getting vertex points of GeometryModel3D to draw a wireframe

getting vertex points of GeometryModel3D to draw a wireframe

I've loaded a 3d model using Helix toolking like this
modelGroupScull = importer.Load("C:\\Users\\Robert\\Desktop\\a.obj");
GeometryModel3D modelScull = (GeometryModel3D)modelGroupScull.Children[0];
and I also have _3DTools that can draw lines from point-to-point in 3d
space. now to draw a wireframe of my GeometryModel3D I guess I have to
cycle to its vertexes and add them to ScreenSpaceLines3D.
ScreenSpaceLines3D wireframe = new ScreenSpaceLines3D();
// need to cycle through all vertexes of modelScull as Points, to add them
to wireframe
wireframe.Points.Add(new Point3D(1, 2, 3));
wireframe.Color = Colors.LightBlue;
wireframe.Thickness = 3;
Viewport3D1.Children.Add(wireframe);
But... how do I actually get this vertex points?
Thanks.