On Sept. 24, 2020, a team of scientists announced they had created the first atlas of human heart cells, a collection of maps showing nearly half a million heart cells and identifying the role of each in the heartﾒs symphony. The researchers examined six regions in 14 healthy donor hearts, creating a detailed database that provides a new basis of comparison for studying heart disease, the leading cause of death worldwide.

To understand what’s going wrong in various forms of heart disease, “first we need to know what is normal,” says Howard Hughes Medical Institute Investigator Christine Seidman, a cardiovascular geneticist at Harvard University and director of the Cardiovascular Genetics Center at Brigham and Women’s Hospital. Seidman and colleagues describe the new heart atlas in the journal Nature.

Heart cells have proven particularly difficult to study. Unlike some cancer cells and other tissues, there are no heart cells that can be grown indefinitely in the laboratory and studied. Instead, much cardiac research is done using mice, whose hearts have important differences from human hearts.

Though the researchers studied a relatively small group of hearts (“fourteen people cannot replicate the world’s population,” Seidman says), the new atlas revealed some biological surprises. The team found previously unknown cell diversity in various parts of the heart. They also uncovered differences between the healthy hearts of males and females; females had a greater proportion of heart muscle cells, called cardiomyocytes, than males. That warrants more research, Seidman says, as those cells might hold clues to differences in heart disease between the sexes.

The atlas is part of the Human Cell Atlas initiative, an effort funded by the Chan Zuckerberg Initiative to map all the cell types in the human body. “It takes a big village to do this,” Seidman says. Her group worked with an international team of experts on everything from heart surgery to computational biology in order to create the database atlas.