INTRODUCTION

Mesenchymal stem cells are multipotent adult stem cells that are present in multiple tissues, including umbilical cord, bone marrow and adipose tissue. Mesenchymal stem cells can self-renew by dividing and can differentiate into bone, cartilage, muscle, adipocytes, and connective tissue. The International Society for Cellular Therapy (ISCT) has recommended the use of the name multipotent mesenchymal stromal cell (also abbreviated to MSC) for the in vitro cultured cells, restricting the term stem cell to designate the proposed in vivo precursors/stem cells.

Due to the diversity of characteristics displayed by hMSCs from different sources by various isolation methods, the ISCT recommended minimal criteria to define MSCs: (1) cell adhesion to a plastic surface, (2) specific positive and negative surface markers and (3) in vitro tri-lineage differentiation capability.

The unequivocal identification of MSCs in vivo has been hindered by their extremely low frequency in tissues and the lack of a distinct MSC-specific immunophenotype to enable their identification and isolation. Cultured human mesenchymal stromal cells do express a panel of cell surface markers such as CD73, CD105, CD90 and lack endothelial or hematopoietic cell markers (CD34, CD31 and CD45).

Nevertheless, these are not homogeneously expressed throughout stromal cultures, vary with isolation protocols and passage number, and therefore are not necessarily representative of MSCs in vivo. The microenvironment for MSC in vivo is quite different from 2-D flask culture, though unfortunately 2-D flask culture is part of the working definition for MSC identification.