Making Human Repairs - New tissues for the body, medical engineering         Tissue engineering is an exciting new field in which researchers are developing techniques to re-grow tendons and cartilage and restore bone loss. This is done using the body's own supercell, the mesenchymal stem cell, which is found in the bone marrow and is a primordial cell that has the ability to change into different types of tissues.     In a newborn infant, mesenchymal stem cells number one in every 10,000 cells in the bone marrow. By age 50, the number drops to one in 400,000 cells and continues to drop further as the person ages. The ability of the mesenchymal stem cell to differentiate into so many types of tissues makes them ideal for use in medical tissue engineering. Fortunately, this is not science fiction any longer.     Bone marrow is removed from the pelvic bone and mesenchymal stem cells are extracted from the bone marrow and purified. Purified mesenchymal stem cells are placed in a petri dish with fetal calf serum which allows the cells to multiply hundreds of millions of times. It is challenging to figure out how to reinsert the cells into the body to regenerate tissues that the body cannot regenerate itself. The multiplied cells must be placed on various types of delivery vehicles and inserted into the body. These vehicles include ceramic artificial bone, gel, and gel coated structures.         Mesenchymal stem cells receive signals from surrounding cells and transform into those surrounding cells. If exposed to bone, they can become bone. If they are exposed to cartilage, they can become cartilage, and if they are exposed to tendons, they can become tendons.     The body is constantly and continuously regenerating tissues. An example of this is the red blood cell. It lives for approximately 90 days and then dies. Primitive cells in the bone marrow called hematopoietic stem cells make new blood cells and they enter the blood stream as replacements.     But there is a second, rarer type called the mesenchymal stem cell with the much more diverse job description. The cell has the ability to give rise to bone, cartilage, tendon, teeth, fat, and skin. It is also responsible for the scaffolding or connective tissue in the bone marrow.     Scientists are working on development of a scaffolding upon which new bone can be grown. It is a polymer material, porous like a sponge, which can accommodate a large number of cells providing a surface for the cells to grow on and then eventually the material is degraded. The new tissue would develop its own supportive matrix in time and does not need the scaffolding any longer, therefore biodegradable supports are essential.     Cells communicate with each other chemically and respond to cues from their environment. The cues important to tissue engineers are those that tell cells to divide, migrate, and differentiate into different types of tissue.     Natural substances called growth factors contain cues for getting cells to multiply and to migrate.     The first tissue successfully engineered and made available to humans was skin. Skin substitutes have been made available for almost 20 years.              A paper was presented recently at the Orthopaedic Research Society meeting in Dallas in which a researcher reported that he grew a ligament similar to anterior cruciate ligament using a patient's own cells. They used the patient's bone marrow stromal cells, which they placed on a specially designed silk matrix, then loaded it into a tube-like structure. The growing tissue was loaded in torsion and in tension reflecting the in vivo environment of the anterior cruciate ligament.     Ultimately the tissue seemed to match the material properties of a normal ligament. The researchers were able to get the cells to elongate into an appearance similar to ligament cells. The cells also produced a matrix component, a protein found in anterior cruciate ligaments. There is no way to know how long this may take to be something that could be used in clinical practice. It is a very interesting attempt to produce a tissue that could be substituted for the anterior cruciate ligament.