Proliferative Diabetic Retinopathy is Associated with a Low Level of the Natural Ocular Anti-angiogenic Agent Pigment Epithelium-derived Factor (PEDF) in Aqueous Humor. A Pilot Study - Boehm , B O; Lang , G; Feldmann , B; Kurkhaus , A; Rosinger , S; Volpert , O; Lang , G K; Bouck , N (Hormone and Metabolic Research, 2003)