CHARACTERIZATION OF STROMAL PROGENITOR CELLS ENRICHED BY FLOW-CYTOMETRY

The progenitors for cells of bone, cartilage, fat, and muscle are thou ght to be derived from mesenchymal stem cells but despite extensive st udy of stromal cell differentiation, neither mesenchymal stem cells or the more committed, tissue-specific progenitors have been well-charac terized. In this study we used flow cytometry to isolate from fetal ra t periosteum a population of small, slowly cycling cells with low cyto plasmic granularity (S cells) that display stem cell characteristics. On plating, S cells exhibited a 90% higher labeling index with [H-3]-t hymidine compared to unsorted cells and when grown in culture generate d cartilage, adipocyte, and smooth muscle phenotypes, in addition to b one. Only the S-cell population showed extensive self-renewal of cells with osteogenic potential. Electron microscopy showed that S cells ha ve high nuclear:cytoplasmic ratios with large condensed nuclei and a p aucity of cytoplasmic organelles. Freshly sorted suspensions of immuno cytochemically stained S cells did not express differentiation-associa ted markers such as type I, II, and III collagens, alkaline phosphatas e, or osteopontin, However, after attachment, S cells became immunopos itive for collagens I, II, III, osteopontin, and also for the cell sur face receptor CD44, which mediates cell attachment to hyaluronan and o steopontin. These studies show that viable osteogenic precursor cells with the stem cell characteristics of self-renewal, high proliferative capacity, and multipotentiality can be enriched from heterogeneous st romal cell populations with simple flow cytometric methods. These cell s may be useful for regeneration of stromal tissues. (C) 1997 by The A merican Society of Hematology.