They were a couple of hard-luck cases: Two horses with torn suspensory ligaments. For close to two years they were lame, their injuries unresponsive to conventional therapies, with no effective means of resolution in the offing. Recalls Doug Herthel, DVM, Alamo Pintado Equine Medical Center, Los Olivos, Calif., "I was disgusted with laying up horses for six to 12 months and having them come back lame. At that time, veterinarians tried laser treatments, infrared treatments, ultrasound therapies, and just about every kind of injection known to mankind. So we thought, "Why not inject regenerative cells containing growth factors and fibrin into the torn ligaments and see what happens. "

What happened, back in 1995, was the two horses became better within a couple of months. Since then, Herthel has used this means of treatment, now known as stem cell therapy, on more than 2,000 horses, with the majority demonstrating remarkable improvement. Other veterinarians have followed suit.

Stem Cell Therapy 101

Stem cells are immature, vigorous cells that mature into any of the hundreds of various body cell types--nerve, blood, heart, muscle, fat, bone, cartilage, etc. They are what turns an embryo into a fetus into a baby into an adult. They have plasticity, meaning they are adaptable and capable of being manipulated into becoming different types of cells.

"After harvesting and concentrating the stem cells in a laboratory, the stem cells and bone marrow are transferred into the damaged ligament or tendon," Herthel says. "They reproduce into normal, healthy tissue, thus improving healing and providing stability to the injured or diseased area. They also reduce inflammation and relieve pain."

It's this type of adaptability that is so promising for treating equine tendons, ligaments, joints, and bones. Often, injuries at these sites cannot repair themselves to a stable, pre-injured status. Adult cells are geared toward maintaining the status quo, possessing a limited capacity for replicating normal cells. Often instead of producing new, healthy, normal tissue after an injury, scar tissue is produced, and scar tissue is often a weaker, inferior tissue, making the recently healed site prone to re-injury.

Explains Rich Redding, DVM, MS, Dipl. ACVS, clinical associate professor of equine surgery, Veterinary Teaching Hospital, North Carolina State University (NCSU), "It's preferable to have everything heal by intrinsic processes in which the normal cells (tenoblasts and tenocytes) that reside in the tendon tissue proliferate and produce normal collagen and matrix to repair the tendon. Unfortunately, many tendon and ligament injuries are so extensive that they must heal by extrinsic pathways in which cells from outside of the tendon/ligament migrate into the damaged structure and predominate the repair process."

In the superficial digital flexor tendon, for example, healing occurs when cells migrate from paratendinous tissue (tissues adjacent to the tendon) into the damaged tendon, filling the defect or injury with granulation tissue (immature collagen and capillaries). "Most of this collagen is a weaker type of collagen," Redding says. "In reality, it is not that the tendon cannot heal--it can. The problem is that the injury heals with a tissue that has different mechanical properties than normal tendon tissue, typically with less elasticity. It is the junction or interface between normal and abnormal tissue that becomes the weak link and the site of re-injury in the future."

But when manipulated stem cells are injected into the injured site, they grow into normal or near-normal tissue.

"The advantage to stem cell therapy is the quality of healing that can be obtained in the implanted tissue," says Christopher Johnson, DVM, MS, Dipl ACVS, of the Equine Division of Woodford Veterinary Clinic, Versailles, Ky. "The goal of stem cell therapy is to improve the quality of the tissue in the injured area during the healing process--as close to normal tissue as possible."


Despite the promising outlook for stem cell therapy in horses, the political and media fixation occurring on the human side of stem cell therapy research has lead to a somewhat constipated state in research. In human medicine, stem cell therapy is an emotional issue, swirling around the ethics of using stem cells harvested from embryonic tissues that are frozen, days-old human embryos from fertility clinics. But stem cells reside in--and can be harvested from--a variety of sources, not just from fertilized eggs and embryos. They can be harvested from bone marrow, fat, and umbilical cords--a point that's sometimes lost in the political maelstrom.

In fact, it appears that embryonic tissues are unlikely to be the best source of stem cells, the big reason: Tissue rejection. Because embryonic tissue comes from a donor, it is foreign tissue, and foreign tissue is sometimes rejected by the host patient.

