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Wound Healing Response of the MCL During Hindlimb Unweighting in Rodents.  D.A. Martinez¹, R. Vanderby, Jr.², R.E. Grindeland³, K. Dave¹, A.K. Lee¹, H.Y. Hoang¹, P. Provenzano², T. Wang³, and A.C. Vailas¹. ¹Connective Tissue Physiol. Lab, Univ. of Houston, TX, ²Dept. of Surgery, Univ. of Wisconsin, Madison, WI, ³NASA-Ames, Moffett Field, CA.

   It is well known that exposure to microgravity or simulated micro-gravity causes bone and muscle atrophy. It is less well known whether or not microgravity has a great impact on the strength and composition of dense fibrous connective tissues. In simulated microgravity, the extracellular matrix of ligament and tendon lose mass, strength and decrease their adhesion to bone at the insertion site. The little data from spaceflight indicate that muscle and bone do not repair well after injuries, strongly suggesting that ligaments may not repair optimally. Thus, the objective of this study is to investigate the molecular, cellular and biomechanical properties of the collagen extracellular matrix in response to medial collateral ligament (MCL) injury repair in hindlimb unweighted (HLS) rodents. Two studies were performed comparing MCL wound repair during 3 weeks and 7 weeks of HLS. Each study consisted of 3 groups: hindlimb uweighted+lesion (HLS+lesion, n=24), ambulatory+lesion (Amb+lesion, n=24) and sham animals (Sham, n=24). Following suspension, rats were euthanized and the hindlimbs were frozen at –85˚ C. Bone-MCL-bone preparations were tested biomechanically for ligament strength. All other MCLs were isolated and prepared for molecular or biochemical analyses. Ultimate stress and elastic modulus tests demonstrated that the 3 week Amb+lesion MCLs were significantly stronger (P<0.05) compared HLS+lesion animals. Hydroxyproline concentration was substantially reduced in both the 3 and 7 week HLS+lesion groups versus the other groups. MCL collagen gene expression of type I  (Col1A2), type III (Col3A1) and type V (Col5A1), measured by real time Q-PCR, showed significant differences in the HLS+lesion versus Amb+lesion groups. In conclusion, wound repair in the rodent MCL is significantly impacted by hindlimb unweighting. 

     (Supported by NASA Grant: NAG9-1152)