'Embryonic' Stem Cells and Tendonitis Repair (AAEP 2010)

Fifteen percent of racehorse musculoskeletal injuries are related to tendons, with half of these occurring in the forelimb, and 75% of the forelimb tendon injuries affecting the superficial digital flexor tendon. In eventing horses, 50% of injuries involve a tendon or ligament. Unfortunately, there is a high recurrence rate in affected animals, with 50% of horses reinjuring their tendons within two years. This is likely because tendons heal by repair through scar tissue formation rather than regeneration of tendon tissue. At the 2010 American Association of Equine Practitioners Convention, held Dec. 4-8 in Baltimore, Md., Ashlee Watts, DVM, Dipl. ACVS, described a new approach for regenerative therapy using fetal-derived "embryonic-like" stem cells for tendon repair.

Historically, regenerative therapy has involved the use of active molecules in the horse's own blood such as those found in platelet-rich plasma (PRP), with its high concentration of trophic (nutritional) constituents and growth factors, and stem cells derived from the horse's own bone marrow, or fat. These adult-derived stem cells can form tissues from a single germ line, the mesoderm (the embryonic origin of all musculoskeletal tissues, including tendon). Although targeting the germ line responsible for tendon development seems beneficial, it is possible that a stem cell with greater potency, or ability to form all three developmental germ layers, could lead to improved outcomes.

Embryonic stem cells (ESCs, cells derived from embryos or generated using genetic means) have improved growth potential and are pluripotent, meaning they can form any tissue type to produce optimal healing. Unlike stem cells derived from an adult horse, ESC sources are available off the shelf, or over the counter, with improved uniformity in the product. This is in contrast to cells isolated from the horse's own bone marrow or fat, where the horse's age and health status can affect the ability of the stem cells to repair injury. Embryonic stem cells are homogeneous (the same throughout) and, according to Watts, their greater potency might better contribute to repair and self-renewal for stronger tendon healing with less scar tissue.

Because of the difficulty in isolation of equine ESCs, Watts and colleagues used a cell line derived from fetal tissue. Traditionally classified as an adult stem cell, fetal-derived stem cells are embryoniclike in that they can be induced to express genes and proteins of pluripotency, essentially making them act like an ESC. Because of these similarities to ESCs, fetal-derived embryonic stem cells (fdESCs) have a lower stage of differentiation (are less committed to a single embryonic germ layer) and a greater self-renewal and expansion potential compared to adult stem cells from bone marrow or fat. FdESCs might also share beneficial characteristics with the adult derived stem cell, such as immune privilege, allowing 'non-self' cells to be safely used without risk of immune rejection.

Scientists isolate cell lines of fdESC from early fetal tissue, specifically the brain. In their study, Watts and colleagues Injected male fdESCs into tendon lesions of Thoroughbred and Thoroughbred-cross mares--this enabled tracking of fdESCs. Horses in the study were trained or raced animals that were treated one week after inducing tendonitis.

The team members performed sequential ultrasound examinations at regular two-week intervals until the study horses were euthanized at eight weeks after treatment injection. Ultrasound at four weeks revealed that tendon lesions had already reduced in size compared to placebo control treated tendons, but were not yet different in their accumulation of more normal fiber patterns. By eight weeks, there were significant improvements in fiber pattern and architecture in addition to reduced tendon and lesion size in the fdESC treated tendons, indicating improved healing as compared to control tendons. Post-mortem MRI exams on tendonitis samples demonstrated reduced size of the lesions and more normal tissue patterns compared to the placebo control group.

Treatment with fdESC did not cause post-injection inflammation, and investigators did not note any adverse reactions in the treated horses. New tendon cells were evident at eight weeks along with improved tendon architecture, as evidenced by better cell shape and orientation, filling in of tendon lesions, and correct fiber alignment. Watts stressed that fdESC treatment might elicit tendon regeneration rather than repair with connective scar tissue but that longer-term studies are required to confirm this finding.

Relevant to the equine athlete, she explained, better tendon healing could lead to reduced reinjury rates.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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