Connective Tissue Mechanotransduction Responses To Stretch and Acupuncture: From Ex Vivo Fibroblast Cytoskeletal Morphology to In Vivo Ultrasound Elasticity Imaging
Helene Langevin, M.D., L.Ac., Research Assistant Professor, University of Vermont, Department of Neurology, Burlington, Vermont
A common feature of manual therapies is the therapeutic application of mechanical forces (e.g. stretching, pressure) on either “dense” (tendons, ligaments, joint capsules) or “loose” (fasciae, subcutaneous, interstitial) connective tissues. Tissue viscoelastic responses to mechanical forces are determined by their connective tissue matrix composition (collagen, glycosaminoglycans (GAGs), water content) and architecture. Connective tissue fibroblasts play a pivotal role in both immediate and long term connective tissue responses to mechanical forces by 1) secreting matrix components and thus regulating matrix composition and 2) actively responding to mechanical forces via mechanotransduction—or mechanisms that directly link mechanical forces to active changes in cell shape, intracellular signaling mechanisms and/or expression of specific mechanosensitive genes (including collagen, GAGs, metalloproteinases and growth factors).
Thus mechanical forces actively participate (via cellular responses) in remodeling of connective tissue’s extracellular matrix. The effect of mechanical forces on connective tissue fibroblasts may be key to the therapeutic mechanism of manual therapies by causing important cellular effects both immediate (activation of signaling mechanisms) and delayed (gene expression, modification of extracellular matrix composition) which may affect future biomechanical tissue behavior during movement.
Ex vivo and in vivo animal models can be used to study the effect of mechanical forces on fibroblasts in whole tissue. Using histochemistry and confocal microscopy, we found that both tissue stretch and acupuncture induced a dynamic, reversible change in fibroblast morphology within 30 minutes. During acupuncture, connective tissue mechanical stimulation is caused by winding and pulling of collagen during needle manipulation. With both types of mechanical stimulation, fibroblasts cell bodies became large, flat and “sheet-like” in contrast to the small cell bodies and long branching processes seen without stretch. These changes in cell shape required the presence of both intact microtubules and microfilaments, implying an active, cytoskeletal-dependent mechanism. Results obtained with these cell imaging techniques therefore suggest that acupuncture may share common cellular mechanotransduction mechanisms with a range of manual therapies. Further studies will be needed to examine the effects of varying force amplitude, frequency and duration on these mechanisms, and how these effects may differ across treatment modalities.
In vivo ultrasound elasticity imaging techniques can be used to study the effect of applied mechanical forces on tissues in humans. Using a combination of ultrasound elasticity imaging and robotic acupuncture needling, we quantified spatial and temporal tissue displacement and strain patterns during acupuncture needling in humans. We found that rotation of the acupuncture needle preconditioned the tissue and modified its biomechanical behavior during subsequent axial needle motion. Ultrasound elasticity imaging is emerging as a powerful non-invasive technique to quantify biomechanical tissue behavior that may be applied to investigating the mechanism of manual therapies as well as acupuncture.
Combined approached using these ex vivo and in vivo techniques may ultimately allow translation of findings from animal models leading to mechanistic studies of therapeutic mechanisms in humans.