of microglial cells and astrocytes. Thus, antero
grade degeneration is a sufficient stimulus to induce axon incorporati
on by both astrocytes and a subpopulation of microglial cells. (C) 199
7 Wiley-Liss, Inc.