The Knee Stem Cell Reserve: Keeping It Topped Up
I’ve been snapping slides while at the Havemeyer CSU symposium that catch my eye and this is a great one. Above you’ll see a summary of all of the areas where the stem and progenitor cells live in the knee. Even though I’ve read most of these studies before, seeing it all in one place was amazing. This brings up a theory of one mechanism for how knee stem cell therapy works, by topping up the tank. Let me explain.
You Have Stem Cells in Almost All of Your Tissues
We hear about stem cells taken from bone marrow and fat. However, remember that mesenchymal and other stem and progenitor cells are the maintenance men of the body. Hence, you have stem cells in your teeth, muscles, ligaments, tendons, and just about everywhere else.
The Stem Cells in Your Knee
Looking up at the slide, there are stem cells in the following areas:
Bone Marrow: This one will make sense to you, as you’ve likely heard there are stem cells in your bone marrow. However, there are far fewer stem cells in the knee bone marrow than in the pelvis.
Synovium: This is the stuff that surrounds the joint. It also acts as a major stem cell reserve for the knee.
Synovial Fluid: This is the fluid inside the joint. Stem cells that live in the synovium can be drawn into the fluid. In fact, we have cultured stem cells out of synovial fluid.
Meniscus: This is the spacer tissue in the knee that protects the cartilage. Stem cells here help to maintain the structure when you’re young.
Fat Pad: This is the fat that lives behind your knee cap and around various structures. It’s different than the fat that surrounds your belly, but like that fat, it has stem cells that are also believed to help maintain the joint.
Ligament: The ligaments in all of your joints act as the duct tape that holds the structure together. These do have stem cells that maintain their integrity.
Cartilage: This is the stuff that protects the end of your bones. There are progenitor cells that live on the surface of the cartilage and cartilage communicates with stem cells in the underlying bone.
Filling the Tank
One theory for why we would want to transplant stem cells from your bone marrow to your knee is that there are too few stem cells in your knee due to aging or injury. This lack of stem cells or lack of good stem cells is likely one of the many things that begins the degenerative cascade. So in some respects, when we inject stem cells into various structures, we’re filling the tank!
The upshot? The knee has a bunch of native stem cells in many different structures. However, some of these cells may die or become less functional with age, inflammation, or disease. Hence, one possible mechanism for why stem cell therapy works is by replacing these cells.