title: Transient expression of myofibroblast-like cells in rat rib fracture callus.
abstract: OBJECTIVE
  We have previously shown that early fracture callus of rat rib has viscoelastic and contractile properties resembling those of smooth muscle. The cells responsible for this contractility have been hypothesized to be myofibroblast-like in nature. In soft-tissue healing, force generated by contraction of myofibroblasts promotes healing. Accordingly, we tried to identify myofibroblast-like cells in early fibrous callus.
  
  
  METHODS
  Calluses from rat rib fractures were removed 7, 14, and 21 days after fracture and unfractured ribs acted as controls. All tissues were analyzed using qPCR and immunohistochemistry. We analyzed expression of smooth muscle- and myofibroblast-associated genes and proteins including alpha smooth muscle actin (αSMA), non-muscle myosin, fibronectin extra domain A variant (EDA-fibronectin), OB-cadherin, connexin-43, basic calponin (h1CaP), and h-caldesmon.
  
  
  RESULTS
  In calluses at 7 days post-fracture, there were statistically significant increases in expression of αSMA mRNA (2.5 fold), h1CaP mRNA (2.1 fold), EDA-fibronectin mRNA (14 fold), and connexin-43 mRNA (1.8 fold) compared to unfractured ribs, and by 21 days post-fracture mRNA expression in calluses had decreased to levels approaching those in unfractured rib. Immunohistochemistry of 7 day fibrous callus localized calponin, EDA-fibronectin and co-immunolabeling of OB-cadherin and αSMA (thus confirming a myofibroblastic phenotype) within various cell populations.
  
  
  CONCLUSIONS
  This study provides further evidence that early rat rib callus is not only smooth muscle-like in nature but also contains a notable population of cells that have a distinct myofibroblastic phenotype. The presence of these cells indicates that in vivo contraction of early callus is a mechanism that may occur in fractures so as to facilitate healing, as it does in soft tissue wound repair.
authors.affiliation: Tissue and Cell Biology Group, Musculoskeletal Research Centre, La Trobe University, Victoria, Australia.
authors.forename: Stuart J
authors.initials: SJ
authors.lastname: McDonald
authors.forename: Philip C
authors.initials: PC
authors.lastname: Dooley
authors.forename: Aaron C
authors.initials: AC
authors.lastname: McDonald
authors.forename: Johannes A
authors.initials: JA
authors.lastname: Schuijers
authors.forename: Alex R
authors.initials: AR
authors.lastname: Ward
authors.forename: Brian L
authors.initials: BL
authors.lastname: Grills
mesh_terms: D000818:Animals; D015415:Biomarkers / Q000378:metabolism*; D002146:Bony Callus / Q000378:metabolism / Q000473:pathology / Q000503:physiopathology*; D017102:Fracture Healing / Q000502:physiology; D005786:Gene Expression Regulation; D007150:Immunohistochemistry; D008297:Male; D009130:Muscle, Smooth / Q000378:metabolism*; D058628:Myofibroblasts / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012253:Rib Fractures / Q000378:metabolism / Q000473:pathology / Q000503:physiopathology*; D018709:Statistics, Nonparametric; D013997:Time Factors; D015854:Up-Regulation
chemical_list: D015415:Biomarkers