From EurekAlert!: "Human adult stem cells injected around the damage caused by a heart attack survived in the heart and improved its pumping efficiency for a year in a mouse model ... Injection of a patient's own adult stem cells into the heart has shown some efficacy in assisting recovery after a heart attack in early human clinical trials, [but] nobody knows how they work, or how long the stem cells last and function in the heart. This study shows how one type of adult stem cell works. ... The team's research focused on adult stem cells – those that can differentiate into a limited variety of tissues – that circulate in the blood and are distinguished by the presence of the CD34 protein on the cell surface. ... CD34+ cells are capable of becoming heart muscle cells, called cardiomyocytes, blood vessel cells and smooth muscle cells. ... The CD34+ cells survived in the left ventricle of the heart for 12 months or longer. Left ventricular ejection fraction - a measure of how much blood is pumped from the heart to other organs at each contraction - improved in treated mice compared with controls for 52 weeks. ... This improvement was the result of increased blood vessel formation in and around the injured area, or paracrine signaling by the stem cells to other nearby cells, rather than formation of new heart muscle."