Notes Herthel, "Horses tend to reject embryonic stem cells. (When that happens, the horse experiences swelling, pain, and lack of structural integrity.) In an unpublished study we did in conjunction with the University of Miami, we compared the use of embryonic equine stem cells, bone marrow stem cells, platelet-rich plasma, and a control group in treating ligament tears. The embryonic stem cell treatment gave us extremely rapid healing on ultrasound exam, but within six months, every case exhibited a rejection phenomena. The platelet-rich plasma technique caused significant scarring of the ligaments, while the stem cell bone marrow procedure enhanced healing without causing any scarring."

Cell Sources

The reason fertilized eggs and embryos from early developing fetuses were considered the best source of stem cells is because of their plasticity. Adds Herthel, "Embryonic cells were also considered to be less antigenic--that is, less likely to produce a rejection response--because these cells are so young. But it turns out they are potentially extremely antigenic and not any more plastic than mature stem cells."

Stem cells from the umbilical cord or the blood within the umbilical cord at the time of birth are other potential sources. These cells are extracted, then frozen until needed. Peter Clegg, Vet MB, PhD, Cert EO, Dipl. ECVS, MRCVS, professor at the University of Liverpool, writes, "These may be more efficient stem cells than those we can obtain from bone marrow in the adult horse." ("The Brave New World Of Stem Cell Therapy," Horse & Hound,

The obvious downside is this is a banking option available only at the time of birth.

Cells harvested from the patient's own bone marrow are showing great promise for several reasons. First, there is no risk of tissue rejection. Second, "many, many studies coming out now are showing that the adult mesenchymal stem cell (from which arises connective tissues of the body, blood vessels, and lymphatic vessels) in the human has tremendous plasticity," says Herthel. "The adult stem cell can become a muscle cell or cardiac cell or tendon cell, etc. It is just as plastic--maybe more so--than embryonic stem cells."

Third, blood marrow contains fibronectin (which aids cell migration through the area of injection) and growth factors (which enhance healing). Finally, this therapy could provide very quick pain relief, as was found on the human side when Herthel's group assisted several human orthopedic surgeons in using this procedure for Achilles tendon tears, patellar tendon (tendon that connects the patella to the tibia), quadriceps tendon (tendon that connects the long quadriceps muscle to the patella), and lateral epicondylitis tendonitis.

"In 2001, Alamo Pintado Equine Medical Center published and presented a scientific paper to the American Association Of Equine Practitioners (AAEP) in San Diego," says Herthel. "This paper reported the results of 100 suspensory injuries that were not treated with bone marrow stem cells and compared the results to 100 horses that had been treated with bone marrow stem cells. In the untreated group, less than 30% went back to work and stayed sound; in the treated group, 80% went back to work and stayed sound for at least one year."

Using a relatively simple surgical procedure, stem cells are extracted from the patient's bone marrow. In previous reports, only a limited number of stem cells, about five to 10, are harvested from a typical bone marrow aspirate, says Redding. "At Vet-Cell in Great Britain, these cells are grown out to sufficient numbers (millions) in a cell culture laboratory, which takes several weeks. This delays the injection until well after the acute stage of injury."

At Herthel's lab, the procedure is a little different. "We do not grow the cells--it takes too long and there is no assurance of what they will do in vivo after being propagated," says Herthel. The latest technique is to concentrate the native stem cell population in bone marrow or fat at the time of surgery, then inject them along with bone marrow into the damaged ligament or tendon."

Fat is another viable source of stem cells. "These cells are harvested from the tail head of the patient horse, then sent to Vet-Stem, the only commercially available laboratory in the United States that does this procedure," says Redding. "There are many types of cells other than stem cells in the sample. Vet-Stem utilizes a proprietary flotation technique to recover the stem cells (and other cells) from the fat sample. The sample is then sent back for use usually in 48 hours. If a large sample of fat is harvested, the extra cells can be frozen for later use."

The short turnaround time allows the cells to be injected soon after the initial injury. A recent research study evaluated tendon healing after treatment with fat-derived stem cells at six weeks using a collagenase induced tendon injury model. This study found that the fat-derived stem cells developed improved overall healing scores over controls. However, more clinical research needs to be performed to assess the optimum use and the effectiveness of fat-derived stem cells in clinical practice.

Best Uses for Stem Cells

Although stem cell therapy is still undergoing exploration, the therapy is available to horse owners who either have the funds or the insurance to pay for the procedure. Vet-Stem lists around 100 veterinarians at their web site who use this technology. "Any vet with a diagnostic ultrasound machine can harvest and inject the stem cells," reports Redding.

