Cytori Therapeutics Inc. said on Thursday that it will begin the first clinical trial to test whether stem cells derived from fat can be used to regenerate breast tissue in Japan next month. The 20-patient study, which will include women who have undergone partial mastectomies due to breast cancer, was approved by Japanese regulators, the company said. Cytori, based in San Diego, said it has developed a system for isolating and concentrating a patients' own stem cells from tissue removed from one of the body's normal fatty deposits