The Active Components of Connective Tissue
July 1, 2012
A few years ago I presented a talk at a prestigious college in Pennsylvania. I was invited to speak about connective tissue research to the kinesiology department and students. I was only a few minutes into my presentation when a man sitting in the second row raised his hand. I was quite surprised that there were already questions when I had barely begun. The man in the second row turned out to be their tenured anatomy professor who was less than convinced that connective tissue was worth discussing at all. His exact words were, “You are not going to tell us that the connective tissue has active components are you?”
“Oh wait until I get going. I am going to propose many interesting ideas, hypothesis, and some new relevant science about this understudied tissue,” I said filled with my usual excitement about this research.
Needless to say, he poo-pooed the notion but continued to say what I was talking about was a complete fallacy. Regardless of his closed view as an anatomist, I continued.
I discussed many aspects about connective tissue but one idea that, at the time was hit with a great deal of controversy was the science on fibroblasts. Fibroblasts are the primary cell of connective tissue. They are cells responsible for making “all the stuff” that is outside of our cells. This stuff is called the extracellular matrix.
Not only do fibroblasts create all of the ground substance, collagen, elastin, hyaluronic acid, and other molecular components, fibroblasts also sense force-induced deformations (strains) in their ECM. Recent findings have indicated that the integrins within cell-matrix adhesions can act as 'strain gauges', triggering chemical pathways in response to changes in mechanical stress.
Mechanical forces are important regulators of connective tissue homeostasis. Mechanical stress can regulate the production of ECM proteins indirectly, by stimulating the release of cell signaling agents like clotting and growth factor, or directly, by triggering an intracellular signaling pathway that activates the genes in the ECM. This can be good and bad. It’s good when it aids in healing, bad when it causes excessive adhesions. Some signaling molecules degrade very quickly, limiting the scope of their effectiveness to the immediate surroundings. Others affect only nearby cells because they are taken up quickly, leaving few to travel further, or because their movement is hindered by the ECM.
The main function of fibroblasts is to maintain the structural integrity of connective tissues by continuously secreting the molecular components of the extracellular matrix. The composition of the extracellular matrix determines the physical properties of connective tissues.
It seems comical at this stage to argue that there are active components in connective tissue. In fact, it’s undeniable that something is reacting in our connective tissue when we move, stretch, compress, or change our body basically every minute of everyday. To say connective tissue is an inactive packing material is as convoluted as the professor sitting in the second row arguing the point.
We need to stop questioning the science and simply keep the scientific studies moving. We don’t have all of the answers however, more answers are there to be had. So for all of the anatomists, like myself who for years were not looking at the connective tissue, it’s time we took a new view on the most abundant material in the entire human body and learn more about how to keep it active, healthy, and hydrated for a lifetime. It is our framework afterall.