Adult stem cells and repair through granulation tissue

Macrophage recruitment and proliferation of both small vessels ( endothelium and pericytes) and fibroblast-myofibroblasts are the fundamental and provisional cellular findings in repair through granulation tissue (RTGT). Endothelium and pericytes of preexisting microvasculature may act as progenitor cells of new endothelial cells and new pericyte-fibroblast-myofibroblasts, respectively. Likewise, fibroblasts may be progenitors of themselves, and of myofibroblasts and pericytes. Moreover, all these cells may originate from circulating progenitor cells or other progenitor cells.. According to this extensive cellular plasticity, this work reviews the adult stem cells (ASC) and transit-amplifying cells (TAC) related to the principal cellular components of RTGT. Moreover, we hypothesize that the perivascular region, with a heterogeneous pericyte-like cellular population, including pericytes, perivascular fibroblasts and homing cells from the bone marrow (fibrocytes and bone marrow mesenchymal cells), is the niche of progenitor cells in RTGT and the substrate of regulatory mechanisms ( perivascular niche hypothesis). We also highlight RTGT as a "paracrine transitional organ" during involutive phenomena and cellular differentiation. Furthermore, we consider the combined role of both systems (ASC-TAC and RTGT) in tissue engineering and in pathological processes, such as fibrosis, organization, atherosclerosis, and tumor stroma.