Objective: Hypocellularity resulting from chondrocyte death in the aftermath of mechanical injury is thought to contribute to posttraumatic osteoarthritis. However, we observed that nonviable areas in cartilage injured by blunt impact were repopulated within 7–14 days by cells that appeared to migrate from the surrounding matrix. The aim of this study was to assess our hypothesis that the migrating cell population included chondrogenic progenitor cells that were drawn to injured cartilage by alarmins.Methods: Osteochondral explants obtained from mature cattle were injured by blunt impact or scratching, resulting in localized chondrocyte death. Injured sites were serially imaged by confocal microscopy, and migrating cells were evaluated for chondrogenic progenitor characteristics. Chemotaxis assays were used to measure the responses to chemokines, injury‐conditioned medium, dead cell debris, and high mobility group box chromosomal protein 1 (HMGB‐1).Results: Migrating cells were highly clonogenic and multipotent and expressed markers associated with chondrogenic progenitor cells. Compared with chondrocytes, these cells overexpressed genes involved in proliferation and migration and underexpressed cartilage matrix genes. They were more active than chondrocytes in chemotaxis assays and responded to cell lysates, conditioned medium, and HMGB‐1. Glycyrrhizin, a chelator of HMGB‐1 and a blocking antibody to receptor for advanced glycation end products (RAGE), inhibited responses to cell debris and conditioned medium and reduced the numbers of migrating cells on injured explants.Conclusion: Injuries that caused chondrocyte death stimulated the emergence and homing of chondrogenic progenitor cells, in part via HMGB‐1 release and RAGE‐mediated chemotaxis. Their repopulation of the matrix could promote the repair of chondral damage that might otherwise contribute to progressive cartilage loss.

We tested whether inhibiting mechanically responsive articular chondrocyte mitochondria after severe traumatic injury and preventing oxidative damage represent a viable paradigm for posttraumatic osteoarthritis (PTOA) prevention. We used a porcine hock intra-articular fracture (IAF) model well suited to human-like surgical techniques and with excellent anatomic similarities to human ankles. After IAF, amobarbital or N-acetylcysteine (NAC) was injected to inhibit chondrocyte electron transport or downstream oxidative stress, respectively. Effects were confirmed via spectrophotometric enzyme assays or glutathione/glutathione disulfide assays and immunohistochemical measures of oxidative stress. Amobarbital or NAC delivered after IAF provided substantial protection against PTOA at 6 months, including maintenance of proteoglycan content, decreased histological disease scores, and normalized chondrocyte metabolic function. These data support the therapeutic potential of targeting chondrocyte metabolism after injury and suggest a strong role for mitochondria in mediating PTOA.

Objective: Articular cartilage damage after joint trauma seldom heals and often leads to osteoarthritis. We previously identified a migratory chondrogenic progenitor cell (CPC) population that responds chemotactically to cell death and rapidly repopulates the injured cartilage matrix, which suggests a potential approach for articular cartilage repair. This study was undertaken to determine whether recombinant human stromal cell–derived factor 1α (rhSDF‐1α), a potent CPC chemoattractant, would improve the quality of cartilage regeneration, hypothesizing that increased recruitment of CPCs by rhSDF‐1α would promote the formation of cartilage matrix upon chondrogenic induction.Methods: Full‐thickness bovine chondral defects were filled with hydrogel, composed of fibrin and hyaluronic acid and containing rhSDF‐1α. Cell migration was monitored, followed by chondrogenic induction. Regenerated tissue was evaluated by histology, immunohistochemistry, and scanning electron microscopy. Push‐out tests and unconfined compression tests were performed to assess the strength of tissue integration and the mechanical properties of the regenerated cartilage.Results: Use of rhSDF‐1α dramatically improved CPC recruitment to the chondral defects at 12 days. After 6 weeks under chondrogenic conditions, cell morphology, proteoglycan density, and the ultrastructure of the repair tissue were all similar to that found in native cartilage. Compared with empty controls, neocartilage generated in rhSDF‐1α–containing defects showed significantly greater interfacial strength, and acquired mechanical properties comparable to those of native cartilage.Conclusion: This study showed that stimulating local CPC recruitment prior to treatment with chondrogenic factors significantly improves the biochemical and mechanical properties of the cartilage tissue formed in chondral defects. This simple approach may be implemented in vivo as a one‐step procedure by staging the release of chemokine and chondrogenic factors from within the hydrogel, which can be achieved using smart drug‐delivery systems.

The accuracy of quantitative real-time RT-PCR (qRT-PCR) is often influenced by experimental artifacts, resulting in erroneous expression profiles of target genes. The practice of employing normalization using a reference gene significantly improves reliability and its applicability to molecular biology. However, selection of an ideal reference gene(s) is of critical importance to discern meaningful results. The aim of this study was to evaluate the stability of seven potential reference genes (Actb, GAPDH, 18S rRNA, CycA, Hprt1, Ywhaz, and Pgk1) and identify most stable gene(s) for application in tissue culture research using the rat and rabbit intervertebral disc (IVD).