Here's a link to a video of a surgical procedure at the New England Equine Practice in Patterson, New York. This is one type of procedure for stem cell therapy. The surgeon extracts bone marrow from the horse's sternum to harvest stem cells, which are then injected into the tendon or ligament that is damaged.
The video is posted at this link:
More information on this type of therapy, which was developed by surgeon Roger Smith at the Royal Veterinary College in England, is at the web site of the Vetcell company, marketers of the procedure.
You can also read an article about New England Equine Practice's new surgical facilities and hospital.