Fig. 3. Netrin 1 and Dcc regulate process outgrowth by immature oligodendrocytes in vivo. (A-C) Transverse sections of brachial spinal cord from E18 Ntn1^-/- mice stained with antibodies to Cnp (green) and Nfm (red) to label premyelinating oligodendrocytes and neuronal cell bodies and axons, respectively. (A) Arrows indicate Cnp-immunopositive premyelinating oligodendrocytes. (B) As premyelinating oligodendrocytes search for and initially contact axons, they extend one or two major processes (arrows). (C) Netrin 1 expression, determined by β-galactosidase (β-Gal) reporter expression in a transgenic Ntn1^+/- mouse, shown at the E18 spinal cord floor plate (arrow). Premyelinating oligodendrocytes are not immunopositive for β-galactosidase, but are adjacent to cells expressing netrin 1. (D) Dashed lines illustrate the measurement of extending processes of premyelinating oligodendrocytes in Ntn1^+/+ and Ntn1^-/- embryos. (E,F) Oligodendrocytes in the spinal cords of Ntn1^-/- or Dcc^-/- mice exhibit significantly shorter processes than wild-type or heterozygous littermates. ^*P<0.05, ^***P<0.0001 versus control. The number of cells analyzed in each condition is indicated in parentheses. Error bars indicate s.e.m. A, 20x0.5 n.a. objective; B-D, 40x0.75 n.a. objective. Scale bars: 40 µm in A; 20 µm in B-D.