It seems like science fiction, but researchers have actually grown organs from stem cells, organs that were successfully transplanted into humans. Two years ago, a man received a new trachea to replace his, damaged by cancer—the trachea was made by Swedish researchers who infused a synthetic scaffold with the patient’s own stem cells. Earlier, in 2006, scientists at Wake Forest used stem cells to successfully implant laboratory-grown bladders in young patients with spina bifida, a developmental birth defect.

Now, science has set its sights on even bigger lab-grown organs: hearts. Researchers are currently growing them in labs using scaffolds made of biomaterial which guide stem cells into becoming cardiomyocytes, the contracting cells that are basis of cardiac muscle.

Such stem cell research in humans comes with a host of ethical problems. However, a new study, published yesterday in the Journal of Clinical Investigation, suggests a different type of cell could do the job when it comes to artificially engineering new tissue. It involves a biological process that doesn’t exist in mammals: parthenogenesis

Parthenogenesis is a form of asexual reproduction that occurs naturally in plants, insects, fish, amphibians and reptiles. During this process, unfertilized eggs begin to develop as if they’ve been fertilized. For example, the entire species of marmorkrebs, a type of crayfish, is female, and the offspring produced, without any male contribution, are genetically identical to the mother.

In 2007, researchers induced human egg cells with chemicals mimicking fertilization so they would undergo the process. The result were parthenogenetic cells that share the same properties as embryos, except that they can’t grow further. The cells are akin to pluripotent stem cells derived from embryos, which means they have the ability to develop into different types of cells—including heart cells.

The German researchers in the new study used this knowledge to turn body cells of mice into parthenogenetic stem cells, which were then grown into mature, functional cardiomyocytes. Researchers used these cells to engineer myocardium–heart muscle–with the same structure and function of normal myocardium. The muscle was then grafted onto the hearts of the mice that had contributed the original eggs for parthenogenesis, where it worked the same way as existing muscle.

For humans, building heart muscle from parthenogenetic stem cell-derived cardiomyocytes in this way could overcome several hurdles, according to a new paper examining the implications of the German team’s discovery. A heart attack can destroy up to one billion cardiomyocytes. These cells can be regrown naturally by the body, but not quickly and not in significant quantities, which means tissue-engineered heart repair may become crucial for a full recovery.

Regeneration via stem cells could also mean the difference between life and death for heart transplant candidates. Approximately 3,000 people in the United States are on the waiting list for a new heart on any given day, but only 2,000 donor organs are available each year. But even if a person receives a new heart from a donor, there’s no guarantee the body will accept the new organ. A person’s immune system sees the new organ as a foreign object, which triggers a chain of events that can damage the transplanted organ. To prevent transplant rejection, patients are treated with immunosuppressive drugs, which can increase cancer risk, and most stay on at least one type of the medication for the rest of their lives. Hearts regrown from parthenogenetic stem cells, however, will likely eliminate organ rejection.

Parthenogenetic stem cells, which can be derived from cells readily made in the blood or skin, contain a genome inherited from only one individual—in this study, the mouse, and potentially in the future, a human patient. This means the cells are likely to be more compatible to the patient’s immune system—the body is less likely to reject organs grown from its own cells.

In humans, the process could remove embryonic stem cells from the equation, taking associated ethical questions with them.