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New therapies for the treatment of equine lameness (Proceedings)

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Oct 01, 2008

Stem Cell Therapy

Stem cells are primitive cells present in almost every tissue that have the ability to differentiate into different types of tissue. There are basically two types of stem cells that can be utilized: embryonic and adult stem cells. The source of embryonic stem cells is the inner mass of the embryo, so that in the process of recovering the cells, the embryo is destroyed. These cells have the ability to differentiate into all tissues (toti-potent). When used in clinical situations, the use of embryonic stem cells introduces foreign DNA, so some form of immunosuppression would be necessary. Experimentally, embryonic stem cells are unpredictable and can form teratomas in-vivo. The source of adult stem cells is any adult tissue. These cells can differentiate into many, but not all tissues (multi-potent). The tissues that are of most interest to equine practitioners include bone, muscle, tendon, and ligament. These cells have never been seen to form teratomas or tumors in vivo.

Stem cell therapy has been coined "regenerative medicine" because the goal is to regenerate tissue and provide tissue homeostasis, rather than just healing by fibrosis. Tissue regeneration should maximize the strength, range of motion, and performance of the tissue. At the same time regeneration of tissue reduces scarring, pain, and re-injury. Regenerative medicine supplies the three major components necessary for tissue repair: regenerative cells, extra-cellular matrix, and growth factors. The regenerative cells also recruit other cells necessary for tissue healing.

The two major sources of adult stem cells include bone marrow and adipose tissue, although muscle, skin, brain and other tissues contain adult stem cells. It is estimated that 1 in 50 cells (2%) in adipose tissue are stem cells, whereas 1 in 100,000 cells (0.001%) in bone marrow are stem cells. In addition to the high percentage of stem cells in adipose tissue, fat tissue is easy to access in the horse, is a renewable resource, and because fat floats, it is easy to separate the cells. In addition to providing mesenchymal stem cells, adipose tissue can also provide endothelial precursor cells, growth factor producing cells, B and T lymphocytes, macrophages, natural killer cells, preadipocytes, fibroblasts, smooth muscle cells, and endothelial cells. It is not known what role these other cells play in tissue regeneration. Currently, the FDA does not regulate the use of autologous adipose derived stem cells if they are minimally manipulated. Culturing of stem cells to propagate a cell line is not considered minimally manipulating cells so therefore is regulated by the FDA.

The potential uses for stem cell therapy in equine medicine include soft tissue disorders (tendon/ligament), osteoarthritis, osteochondrosis, fractures, or other degenerative processes such as laminitis. In my practice, I will use adipose derived stem cell therapy primarily for tendon, suspensory, or suspensory branch lesions. I have used these cells for other indications including osteoarthritis, navicular disease, and osteochondrosis, but the results are not as favorable for bone or joint problems as they are for soft tissue disorders.

Fat tissue can be harvested from any site on the horse, but lateral to the tail head seems to be most appropriate. This can be readily done through a small lipectomy incision in the standing animal with local anesthesia. This is the most consistent method of collecting fat, but will leave a scar and a small depression at the surgical site. If cosmetics are of major concern, the inguinal area is also rich in adipose tissue, but this requires general anesthesia. More recently, I have been performing liposuction at the tail head using liposuction cannulas designed for human patients. This technique is most appropriate for horses having at least 1 cm of tail head fat as measured by ultrasound. Liposuction is performed bilaterally through very small punctures in the skin, so the post operative cosmetic outcome is excellent. I use ultrasound to guide my liposuction cannulas so that the contamination with muscle tissue is minimized.

The tissue is sent overnight on ice to the laboratory, where the fat is digested, leaving regenerative cells and their supporting cells and matrix. The dose volume can be requested based on the lesion being treated. Typically, a dose of approximately 5 million cells are recommended to treat a single lesion. If a large quantity of cells is recovered, additional doses can be frozen for later use. The turnaround time from fat harvest to arrival for injection is 3 days. When the cells arrive, they are typically injected into the desired lesion using ultrasound guidance following aseptic preparation of the area. Depending on the site and condition, I often combine the stem cells with platelet rich plasma at the hospital just prior to injecting. The aftercare is minimal, as the cells are autologous so there is no detectable tissue reaction. Follow-up examinations and rehabilitation are tailored to the individual and problem being treated.