Grading system. (a) Grade 0: Control vas graft; normal histology. (b) Grade 1: Loss of stellate lumen but maintenance of periluminal smooth muscle continuity. (c) Grade 2: Reduction in mass and progressive fibrosis of the longitudinal and circular smooth muscle causing discontinuity of muscle layers. (d) Grade 3: Smooth muscle layers are largely replaced by fibrous tissue surrounded by a revascularizing, thickened adventitia.