Bone marrow-derived cells in the healing burn wound--more than just inflammation

Scarring after severe burn is a result of changes in collagen deposition and fibroblast activity that result in repaired but not regenerated tissue. Re-epithelialisation of wounds and dermal cell repopulation has been thought to be driven by cells in the periphery of the wound. However, recent research demonstrated that cells originating from the bone marrow contribute to healing wounds in other tissues and also after incisional injury. We investigated the contribution of bone marrow-derived cells to long-term cell populations in scar tissue (primarily fibroblasts and keratinocytes) after severe burn. Wild-type mice were lethally irradiated and then the bone marrow reconstituted by injection of chimeric bone marrow cells expressing EGFP marker protein. Mice with chimeric bone marrow were then given a burn, either an 1-cm diameter injury (to mimic minor injury) or 2-cm diameter (to mimic moderate injury). Wounds were analysed at days 1, 3, 7, 14, 21, 28, 56 and 120 using FACS and immunohistochemistry to identify the percentage and cell type within the wound originating from the bone marrow. The inflammatory cell infiltrate at the early time-points was bone marrow in origin. At later time-points, we noted that over half of the fibroblast population was bone marrow-derived; we also observed that a small percentage of keratinocytes appeared to be bone marrow in origin. These findings support the theory that the bone marrow plays an important role in providing cells not only for inflammation but also dermal and epidermal cells during burn wound healing. This increases our understanding of cell origins in the healing wound, and has the potential to impact on clinical practice providing a potential mechanism for intervention away from conventional topical treatments and directed instead to systemic treatments affecting the bone marrow response.