Fig. 1. Expression of netrin 1 by oligodendrocytes in vivo. (A,A') Longitudinal section through a P8 rat spinal cord showing Mbp-immunopositive oligodendrocytes (red) that do not express netrin 1, intermingled with netrin-1-expressing cells (green). (B,B') At P8, netrin 1 immunoreactivity (green) was detected in the immediate environment surrounding the cell bodies of Cnp-positive oligodendrocytes (red), but netrin 1 was not expressed by the oligodendrocytes themselves. (C,C') An Mbp-positive netrin-1-negative process, surrounded by netrin-1-expressing neuroepithelial cells (arrows) in P8 spinal cord. (D,D') Longitudinal section through a P8 rat spinal cord showing axons positive for neurofilament medium polypeptide (Nfm), surrounded by netrin-1-expressing cells. (E,E') Cell bodies of netrin-1-immunopositive neurons. (F) Cross-section of a P12 rat dorsal spinal cord showing Cnp-positive, myelinating oligodendrocytes (red) expressing netrin 1 (green). (G-G'') Oligodendrocytes adjacent to the central canal (arrow) express netrin 1 in P12 spinal cord. (H,H') Netrin 1 immunoreactivity (green) associated with a Cnp-immunopositive (red) myelinating oligodendrocyte in the P12 rat spinal cord. (I-I'') By P12, netrin 1 expression (green) colocalizes with Nfm-immunoreactive axons (blue), but is not detected in Gfap-positive astrocytes (red). (J,J') Transverse section of a P22 thoracic spinal cord showing widespread expression of netrin 1 (green) by Cnp-positive, myelinating oligodendrocytes (red). (K-K'') Magnification of the boxed region from J showing Cnp-positive netrin-1-expressing cell bodies. A,F, 20x0.5 n.a. objective; G,J, 40x0.75 n.a. objective; D,E,I,K, confocal microscopy, 40x0.75 n.a. objective; B,C,H, confocal microscopy, 100x1.4 n.a. objective. Scale bars: 10 µm in B',C',H; 20 µm in A',D,E,G,K; 40 µm in F,I',J.