- Mesenchymal stem cells (MSCs) are multipotent stromal cells capable of differentiating into various cell types, including osteoblasts, adipocytes, and chondrocytes.
- They are primarily isolated from bone marrow but also exist in other tissues such as adipose tissue, umbilical cord, and dental pulp.
- Mesenchymal stem cells are defined not only by their plastic adherence and differentiation potential but also by a unique surface marker profile.
- MSCs are characterized by specific surface markers that distinguish them from hematopoietic and other cell types, as well as by their functional capabilities.
- The International Society for Cellular Therapy (ISCT) has proposed minimal criteria for defining human MSCs:
- They must adhere to plastic under standard culture conditions.
- They must express a specific set of surface markers.
- They must differentiate into osteoblasts, adipocytes, and chondroblasts in vitro.
MSCs express a distinct profile of positive and negative surface markers:
- Positive Markers (typically expressed):
- CD105 (Endoglin): A TGF-β receptor accessory protein involved in angiogenesis and cell proliferation. Highly expressed on MSCs.
- CD73 (Ecto-5′-nucleotidase): An enzyme involved in purine metabolism, associated with immunomodulatory functions.
- CD90 (Thy-1): A glycoprotein linked with cell adhesion and signal transduction.
- CD44: A receptor for hyaluronic acid, important for cell–matrix interactions.
- CD29 (Integrin β1): Plays a role in cell adhesion and interaction with the extracellular matrix.
- CD146 (MCAM): A cell adhesion molecule; sometimes used to identify subpopulations of MSCs with enhanced differentiation potential.
- Stro-1: An early marker of MSCs, especially in bone marrow-derived populations.
- Negative Markers (not expressed):
- CD34: A marker for hematopoietic progenitor and endothelial cells.
- CD45: A pan-leukocyte marker.
- CD14/CD11b: Markers for monocytes and macrophages.
- CD79a/CD19: B cell markers.
- HLA-DR: An MHC class II surface receptor; MSCs typically do not express it under baseline conditions but may upregulate it in response to inflammatory cytokines.
- Functional Markers and Emerging Candidates: In addition to surface markers, MSCs may also express markers related to their immunomodulatory and differentiation potential, such as:
- Nestin: An intermediate filament protein expressed in MSCs with neural differentiation potential.
- SSEA-4 (Stage-Specific Embryonic Antigen-4): Sometimes used to identify early progenitor MSCs.
- ALCAM (CD166): Involved in adhesion and migration, also expressed in MSCs.
