Electromechanical and physicochemical properties of connective tissue

This review has dealt primarily with the electromechanical and transport properties of the extracellular matrix, which generally contains ionized charged groups under physiological conditions. Connective tissues are not electrically "active" in the sense of nerve or muscle; that is, electrical signals do not propagate as waves within the tissue. However, we have attempted to show the importance of "passive" electromechanical coupling and the coupling of passive transport mechanisms to the functional health of connective tissues. The effect of mechanical and electrical stresses on cell growth and biosynthesis is a relatively new and exciting area of research that should provide important clues concerning the interactions between cells and the extracellular matrix. While the role of cells in connective tissues is beyond the scope of this review, it is well known that environmental stresses have a direct effect on the structure and composition of connective tissues. Studies have shown that changes in the chemical and mechanical environment of cells can significantly alter cell synthesis of polysaccharide and protein components of the matrix. For example, Gillard et al. studied the glycosaminoglycan and collagen composition of the flexor digitorium profundus tendon of the rabbit. In regions where the tendon is subject to tensile forces, the tissue GAG content is approximately 0.2% of the dry weight, a value not unlike other tendons. However, in the small sesamoid region where the tendon hooks around the heel bone, the tendon is subjected to high compressional stresses. In this region, the GAG concentration is 15 to 20 times higher and the GAG composition is similar to that of articular cartilage. Gillard et al. found that manipulation of the tendon so as to release the compressional forces lead to a decrease in GAG content by more than 60%. Subsequent replacement of the tendon to its original position caused a concomitant increase in the GAG content. These results can be interpreted to be directly linked to the influence of mechanical forces on cell synthesis. The recent finding that cell synthesis is also affected by imposed electrical fields may suggest that electrical, mechanical and chemical signals are somehow interpreted by the cells along common pathways. The fact that electrical potentials are naturally produced near cells by deformation of the extracellular matrix provides additional support for such hypotheses.