The aggregate nature of human mesenchymal stromal cells in native bone marrow.

BACKGROUND AIMS: The clinical application of mesenchymal stromal cells (MSC) faces several obstacles, such as the lack of a standard method for direct isolation as well as a low frequency and concern about the safety of their in vitro expansion. Although the density-gradient separation technique is used as the first step in most methods of MSC isolation to enrich mononuclear cells, the efficiency of this method has not so far been examined. This study was designed to address this issue.
METHODS: Human bone marrow (BM) samples were laid over Ficoll-Paque, and after centrifugation the upper and lower fractions were cultured separately. Surface markers, differentiation potential and the number of emerged cells were determined.
RESULTS: The isolated cells from both the upper and lower fractions were characteristic of MSC. Although it is commonly believed that MSC are single suspending mononuclear cells and so are enriched in the upper fraction of Ficoll-Paque after density-gradient separation, our data showed that considerable numbers of these cells were accumulated in the lower fraction. Further data indicated that MSC were actually present as cell aggregates in BM and they could be enriched effectively by a simple filtration method.
CONCLUSIONS: The aggregate nature of MSC in BM is in agreement with the concept that they are one of the main elements of the hematopoietic stem cell niche. In addition, the simple filtration method proposed here to isolate cell aggregates may provide opportunities for instant stem cell therapy without the need for extensive in vitro expansion.