Figure 1

Figure 1 (A) Timeline of MEP glial development. MEP glia (purple) are specified in the ventral spinal cord (gray) after 36 hpf, exit through motor exit points at ~50 hpf, divide, and migrate to eventually initiate myelination of motor root axons (pink) starting at 72 hpf. mn: motorneuron, LFP: lateral floor plate. (B–F) Lateral views of the motor exit point showing (B) a nkx2.2a⁺/foxd3⁺ MEP glia (outlined arrowhead) exiting the spinal cord at the motor exit point at 48 hr post-fertilization (hpf); (C) an olig2⁺/foxd3⁺ MEP glia (outlined arrowhead) along motor nerve root axons at 55 hpf; (D) sox10⁺/nkx2.2a⁺ MEP glia (outlined arrowheads) and sox10^-/nkx2.2a⁺ perineurial glia (arrow) at 72 hpf; (E) sox10⁺/nkx2.2a^- MEP glial sheaths (outlined arrowheads) and sox10⁺/nkx2.2a⁺ perineurial cells at 4 days post-fertilization dp (arrowhead) and (F) foxd3⁺ MEP glia making mbp⁺ myelin sheaths (outlined arrowheads) at 4 dpf. Insets show single z-plane images. (G) In olig2:egfp and nkx2.2a:nls-egfp larvae at 55 hpf, olig2⁺ pMN domain cells (teal), OPCs (gray), and MEP glia (purple) are labeled, as are nkx2.2a⁺ LFP cells (yellow), OPCs (gray) and MEP glia (purple). These images were used for fluorescence intensity measurement. (H) Violin plot of mean intensity of GFP fluorescence of olig2:egfp⁺ cells at 55 hpf (OPCs: 54914 ± 1674 arbitrary units (A.U.), MEP glia: 7149 ± 394, pMN domain cells: 29934 ± 1481). (I) Violin plot of mean intensity of GFP fluorescence of nkx2.2a:nls- egfp cells at 55 hpf (OPCs: 22981 ± 2013, MEP glia: 28610 ± 1624, lateral floor plate cells: 24049 ± 1602). (H–I) (n = 28 MEP glia, n = 28 OPCs and n = 28 neural tube cells from seven embryos). Asterisks denote the dorsal root ganglion (DRG) and yellow dashed lines denote the edge of the spinal cord. Scale bar (B–G) 20 µm.