WASHINGTON, Jan 23 (Reuters) - The U.S. Food and Drug Administration has cleared the way for the first trial to see if human embryonic stem cells can treat people safely, a company involved in the controversial research on Friday. Geron Corp (GERN.O: Quote, Profile, Research), a California biotechnology company, said it plans a clinical trial to try to use the stem cells to regrow nerve tissue in patients with crushed, but not severed, spinal cords.