Functional alterations in mechanical loading of condylar cartilage induces changes in the bony subcondylar region

Bone remodeling is orchestrated by cells of the osteoblast lineage and involves an intricate network of cell-cell and cell-matrix interactions. This dynamic process engages systemic hormones, locally produced cytokines and growth factors, as well as the mechanical environment of the cells. In growing subjects, the mandibular condyle consists of both articular and growth components and the presence of progenitor cells is verified by their anabolic responses to growth hormones. The pathways of chondrocyte and osteoblast differentiation during endochondral bone formation are interconnected and controlled by key transcription factors. The present study was undertaken to explore the possibility and the extent by which the mechano-transduction events in chondrocytes are 'sensed' in the subchondral bony area under altered functional loading. To this end, the involvement of the JNK/ERK-AP-1/Runx2 signaling axe was investigated by immunohistochemistry in temporomandibular joints of young rats subjected to different functional mastication loads. Our results showed that mechanical load triggers differentiation phenomena through the induction of master tissue regulators, namely the expression and/or activation of the JNK-c-Jun signaling pathway components and c-Fos in subchondral osteoblasts, as well as the activation of ERK/MAPK and the cellular expression of the transcription factor Runx2 in subchondral osteoblasts.