Stem Cell Homing: What Happens Post-Injection?

Stem Cell Homing: What Happens Post-Injection?

Although stem cell research has advanced at a phenomenal rate, what those cells actually do once injected into the horse's body and whether they stay put remain a mystery.

Photo: The Horse Staff

There's no place like home, but where is home for a stem cell, exactly? Although stem cell research has advanced at a phenomenal rate (equine veterinarians can now isolate, process/culture, and inject stem cells into horses to treat a growing number of conditions), what those cells actually do once injected into the horse's body and whether they stay put remain a mystery.

At the 2011 North American Veterinary Regenerative Medicine Association Conference, held June 2-4, in Lexington, Ky., several researchers presented their data on stem cell "homing," and the results aren't quite what any of the groups expected.

Alan Nixon, BVSc, MS, Dipl. ACVS, and Ashlee Watts, DVM, Dipl. ACVS, both surgeons at Cornell University's College of Veterinary Medicine, discussed a recent study in which they hypothesized the stem cells would "engraft" to damaged cartilage in arthritic joints, but not the healthy cartilage in normal joints.

After tracking injected stem cells labeled with fluorescent nanoparticles that they injected into 29 joints--17 healthy and 12 osteoarthritic fetlock or stifle joints--from 10 horses, the team found the stem cells didn't engraft to the cartilage in any of the horses. Instead of engrafting to cartilage, the cells were found in the lining of the joint, called the synovial membrane.

One consequence of this finding could be that stem cells are not useful for treating osteoarthritis; however, according to the researchers, another possibility is that the cells work differently than originally thought, such as possessing secreting factors that influence the joint.

"If MSCs (stem cells) by intra-articular injection are effective in reducing joint disease, it may be through modulation of synovial fluid constituents, inflammation, or cytokine (inflammatory mediator) profile," concluded Nixon and Watts.

Another group of researchers, led by Roger Smith, VetMB, PhD, DEO, Dipl. ECVS, MRCVS, a professor of equine orthopedics at the Royal Veterinary College in the United Kingdom, also found that the stem cells they tracked didn't remain where the scientists believed they would.

Smith et al. radioactively labeled cells with either technetium-99m or a fluorescent red dye before injecting the cells into horses with naturally-occurring tendon lesions.

Key findings include:

  • A large number of cells were lost within the first 24 hours after injection into the lesion;
  • Cells persisted in the damaged tendon for up to 160 days post-injection, but only in small numbers; and
  • The cells did not spread throughout the tendon injury as much as expected, but remained in focal areas.

Because the cells appeared to show limited migration throughout the damaged site of the tendon, the researchers considered that this could limit stem cells' efficacy for treating tendon injuries.

"Alternatively, these findings could mean that stem cells function by signaling other cells to help repair the damaged tissue," Smith added.

Smith's future research will look at increasing the spread and retention of stem cells in a tendon lesion, different delivery techniques, and selecting the "right" cells from the bone marrow-derived cell cultures in the laboratory to inject into injured horses.

Although mixed results came from the studies presented, it's important to remember that stem cell therapy--and regenerative medicine as a whole--is still in its infancy. Researchers are working tirelessly to better understand how and why regenerative treatments work the way they do.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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