Mesenchymal stem cells: the 'other' bone marrow stem cells …

What can mesenchymal stem cells do?

Mesenchymal stem cells (MSCs) are an example of tissue or 'adult' stem cells. They are multipotent, meaning they can produce more than one type of specialized cell of the body, but not all types. MSCs make the different specialized cells found in the skeletal tissues. For example, they can differentiate or specialize into cartilage cells (chondrocytes), bone cells (osteoblasts) and fat cells (adipocytes). These specialized cells each have their own characteristic shapes, structures and functions, and each belongs in a particular tissue.

Some early research suggested that MSCs might also differentiate into many different types of cells that do not belong to the skeletal tissues, such as nerve cells, heart muscle cells, liver cells and endothelial cells, which form the inner layer of blood vessels. These results have not been confirmed to date. In some cases, it appears that the MSCs fused together with existing specialized cells, leading to false conclusions about the ability of MSCs to produce certain cell types. In other cases, the results were an artificial effect caused by chemicals used to grow the cells in the lab.

Mesenchymal stem cell differentiation: MSCs can make fat, cartilage and bone cells. They have not been proven to make other types of cells of the body.

MSCs were originally found in the bone marrow. There have since been many claims that they also exist in a wide variety of other tissues, such as umbilical cord blood, adipose (fat) tissue and muscle. It has not yet been established whether the cells taken from these other tissues are really the same as, or similar to, the mesenchymal stem cells of the bone marrow.

The bone marrow contains many different types of cells. Among them are blood stem cells (also called hematopoietic stem cells; HSCs) and a variety of different types of cells belonging to a group called mesenchymal cells. Only about 0.001-0.01% of the cells in the bone marrow are mesenchymal stem cells.

It is fairly easy to obtain a mixture of different mesenchymal cell types from adult bone marrow for research. But isolating the tiny fraction of cells that are mesenchymal stem cells is more complicated. Some of the cells in the mixture may be able to form bone or fat tissues, for example, but still do not have all the properties of mesenchymal stem cells. The challenge is to identify and pick out the cells that can both self-renew (produce more of themselves) and can differentiate into three cell types bone, cartilage and fat. Scientists have not yet reached a consensus about the best way to do this.

No treatments using MSCs are yet available. However, several possibilities for their use in the clinic are currently being explored.

Bone and cartilage repair The ability of MSCs to differentiate into bone cells called osteoblasts has led to their use in early clinical trials investigating the safety of potential bone repair methods. These studies are looking at possible treatments for localized skeletal defects (damage at a particular place in the bone).

Other research is focussed on using MSCs to repair cartilage. Cartilage covers the ends of bones and allows one bone to slide over another at the joints. It can be damaged by a sudden injury like a fall, or over a long period by a condition like osteoarthritis, a very painful disease of the joints. Cartilage does not repair itself well after damage. The best treatment available for severe cartilage damage is surgery to replace the damaged joint with an artificial one. Because MSCs can differentiate into cartilage cells called chondrocytes, scientists hope MSCs could be injected into patients to repair and maintain the cartilage in their joints. Researchers are also investigating the possibility that transplanted MSCs may release substances that will tell the patients own cells to repair the damage.

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