Recruitment and attachment defects of HS-deficient MCs in embryonic skin. MCs of the skin vasculature were visualized with NG2 (green, A-F) in E13.5 control (A-B) and EXT1^MCko embryos (C-F). Blood vessels in the mutant display a reduced number of MCs in the vascular plexus (C) compared with the control (A). White arrows represent MCs that attach to the vessel and are in close contact to the endothelium (red; D,F) as seen similarly in the control (B). However, EXT1^MCko vessels are dominated by rounded NG2-positive cells in close proximity to the vessel (arrowhead; D-E), indicating progenitor cells that fail to differentiate; n > 5. Embryonic transverse section (G-H) confirms the presence of MCs; however, MC organization is chaotic (H, ) compared with the tight alignment of MC in control cardinal vein (G).