Figure 5.

CX3CR1-EGFP⁺ cells are widely distributed and intermingle with neuroblasts in the SVZ/RMS, yet microglial phagocytosis of neuronal precursors is uncommon. a, In the RMS, CX3CR1-EGFP⁺ cells (gray) intermingle with neuroblasts (DCX⁺, magenta). While some exhibit phagocytic bulges (white arrowheads), optical sectioning revealed that these bulges did not contain neuroblast fragments (b, white arrows and arrowheads). Newborn cells were also labeled by BrdU administration (150 mg kg⁻¹, i.p.). Brain sections were obtained from CX3CR1-EGFP mice (P60) killed 1, 3, 12 h and 5 and 7 d after a single dose of BrdU. d, e, At 3 h, BrdU⁺ cells are distributed along a region that contains neuroblasts (DCX, magenta) surrounded by microglia (EGFP, green). Microglia processes contact some BrdU⁺ cells; however, orthogonal views elucidate that CX3CR1-EGFP⁺ cells are not engulfing neuroblasts (arrows and arrowheads, inset). f, Immunolabeling for TREM2 (gray) in the SVZ. g, Within the SVZ, neuroblasts (DCX, magenta) are apposed to microglia (EGFP, green), but these cells do not express TREM2 (red), as shown in optical sections. Different observations were made in the OB within the glomerular layers (gl), 7 d after a single BrdU pulse. There, CX3CR1-EGFP⁺ cells were noted to have incorporated BrdU (h, i). However, in these cells BrdU is located in the microglial cytoplasm, and associated with pyknotic cell nuclei, as may be inferred from the pattern of DAPI staining (blue, inset). j, TREM2-immunoreactive cells are detected in the glomerular layer. k, TREM2 is detected in those CX3CR1-EGFP⁺ cells that have engulfed DCX⁺ cells, which in turn have pyknotic nuclei (DAPI staining, blue arrowhead). Schematics represent parasagittal sections of analyzed areas, RMS (orange), and OB (light red). lv, Lateral ventricle; Str, striatum. Scale bars: 10 μm.