title: Response of Chondrocytes to Local Mechanical Injury in an Ex Vivo Model.
abstract: BACKGROUND
  Our goal was to set up an ex vivo culture system to assess whether cartilage wounding (partial-thickness defects) can induce morphological changes in neighboring chondrocytes and whether these cells can translocate to the surface of the defect.
  
  
  METHODS
  Two-millimeter partial-depth defects were created in human osteochondral explants followed by culture for up to 4 weeks. Frozen sections of defects and defect-free regions were labeled using immunofluorescence for a plasma membrane protein, CD44, and actin with TRITC-phalloidin. Viable nuclei were detected with Hoechst 33342. Differential interference contrast (DIC), confocal, and transmission electron microscopy (TEM) were used to examine process extension.
  
  
  RESULTS
  Significant changes in cell morphology occurred in response to wounding in the superficial and deep cartilage zones. These included cell flattening, polarization of the actin cytoskeleton, extension of pseudopods projecting towards the edge of the defect, and interactions of these filopodia with collagen fibers. Cell density decreased progressively in the 300-µm zone adjacent to the defect to an average of approximately 25% to 35% after 3 weeks. Concomitant increases in cell density in the defect margin were observed. By contrast, minimal changes were seen in the middle cartilage zone.
  
  
  CONCLUSIONS
  These novel observations strongly suggest active cartilage cell responses and movements in response to wounding. It is proposed that cartilage cells use contact guidance on fibrillated collagen to move into and populate defect areas in the superficial and deep zones.
authors.affiliation: The Lyman Knee Clinic, Scottsdale, AZ, USA.
authors.forename: Jeffrey R
authors.initials: JR
authors.lastname: Lyman
authors.affiliation: Wake Orthopaedics, Raleigh, NC, USA.
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Chappell
authors.affiliation: Massachusetts General Hospital and Harvard Medical School, Boston, MA, USA.
authors.forename: Teresa I
authors.initials: TI
authors.lastname: Morales
authors.affiliation: Department of Orthopaedics, Duke University, Durham, NC, USA.
authors.forename: Scott S
authors.initials: SS
authors.lastname: Kelley
authors.affiliation: Department of Orthopedics and Thurston Arthritis Research Center, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA.
authors.forename: Greta M
authors.initials: GM
authors.lastname: Lee
keywords: cartilage; collagen; motility; pseudopod; repair