IMAGE: THE SCAR IN THE MURINE HEART AFTER MYOCARDIAL INFARCTION. SCAR TISSUE IS SHOWN IN RED. HEART MUSCLE CELLS ARE SHOWN IN GREEN. view more
CREDIT: © HUBRECHT INSTITUTE
Scientists know a lot about human body and the cells, tissues and organs that make it up. However, this knowledge is not complete. We are still learning about even important organs such as the heart.
When a heart attack occurs, blood supply to part of the heart muscle is cut off. That part dies. Basically, heart is a pump that maintains the blood circulation through our blood vessels. Death of a part of the heart muscle is a life- threatening event.
When any tissue damages, stem cells that reside in the tissues come forward; they multiply swiftly forming large numbers of daughter cells which quickly replace the damaged cells
For two decades researchers and clinicians have searched for cardiac stem cells, stem cells that should reside in the heart muscle and that could repair the heart muscle after a myocardial infarction. Many research groups have claimed that they identified cardiac stem cells, yet none of these claims have held up. See for instance the following recent press release: “US governments halts heart stem-cell study”. The jury is still out on the existence of cardiac stem cells and their significance for adult hearts. It remains therefore hotly debated.
To solve this debate, researchers from the Hubrecht Institute inUtrecht, the Amsterdam University Medical Center, the École Normale Supérieure(ENS) de Lyon and the Francis Crick Institute London, led by Hans Clevers,focused on the broadest and most direct definition of stem cell function in themouse heart: the ability of a cell to replace lost tissue by cell division. Inthe heart, this means that any cell that can produce new heart muscle cellsafter a heart attack would be termed a cardiac stem cell. The authors generateda ‘cell-by-cell’ map of all dividing cardiac cells before and after amyocardial infarction; they used advanced molecular and genetic technologies.
The study establishes that many types of cells divide upon damage of the heart, but that none of these are capable of generating new heart muscle. In fact, many of the ‘false leads’ of past studies can now be explained: cells that were previously named cardiac stem cells now turn out to produce blood vessels or immune cells, but never heart muscle. Thus, the sobering conclusion is drawn that heart stem cells do not exist. In other words, heart muscle that is lost due to a heart attack is irreplaceable . This finding -while disappointing- settles a long-standing controversy.
The authors saw that amazing events do happen post an infarction. Connective tissue cells (also known as fibro blasts) that are intermingled with heartmuscle cells respond vigorously to a myocardial infarction by undergoing multiple cell divisions. In doing so, they produce scar tissue that replaces the lost cardiac muscle. While this scar tissue contains no muscle and thus does not contribute to the pump function of the heart, the fibrotic scar ‘holds together’ the infarcted area. Indeed, when the formation of the scar tissue is blocked, the mice succumb to acute cardiac rupture. Thus, while scar formation is generally seen as a negative outcome of myocardial infarction, the authors stress the importance of the formation of scar tissue for maintaining the integrity of the heart.
ksparthasarathy 