Mesenchymal Stem Cells – A Lay Review
Introduction

Mesenchymal Stem Cells (MSCs) were first recognized in bone marrow as a non-hematopoietic stem cell by a German pathologist, Julius Cohnheim in 1867. MSCs are generally defined as clonogenic, non-hematopoietic stem cells that are most commonly found in the bone marrow. They are distinguished by their natural ability to differentiate into multiple mesodermal lineages, such as chondrocytes, osteoblasts, adipose tissue, and endothelial cells.

MSCs are adult, non-hematopoeitic stem cells that differentiate along multiple lineages to form bone, cartilage, adipose tissue (fat cells), tendons, and muscle (1). The differentiated cells have clinical potential to repair tissues (Figure 1). Cellular differentiation refers to the process whereby a stem cell undergoes a process of maturation to form specialized cells. Osteoblasts control the formation and degradation of bone, chondrocytes generate cartilage, tendon, and ligament, and adipocytes produce fat cells.

MSCs reside in diverse host tissues and organs, e.g., adult and fetal bone marrow, spleen, amniotic fluid, cartilage, muscle tendons, placenta, adipose tissues, fetal tissues, muscle tendons, periosteum, synovial fluid, synovium and thymus (2-5). The frequency of MSCs in bone marrow is low, representing approximately 0.001-0.1% of nucleated cells (6). As compared to adult bone marrow, it appears that the frequency of MSCs in umbilical cord blood is low (7,8). The advantage of cord blood is that it is a non-invasive procedure.

The functions of MSCs are partly controlled by the microenvironment. A microenvironment is complex and comprises multiple factors, which interact with the stem cells independently, or in combination. During insult to the tissues or organs, the microenvironment is changed and at times, these changes are drastic. In this regard, the stem cells could find themselves within a microenvironment where homeostasis is maintained to one where the changes are significant. Examples of conditions that could cause microenvironmental changes are surgical and stress-induced trauma, inflammation and cell death (9). It should be noted that the aforementioned conditions are only few examples of tissue/organ insult.

Microenvironmental molecules can affect the fate of the MSCs. The most studied are cytokines, which include the interleukins, chemokines and interferons. In addition to soluble factors, MSCs could be affected by intercellular communication. In the bone marrow, it appears that MSCs are in contact with nerve endings and might be stimulated through neural activity. MSC research has received considerable recognition due to many cellular attributes such as: ease of isolation, expansion potential, compatibility with multiple delivery methods, and immunosuppressive properties, all allowing for great therapeutic potential in cell based medicine.

Laboratory Isolation and Quality Control

The functions of MSCs are fraught with contradiction. The differences in functions might be explained by the method used in their culture conditions. The surface and the culture media are critical for the propagation of MSCs. This section briefly describes the laboratory method used to culture MSCs. In human, bone marrow aspirates are obtained from the superior iliac crest of the pelvis. In mice, MSCs are to be scraped from the endosteal region of femurs.

The bone marrow is also home to hematopoietic cells (refer to molecule of the month, HSCs). While HSC lineages are not expected to survive during culture of MSCs, their presence within the primary MSCs needs to be eliminated if one cares to perform experiments with freshly isolated MSC. This is generally done by labeling the bone marrow cells with fluorochrome-tagged antibodies and then positively or negatively selecting the MSC subset.

Another concern is whether the cells are indeed pluripotent, or if they are partly differentiated. Since studies with MSCs should be done with stem cells, quality controls are needed at various times during culture. The following, although recommended, could be supplemented with other methods: Lineage-differentiated studies for adipogenic and chondrogenic cells, flow cytometry for specific markers (CD29, CD44, CD105), or the absence of CD31 (marker of endothelial cells) and CD14 (marker of macrophages). MSCs can be subjected to multiple passages without differentiation, if done under the right conditions.

Potential Therapeutic Applications

An advantage for MSCs in clinical application lies in their immune property. MSCs have been reported to exhibit immunosuppressive functions and could suppress graft vs. host response (8). To be specific, MSCs from one individual can be placed in another person without significant concerns for rejection. MSCs express the major histocompatibility complex-II (MHC-II), which could be downregulated during inflammation (11). This molecule, however, might be re-expressed in the host. Future research studies should dissect the immune properties to determine how, and if, rejection could be circumvented.

MSCs have been shown to generate pancreatic, kidney, heart, and neural cells, under specific laboratory conditions (1). The reduced ethical concerns, ease in their isolation, expansion, engraftment, and immunosuppressive properties indicated potential for cell-based therapies by MSCs. Several reports show MSCs being able to regenerate muscle, bone, teeth, tendon, cartilage, and cardiac muscle (6). MSCs may facilitate the healing of bone defects caused by trauma, osteonecrosis, or tumor excisions, which are repaired with tissue graft from the patient or a donor. These grafts would have to be presented in a “scaffold” orientation for proper implantation (12,13). The concept of connective tissue regeneration could also be applied to cartilage defects with the aid of genetic engineering. MSCs could possibly invade and regenerate damaged muscle tissue. There is also the possibility of using MSCs to repair peripheral nerves, although the mechanism by which this could occur is unclear. This area of research is undergoing intensive scientific investigation.

