A study published on Monday in the Proceedings of the National Academy of Sciences reached a disappointing conclusion: heart steam cells that might have repaired the heart muscle after a heart attack did not exist in adults.
It offered scientific evidence to a decision made in October by the United States health authorities to pause a clinical trial using stem cells to treat heart failure over concerns over the lack of "scientific foundations" of trial.
Researchers from the Hubrecht Institute in the Netherlands, the Ecole Normale Superieure de Lyon in France and the Francis Crick Institute London generated a cell-by-cell map of all dividing cardiac cells before and after a heart attack using advanced molecular and genetic technologies.
They recorded many types of cells divide upon damage in the mouse heart, but that none of these are capable of generating new heart muscle.
During a heart attack or myocardial infarction, the blood supply to part of the heart muscle is cut off, and those muscles might die.
Researchers and clinicians have previously searched for two decades for cardiac stem cells that are able to to replace lost tissue by cell division.
A clinical trial started in 2015 to test a type of stem cell in the heart called c-kit cells, but it was called a halt in October 29 after Harvard Medical School retracted 31 papers upon which the trial was based.
Many of the "false leads" of past studies called those cells cardiac stem cells which turned out to produce blood vessels or immune cells, but never heart muscle, according to the new study.
In other words, heart muscle that is lost due to a heart attack cannot be replaced.
The researchers also found that connective tissue cells known as fibroblasts, intermingled with heart muscle cells, would respond vigorously to a myocardial infarction by undergoing multiple cell divisions.
Those fibroblasts in mice can produce scar tissue that replaces the lost cardiac muscle, holds together the congested area but contains no muscle to improve the pump function of the heart, according to the study.
However, the scar tissue formation has its own function. When the formation is blocked, the mice suffer from acute cardiac rupture.