Cell depleted areas do not repopulate after diphtheria toxin-induced killing of mandibular cartilage chondrocytes

Growth of mandibular condylar cartilage (MCC) is associated with cell proliferation within the polymorphic cell layer and subsequent differentiation into chondrocytes that reside along the condylar surface and along the cartilage/subchondral bone interface. We examined whether cells in the polymorphic layer would proliferate and repopulate toxin-induced cell-depleted areas in MCCs of adult mice.We induced diphtheria toxin (DTA) expression (ROSA26l-s-lDTA) to cell-autonomously kill large fractions of MCC chondrocytes throughout the cartilage or along the articular cartilage surface with Aggrecan-CreERt2 (AcanCreERt2) or Lubricin-CreERt2 (Prg4CreERt2) Cre-recombinase-inducible mice, respectively. We examined MCCs from these mice shortly after cell killing or several months later with histology and confocal microscopy for evidence of chondrocyte proliferation and repopulation.AcanCreERt2-induced DTA expression killed an average of 53% MCC chondrocytes in adult mice after 1 week (39-66%, 95% confidence interval (CI)). Twelve weeks later, surviving chondrocytes had proliferated but not migrated to cell depleted areas. Prg4CreERt2-induced DTA expression killed an average of 24% surface chondrocytes in mice after 5 weeks (14-34% CI). After thirteen weeks there was 34% fewer surface chondrocytes (4-63% CI) in Prg4CreERt2 DTA-induced mice compared to controls.In adult mice, after diphtheria toxin-mediated chondrocyte killing, cell depleted areas within MCC cartilage are not repopulated by new cells.