Starfish, salamanders, and planarian flatworms share a seemingly magical trait: the ability to regenerate body parts they've lost. While humans may never boast quite the same ability, scientists are perfecting ways to create different types of replacement tissue using stem cells or techniques that kick-start regrowth and development. Thanks to their efforts, the fabled "human spare-parts kit" may become a reality sooner than you think.
Patching Damaged Hearts
Early attempts to grow heart-repair patches with embryonic stem cells kept running into a glitch: because oxygen and nutrients only penetrated the outer edges of the patch, cells in the center died. Researchers at the University of Washington tested a novel strategy, mixing stem cell–derived vascular cells with heart-muscle cells that were also created from stem cells. The resulting cardiac-tissue patches formed networks of blood vessels that kept them alive and nourished. When the tissue patches were transplanted into rats, the new vessel networks linked up with existing vessels, raising hopes that the patches could someday provide a long-term repair solution for damaged human hearts.
Culturing Lab-Grown Lungs
In theory, embryonic stem cells can turn into hundreds of different types of tissue, but coaxing them to become one precise type isn't always easy. Researchers at the Free University of Brussels scored a regenerative coup by turning human stem cells into lung epithelial tissue. Their secret? They grew the stem cells in an air–liquid culture that replicated conditions in the human trachea, which encouraged the cells to differentiate into the kind of tissue appropriate for the environment. If lab-grown lung tissue proves its mettle in clinical studies, the tissue could allow sufferers of cystic fibrosis and other pulmonary diseases to avoid lung transplants.
Regenerating Spinal Cords
Spinal cord injuries have typically been considered permanent because the nervous system generates dense scar tissue when it's damaged, which prevents new nerves from regrowing. Researchers at the Georgia Institute of Technology and Emory University have isolated a stable version of an enzyme that digests this scar tissue over time, clearing the way for the body's natural repair mechanisms to work. "Repairing spinal cord injuries is going to take a combination of therapies—controlling inflammation after injury, overcoming inhibition due to scarring, and stimulating nerves," says biomedical engineer Ravi Bellamkonda. "We've made one useful step in the direction of making it possible."
Growing Back Arms and Legs
The humble zebra fish can regrow damaged fins and other body parts with aplomb, but for many years, no one knew exactly what gave the fish that ability. Now researchers at the Salk Institute for Biological Studies have zeroed in on a crucial developmental sequence that allows the fish to go back in biological time. Scott Stewart and his team discovered that after a fin is amputated, enzymes called demethylases help switch on genes that encode development of the missing fin, mobilizing cells in the area to rebuild the fin from scratch. If similar processes turn out to govern human limb development, this research could bring scientists closer to the holy grail of regrowing amputated arms and legs.
Implanting (Natural) Breast Tissue
Today's breast and soft-tissue implants have drawbacks: Saline breast implants burst, and the body sometimes rejects or reabsorbs foreign tissue implants. Jeremy Mao, a tissue engineer at Columbia University, has discovered an alternate strategy—using adipose (fatty) stem cells to grow the implants. Mao placed the stem cells in tiny channels carved into a hydrogel base and added a blood-vessel growth factor, which ensured the developing soft tissue received enough blood circulation to stay alive. "When you regenerate on a clinically relevant scale, your system may not be able to handle all the new tissue," Mao says, in which case the tissue may wither away due to lack of blood circulation. "Your existing vascular supply can benefit from some extra help."
