New research lends increasing support to the notion that paralysis may not be so permanent after all.

Scientists at the University of California, San Diego have generated stem cells that, when grafted onto the injured spines of rats—traverse through the injury sites, coupling with nerve cells hidden beneath the damaged tissue. These results, published today in the journal Neuron, are a critical next step towards using stem cell-technology to reverse spinal cord injury—a condition that has long been considered irreversible.

The extension of human axons into host adult rat white matter and gray matter three months after spinal cord injury. [Credit: UCSD School of Medicine]

This research team, led by CIRM grantee Dr. Mark Tuszynski, generated stem cells from the skin cells of an adult human male. These so-called induced pluripotent stem cells, or iPS cells, then had the ability to transform into virtually any cell type. With a bit of coaxing, the team transformed them one more time—into early-stage neurons—and grafted them onto the injured rats. After monitoring the animals over a period of three months, what they began to see astonished them.

The most amazing changes came from the cells’ axons—long, spindly projections that connect neurons to each other, allowing them to communicate through transmission of electrical signals. Much to their surprise, the team saw these iPS cell-derived axons began to grow—some extending across the animals’ entire central nervous system.

But it wasn’t just the fact that the axons grew that excited researchers—it’s where they went. They began to pierce through the spinal injury sites, penetrating scar tissue and grey matter and forming connections with existing rat neurons that had been entombed inside. Even more incredibly, the native rat axons began to do the same—growing and piercing through the iPS cell grafts to form connections of their own.

As Tuszynski explained in a news release:

“These findings indicate that intrinsic neuron mechanisms readily overcome the barriers created by a spinal cord injury to extend many axons over very long distances, and that these capabilities persist even in neurons that have been reprogrammed.”

The results of this study are encouraging, say the research team, though they do raise a few questions about the underlying signaling mechanisms that are guiding these axons to grow and become intertwined. Tuszynski elaborated:

“The enormous outgrowth of axons to many regions of the spinal cord and even deeply into the brain raises questions of possible side effects if axons are mis-targeted. We also need to learn if the new connections formed by axons are stable over time, and if implanted human neural stem cells are maturing on a human time frame—months to years—or more rapidly.”

The researchers are now exploring whether using different types of stem cells, such as embryonic stem cells, would yield similar results. Once they hone in on the best method, they hope to take their findings further down the path towards clinical trials.

“Ninety-five percent of human clinical trials fail,” explained Tuszynski. “We want to determine as best we can the optimal cell type and best method for human translation so that we can move ahead rationally and, with some luck, successfully.”