Development process of traumatic heterotopic ossification of the temporomandibular joint in mice.

The exact development process underlying traumatic heterotopic ossification of the temporomandibular joint (THO-TMJ) is largely unclear. In this study, we try to explore the histological development process of THO-TMJ.Condylar cartilage of one-month-old male mice was partially removed from the left joint with small scissors to induce THO-TMJ. The phenotypes were observed using gross observation, microcomputed tomography (micro-CT) scans and histological examination from one month to six months after surgery.The micro-CT examination results showed that the injured condyle integrated with ectopic bone tissue to form an osteophyte and that the volume and density of the osteophyte grew exponentially with time. Hematoxylin and eosin (H&E), safranin O and fast green staining of the THO-TMJ specimens revealed that the ectopic bone tissue was mainly nonmineralized fibrous tissue 1 month after surgery. This tissue gradually transformed into cartilage 3 months after surgery. Finally, the tissues transformed into mature bone tissue 6 months after surgery. Immunofluorescence staining showed VEGF-α expression in the heterotopic tissue 1 month after surgery, and the expression of Sox9 in the heterotopic tissue was obvious 3 months after surgery. Furthermore, OCN expression was evident in most of the heterotopic tissue 6 months after surgery. The results also showed clear hypoxia-inducible factor 1-alpha (Hif-1α) expression in the injured chondrocytes of the condyle, especially in the articular proliferative zone and fibrocartilaginous zone.The THO-TMJ imaging characteristics indicated an exponential change with time. Histologically, the development process of THO-TMJ is an endochondral ossification process and includes three stages, fibroproliferative, chondrogenic and osteogenic stage. In addition, Hif-1α, which was expressed in some of the injured chondrocytes, may play an essential role in the initial THO-TMJ.