Résumé :
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Communication n° 324. Stem cells reside in discrete anatomic niches that control their fate. Stem cell niches are often ill-defined but their organization can be conveniently studied in skeletal muscle where stem cells ensuring postnatal skeletal muscle growth and regeneration are easy to spot anatomically and immunohistochemically. These cells, called satellite cells, are located between basal lamina and sarcolemma of normal muscle fibers. They include stem cells and cells closer to terminal differentiation, and express typical markers, including NCAM/CD56 and muscle precursor cell markers such as M-cadherin, PAX7 and MYF5. Previous electron microscopic and teasing preparation studies have shown that satellite cells are found in increased numbers at motor endplates, and are evenly spaced along the remaining length of muscle fibers. In a previous study aimed at deciphering crosstalks between activated satellite cells and monocyte/macrophages during muscle regeneration, we observed satellite cells close to microvessels at histologic examination of human deltoid muscle (Chazaud, JCB 2003). This finding was virtually never pointed out in previous literature on satellite cells and remained of uncertain significance due to possible chance association between satellite cells and the dense microvascular bed of skeletal muscle. We present a study designed to substantiate this strikingly unappreciated characteristic of the muscle satellite cell niche. Close non-random association between satellite cells and capillaries was firmly established by global spatial analysis of skeletal muscle of different species (we used both Ripley's K function and the quadrat test) and by carefull confocal and electron microscopic studies. GFP+ bone marrow transplantation experiments showed that bone-marrow derived cells found in skeletal muscle within sublaminal niches were M-Cadherin+ Pax7+ NCAM+ and were located at the same distance of vessels than control satellite cells (Dreyfus, AJP 2004). In addition, consistently with the vascular niche of hematopoïetic and neural stem cells, endothelial cells, compared to fibroblasts, smooth muscle cells and other cell types, were shown to exert a specific influence on growth of satellite cells, suggesting invariable organization and function of stem cell niches throughout the body.
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