Characteristic contours of the soma of OPCs. A: Contour of a cell soma approximating a circular shape. The black line marks the level of C90. The dashed gray line indicates a parabola fitted to the cell contour that traces the soma but crops the process. B: Contour of a cell soma protruding into the direction of a process. A parabola (gray dashed line) would crop the protrusion of the soma whereas a polynomial of third degree includes the protrusion but still crops the process.