Osteoprogenitor cells of mature human skeletal muscle tissue: An in vitro study

The presence of osteogenic progenitors in human skeletal muscle is suggeste d by the formation of ectopic bone in clinical and experimental conditions, but their direct identification has not yet been demonstrated. The aims of this study were to identify osteogenic progenitor cells in human skeletal muscle tissue and to expand and characterize them in culture. Specimens of gracilis and semitendinosus muscle were obtained from young adults and dige sted to separate the connective tissue and satellite cell fractions. The ce lls were cultured and characterized morphologically and immunohistochemical ly using antibodies known to be reactive with primitive osteoprogenitor cel ls, pericytes, intermediate filaments, and endothelial cells. Alkaline phos phatase activity and osteocalcin gene expression were also determined. In t he early stages of culture, the connective tissue cells obtained were highl y positive for primitive osteoprogenitor cell and for pericyte markers. Alk aline phosphatase activity was detectable at early stages of culture and ro se as a function of time, whereas primitive osteoprogenitor cell markers de clined and osteocalcin mRNA expression became detectable by reverse transcr iptase-polymerase chain reaction (RT-PCR). It is shown that human skeletal muscle connective tissue contains osteogenic progenitor cells. Their identi fication as pericytes, perivascular cells with established osteogenic poten tial, suggests a cellular link between angiogenesis and bone formation in m uscle tissue. These cells are easily cultured and expanded in vitro by stan dard techniques, providing an alternative source of osteogenic progenitor c ells for possible cell-based therapeutic use in certain conditions. (C) 200 1 by Elsevier Science Inc. All rights reserved.