Recent research advances in stem cell biology have raised the promise of cell-based therapies for the treatment of human diseases. While embryonic stem cells hold great potential for tissue engineering and more successfully managing the pathogenesis of various diseases, numerous political and of isolated from post-embryonic tissues circumvent some of these problems. With regard to fibrotic diseases of the skin, a cardinal feature is the excessive deposition of extracellular matrix by dermal fibroblasts. In one disease, systemic sclerosis (SSc) or scleroderma, the molecular and cellular mechanisms associated with this dysregulation of dermal fibroblast function are not clearly understood, however, it has been hypothesized that one possible mechanism is the abnormal activation of a subset of dermal fibroblasts to produce excess matrix. Since scleroderma is largely adult in presentation could this activation occur at the level of a fibroblast progenitor cell leading to the production a sub-population of hyperactive matrix producing cells? As first step toward addressing this question, basic information on dermal fibroblast progenitor cells must be developed. Are dermal fibroblast stem cells strictly dermal in origin or do they have an extra-dermal origin or a combination of the two? This grant proposal will test the hypothesis that dermal fibroblast stem cells are at least in part extra-dermal in origin and this origin is potentially the bone marrow. Two specific aims have been developed to address these hypotheses. One aim will utilize a parabiotic mouse model to determine if extra-dermal precursors are resident in the peripheral circulation. Parabiotic pairs will be surgically created, with one partner bearing a ColGFP (expressed in the dermal fibroblast) and the other partner being non-transgenic. Following the induction of fibrotic lesions by daily injections of bleomycin, skin samples from both members of the parabiotic pair will be evaluated for GFP expression. The presence of GFP expression in the non-transgenic parabiont is consistent with an extra-dermal origin of dermal fibroblasts and these progenitors freely circulate. To assess the hypothesis that bone marrow may be an extra-dermal source of cells participating in dermal fibrosis, bone marrow chimeras will be created via a bone marrow transplant. Non-transgenic recipients will receive injections of bone marrow cell preparations isolated from transgenic donor mice (ColGFP). After bone marrow chimerism has be successfully established, fibrotic lesions in the skin will again induced by local bleomycin injections. An observation of GFP expression in dermal fibroblasts in the induced fibrotic lesion is consistent with the hypothesis that the bone marrow can be a source of dermal fibroblast progenitors. It is anticipate that data collected in these studies will contribute to our knowledge of the origin of dermal fibroblasts. The long-term objective of this research as it relates to public health is to understand the origin of the dermal fibroblast, a cell important in the maintenance of the integrity of the skin and to determine the cellular and molecular mechanisms associated with differentiation of a dermal fibroblast stem cell into a mature matrix producing cell. We feel this knowledge is essential for the development of novel therapeutic approaches for the management of fibrotic skin diseases such as scleroderma as well as other potentially other conditions involving cells that produce connective tissue.