Figure 3. NG2 cells delimit myelinated bundle of axons in the corpus callosum of a P13 CNP-GFP mouse.

(A–D) Projection of a small stack of confocal images (five consecutives planes with z = 2 μm) illustrating the tendency of bundles of axons to myelinate [MBP (maltose-binding protein) staining in green in (A), grey in (C)] as a whole independently of surrounding axons. Boundaries are delimited by NG2 cells [NG2 staining in red in (A), grey in (B)] (coronal section). Images in (B–D) correspond to the area surrounded by the dashed box in (A). The dashed lines in yellow (B–D) indicate the interface between the myelinated bundle of axons labelled for MBP (C) and NG2 cells labelled for the proteoglycan NG2 (B). The image in (D) shows that bright CNP-GFP⁺ oligodendrocyte processes appear restricted to a single bundle. The yellow asterisk indicates the position of the cell body of one of the oligodendrocyte myelinating the axon bundle delimited by the yellow dashed lines. At this developmental stage, CNP-GFP fluorescence is difficult to detect in NG2 cells, as it is much lower than the fluorescence detected in mature oligodendrocytes.