Background: An isolated vascularized bone marrow transplant model (iVBMT) was previously developed in the rat in order to study the contribution of the bone marrow and its environment in a composite tissue allograft (CTA). It was an intraperitoneal model that required a long laparotomy time. This study aimed to develop an extraperitoneal model of iVBMT. Methods: For evaluating technical feasibility, Sprague-Dawley rats (n=5) were used and sacrificed at 24 hrs. For the long-term study, Lewis-Brown-Norway rats (n=5) were used and sacrificed at various time points from 2 to 8 weeks. The iVBMT model consisted of a left donor femur that was harvested with its nutrient vessels. The graft was anastomosed to the right femoral artery and vein of the recipient in an end-to-end fashion, and placed subcutaneously in the abdominal wall. Four micrometer sections of the graft were H&E stained and examined at 100x. Results: Technique - Mean ischemic time was 103 minutes. Four out of five vessels were patent at 24 hrs. Long term - all animals appeared well, and the position of the graft did not interfere with normal movements. Mean ischemic time was 80 minutes. The right limb functioned normally postoperatively. At sacrifice, five out of five vessels were grossly patent. Histology of the grafts in the long term animals showed a normal appearing bone marrow. Discussion: An extraperitoneal model is possible based on the recipient's femoral vessels and a subcutaneous placement of the graft. One thrombosed graft vessel in the technical feasibility study was due to excess manipulation at time of anastomosis. Abundant collateral vessels in the rat's limb permitted ligation of the femoral vessels without morbidity. Using the iVBMT model, future studies across semiallogeneic and allogeneic barriers will help to define the role of the bone marrow compartment in a CTA for potentially inducing immune tolerance.
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