Regenerative medicine using mesenchymal stem cells for the purposes of tissue repair has garnered considerable public attention due to the potential of returning tissues and organs to a normal, healthy state after injury or damage has occurred. To achieve this, progenitor cells such as pericytes and bone marrow-derived mesenchymal stem cells can be delivered exogenously, mobilised and recruited from within the body or transplanted in the form organs and tissues grown in the laboratory from stem cells. In this review, we summarise the recent evidence supporting the use of endogenously mobilised stem cell populations to enhance tissue repair along with the use of mesenchymal stem cells and pericytes in the development of engineered tissues. Finally, we conclude with an overview of currently available therapeutic options to manipulate endogenous stem cells to promote tissue repair.

We have used immunohistochemical and histochemical techniques to identify patches of hepatocytes deficient in the enzyme cytochrome c oxidase, a component of the electron transport chain and encoded by mitochondrial DNA (mtDNA). These patches invariably abutted the portal tracts and expanded laterally as they spread toward the hepatic veins. Here we investigate, using mtDNA mutations as a marker of clonal expansion, the clonality of these patches. Negative hepatocytes were laser-capture microdissected and mutations iden- A dult tissue-specific stem cells are thought to reside within a specialized microenvironment, known as the niche, and it is here that stem cell behavior is regulated and maintained. 1 In epithelia with ordered structure and in a state of continual cell renewal, there is often a hierarchical organization with stem cells at the beginning of the flux, and terminally differentiated, reproductively sterile cells at the end of the flux, imminently to be lost from the population. Many studies have attempted to identify stem cells and the location of the niche using histological methods based on the premise that stem cells have inherent properties such as DNA label retention, high integrin expression, and abundant detoxifying enzyme activity. 2 However, many uncertainties remain even in comparatively well-defined instances such as the hematopoietic system. 3 It has been proposed that the gold standard of stem cell identification involves marking putative stem cells to identify the niche, and then performing lineage tracing to demonstrate that the proposed "stem cell" has multipotentiality. 4 This approach commonly uses mice genetically engineered to have a steroid-activated version of Crerecombinase knocked into the putative stem cell marker gene, such that Cre activation mediates excision of a roadblock sequence in the Rosa26-lacZ reporter, thus resulting in an irreversible marker in all the descendants of the putative stem cell. Using this technology, it has been shown, for example, that mouse hair follicle bulge cells expressing K15 generate all the epithelial cells in the lower hair follicle, 5,6 whereas in the murine small intestine, long-lived cell clones containing all the intestinal cell lineages can be generated from both leucine-rich repeat-containing G protein-coupled receptor 5 [Lgr5]-expressing 7 and polycomb ring finger oncogene [Bmi1]-expressing Abbreviations: 3D, mtDNA: mitochondrial DNA; PBS, SDH, succinate dehydrogenase. From the

Complex biomolecules absorb in the mid-infrared ( ‫؍‬ 2-20 m), giving vibrational spectra associated with structure and function. We used Fourier transform infrared (FTIR) microspectroscopy to "fingerprint" locations along the length of human small and large intestinal crypts. Paraffinembedded slices of normal human gut were sectioned (10 m thick) and mounted to facilitate infrared (IR) spectral analyses. IR spectra were collected using globar (15 m ؋ 15 m aperture) FTIR microspectroscopy in reflection mode, synchrotron (<10 m ؋ 10 m aperture) FTIR microspectroscopy in transmission mode or near-field photothermal microspectroscopy. Dependent on the location of crypt interrogation, clear differences in spectral characteristics were noted. Epithelial-cell IR spectra were subjected to principal component analysis to determine whether wavenumber-absorbance relationships expressed as single points in "hyperspace" might on the basis of multivariate distance reveal biophysical differences along the length of gut crypts. Following spectroscopic analysis, plotted clusters and their loadings plots pointed toward symmetric ( s )PO 2 ؊ (1,080 cm ؊1 ) vibrations as a discriminating factor for the putative stem cell region; this proved to be a more robust marker than other phenotypic markers, such as ␤-catenin or CD133. This pattern was subsequently confirmed by image mapping and points to a novel approach of nondestructively identifying a tissue's stem cell location. s PO 2 ؊ , probably associated with DNA conformational alterations, might facilitate a means of identifying stem cells, which may have utility in other tissues where the location of stem cells is unclear.

Methods for lineage tracing of stem cell progeny in human tissues are currently not available. We describe a technique for detecting the expansion of a single cell's progeny that contain clonal mitochondrial DNA (mtDNA) mutations affecting the expression of mtDNA-encoded cytochrome c oxidase (COX). Because such mutations take up to 40 years to become phenotypically apparent, we believe these clonal patches originate in stem cells. Dual-color enzyme histochemistry was used to identify COX-deficient cells, and mutations were confirmed by microdissection of single cells with polymerase chain reaction sequencing of the entire mtDNA genome. These techniques have been applied to human intestine, liver, pancreas, and skin. Our results suggest that the stem cell niche is located at the base of colonic crypts and above the Paneth cell region in the small intestine, in accord with dynamic cell kinetic studies in animals. In the pancreas, exocrine tissue progenitors appeared to be located in or close to interlobular ducts, and, in the liver, we propose that stem cells are located in the periportal region. In the skin, the origin of a basal cell carcinoma appeared to be from the outer root sheath of the hair follicle. We propose that this is a general method for detecting clonal cell populations from which the location of the niche can be inferred, also affording the generation of cell fate maps, all in human tissues. In addition, the technique allows analysis of the origin of human tumors from specific tissue sites. STEM

These data clearly demonstrate the involvement of circulating fibroblastic precursor cells in the development of human fibrosing lung lesions and provide evidence that these cells are most probably bone marrow derived. These results may open new venues regarding the prevention of fibrosis in lung transplants and potentially in other organs.