The organ and tissue engraftment processes discussed above are associated with stable, and long-term repair (12,14). MSCs have been shown to migrate at the site of injury when induced into the peripheral circulation. In incidences of bone fracture, stroke and heart attack, the scientific report suggested MSCs homing show specific homing to the site of injury (2,15,16). Though further research is needed to ensure the safety and consistency of MSC engraftment and its regeneration capabilities, MSCs have undeniably demonstrated a wide array of therapeutic capabilities placing them in the spotlight of current and future stem cell research.

References
1. Pountos I, Giannoudis PV, Biology of mesenchymal stem cells. Injury, Int J Care Injured 2005;36:S8-S12.
2. in 't Anker PS, Noort WA, Scherjon SA, Kleijburg-van der Keur C, Kruisselbrink AB, van Bezooijen RL, Beekhuizen W, Willemze R, Kanhai HH, Fibbe WE. Mesenchymal stem cells in human second-trimester bone marrow, liver, lung, and spleen exhibit a similar immunophenotype but a heterogeneous multilineage differentiation potential. Haematologica 2003;88:845-52.
3. Friedenstein AJ, Piatezky-Shapiro II, Petrakova KV. Osteogenesis in transplants of bone marrow. J Embryol Exp Morphol 1996;16:381-90.
4. Hu Y, Lia L, Wang Q, Ma L, Ma G, Jiang X, Zhao RC. Isolation and identification of mesenchymal stem cells from human fetal pancreas. J Lab Clin Med 2003;141:342-49.
5. Romanov YA, Svintsitskaya VA, Smirnov VN. Searching for alternative sources as postnatal human mesenchymal stem cells: candidate MSC-like cells from Umbilical cord. Stem Cells 2003;21:105-10.
6. Pittenger MF, Mackay Am, Beck SC, Jaiswal RK, Douglas R, Mosca JD, Moorman MA, Simonetti DW, Craig S, Marshak DR. Multilineage potential of adult human mesenchymal stem cells. Science 1999;284:143-7.
7. Bieback K, Kern S, Kluter H, Eichler H. Critical parameters for the isolation of mesenchymal stem cells from umbilical cord blood. Stem Cells 2004;22:625-34.
8. Kern S, Eichler H, Stoeve J, Kluter H, Bieback K, Comparative analysis of mesenchymal stem cells from bone marrow, umbilical cord blood, or adipose tissue. Stem Cells 2006;24:1294-301.
9. Palermo AT, Lambarge MA, Doyonnas R, Pomerantz J, Blau HM. Bone marrow contribution to skeletal muscle: a physiological response to stress. Dev Biol 2005;279:336-44.
10. Yamazaki K, Allen TD. Ultrastructural morphometric study of efferent nerve terminals on murine bone marrow stromal cells, and the recognition of a novel anatomical unit: the "neuro-reticular complex . Am J Anat 1990;187:261-76.
11. Chan JL, Tang KC, Patel AP, Bonilla LM, Pierobon N, Ponzio NM, Rameshwar P. Antigen presenting property of mesenchymal stem cells occurs during a narrow window at low levels of interferon-γ. Blood 2006;107:4817-24.
12. Bruder SP, Jaiswal N, Ricalton NS, Mosca JD, Kraus KH, Kadiyala S. Mesenchymal stem cells in osteobiology and applied bone regeneration. Clin Orthop 1998;355:S247-56.
13. Orlic D, Kajstura J, Chimenti S, Jakoniuk I, Anderson SM, Li B, Pickel J, McKay R, Nadal-Ginard B, Bodine DM, Leri A, Anversa P. Bone marrow cells regenerate infractured myocardium. Nature 2001;410:701-5.
14. Digirolamo CM, Stokes D, Colter D, Phinney DG, Class R, Prockop DJ. Propagation and senescence of human marrow stromal cells in culture: a simple colony-forming assay identifies samples with the greatest potential to propagate and differentiate. Br J Haematol 1999;107:275-81.
15. Shake JG, Gruber PJ, Braumgartner WA, Senechal G, Meyers J, Redmond JM, Pittenger MF, Martin BJ. Mesenchymal stem cell implantation in a swine myocardial infarct model: engraftment and functional effects. Ann Thorac Surg 2002;73:1919-25.
16. Wang L, Li Y, Chen X, Chen J, Gautam SC, Xu Y, Chopp M. MCP-1, MIP-1, IL-8 and ischemic cerebral tissue enhance human bone marrow stromal cell migration in interface culture. Hematol 2002;7:113-17.

Acknowledgements

This review was prepared by the following graduate students in the Stem Cell Biology Class, Graduate School of Biomedical Sciences, University of Medicine and Dentistry of New Jersey: George Lambrinos, Rob Margolin, Laila Menon, Julio Ortega, Miral Parikh
(in alphabetical order).

Teaching Assistant: Steven Greco

The review was edited by two stem cell biologists.

Acknowledgements

This review was prepared by the following graduate students in the Stem Cell Biology Class, Graduate School of Biomedical Sciences, University of Medicine and Dentistry of New Jersey:

Roger Diaz, Thomas Finocchio, Shannon Henning, Gaurav Gandhi, Nicole Pannucci (in alphabetical order).

Teaching Assistant: Elaine Wong

The review was edited by two stem cell biologists.