At NCSU, veterinarians have used fat-derived stem cells for a variety of tendon and ligament injuries. "Most of the injuries have been discrete core-type defects in the superficial and deep digital flexor tendons and in the suspensory ligament," Redding reports. "We have also used stem cells for a severe collateral ligament injury of the fetlock in a Belgian cross with a very good outcome and in some meniscal injuries of the stifle joint with marginal success."

NCSU researchers hope to establish an on-site laboratory where they can do bone marrow stem cell recovery and culture.

Based on his investigations and other research, Herthel currently employs bone marrow stem cell therapy for several conditions. "We use this technique on suspensory ligaments, check ligaments, superficial and deep flexor tendons, collateral ligaments of the pastern, coffin, fetlock, and femoral tibial ligaments, sacroiliac ligaments, anterior cruciate ligaments, and menisci of the stifle joint," explains Herthel. "We have also used this as an intra-articular treatment on the pastern joints and stifle and hocks, and for osteochondrosis dissecans (OCD) cysts of the pastern joints, medial condyle of the femur, and fetlock joints."

Additionally, Herthel sometimes combines stem cell therapy with HBOT (hyperbaric oxygen therapy implemented via a high-pressure oxygen chamber) and nutritional therapy when treating tendon and ligament injuries.

"A very recent scientific paper about HBOT therapy in people has shown that the number of circulating stem cells in blood can be increased eightfold with HBOT therapy," he explains.

Costs for bone marrow stem cell therapy is about $1,400 to $1,800, including anesthesia and the procedure, says Herthel.

"There are very few riding or performance horses that cannot justify the procedure based on the results and prognosis," he states. "Besides, insurance has paid for all of the horses that had medical insurance. Insurance companies love this, because it saves them millions in loss of use payouts."


Investigations continue on identifying the best sources for stem cells, accessing the efficacy of stem cell therapies, and uncovering the conditions that are amenable to stem cell therapy.

"The future of this treatment is wide open," Redding says. "Veterinarians are now using stem cells in a variety of clinical conditions beyond tendon and ligament injuries, including severe osteoarthritis of high range motion joints like the stifle, tarsus, and fetlock, and in subchondral bone cysts of the medial femoral condyle of the stifle, fetlock, and pastern. The human literature is filled with basic research utilizing stem cells (both embryonic and adult) in a variety of animal models. Surgically created medial meniscus injuries in goats showed significant improvement in healing when stem cells were injected into the joint when compared to the control joints that did not show any significant response. Cardiac infarction models in calves showed revascularization when cells were injected in the coronary vessels supplying the myocardium; in other words, there may be some application for cardiac disease where it might only need to be injected in the vascular system to supply to the damaged area. Recent reports demonstrated that injection of embryonic stem cells into paralyzed mice (due to spinal cord injury) allowed nerve conduction and the ability to walk.

"However, there needs to be more clinical research to support its use in a variety of injuries and confirm the optimum timeframe for their use," continues Redding. "Prospective clinical trials will be necessary to determine which injuries will benefit from stem cell injection. For example, there may very well be certain injuries that heal sufficiently without stem cell treatment, while others may not show improvement in spite of stem cells."

Herthel is presently studying the benefits of cells by concentrating the native stem cell population of bone marrow and fat with a centrifugation process; this procedure would also allow immediate use after collection of the cells instead of having to waits for days or weeks to begin treatment.

Other new therapies are being evaluated as well, including platelet-rich plasma and ACell. The latter is a matrix material (porcine bladder submucosa) product that is without cells (and thus not a stem cell therapy), Redding states. "ACell therapy has been reported to create considerable pain after injection, which implies it creates an intense inflammatory reaction. It is assumed that this inflammatory reaction may precipitate extrinsic repair. However, this pain response has been reported to be controlled with aggressive anti-inflammatory therapy pre-injection."

Although ACell is currently unavailable in North America due to patent litigation, a company spokesperson said they expect to resume normal business operations in the United States in June or July of this year. Updates concerning the patent litigation are posted at

Concludes Johnson, "The final goal will be not to repair but regenerate normal tissue in damaged areas using stem cell therapy. The current main focus in the equine patient is tendon and ligamentous injuries, but there may be many other surgical and medical conditions that could benefit from advances in stem cell research. I believe stem cell therapy is going to change the way we look at and treat a lot of conditions, but finding the right situations and combining the correct modalities is going to be the key."

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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