The TMJ consists of multiple interacting tissues that are prone to injury- and disease-related degeneration. According to the National Institutes of Health (NIH), an estimated 3% to 5% of Americans suffer from a TMJ disorder. The lack of understanding of the development and function of the TMJ at the molecular level has hampered progress toward the diagnosis and treatment of TMJ disorders.
In the present work, we evaluated the role of sprouty genes during TMJ development. Fgf
genes have previously been shown to have important roles during the development of various organs, including ear, tooth, lens, mandible, palate, and muscle (Shim et al., 2005
; Boros et al., 2006
; Klein et al., 2006
; Goodnough et al., 2007
; Mina et al., 2007
; Welsh et al., 2007
; Yang et al., 2010
; Matsumura et al., 2011
). We observed strong expression of Spry1
in the lateral pterygoid and temporalis muscles. We discovered that the combined inactivation of Spry1
resulted in overgrowth of these muscles, leading to the disruption of normal glenoid fossa development. Surprisingly, Spry1
embryos formed a complete condyle and joint disc, providing the first evidence that the condyle and disc form independently of the fossa.
The failure of glenoid fossa formation in Spry1
embryos may be due to one of three possibilities. First, the absence of the fossa could result from increased Fgf signaling in the absence of Spry1
in the muscle, which may inhibit the ossification of the forming bridge that ultimately gives rise to the temporal bone. Consequently, an overgrowth of the muscle could fill the space that otherwise would have been occupied by the fossa. Second, the lack of fossa may be due to a physical impediment to bone formation, perhaps due to excessive growth of the temporalis muscle that occupies the space between the two individual cartilages, preventing their fusion for fossa formation. This hypothesis is supported by the fact that cranial bones form by the fusion of many individual ossification centers (McBratney-Owen et al., 2008
). We observed the formation of an isolated lateral bony fragment as early as E14.5. At E15.5 in control animals, it becomes part of the fossa. By contrast, in Spry1
embryos, the small bony fragment remains isolated in a position equivalent to the lateral distal tip of the fossa. Because this fragment expresses ColI
, it is likely that bone has formed via
endochondral ossification. In fact, only this portion of the fossa is missing in a Sox9
conditional knockout (Wang et al., 2011
), suggesting that more than one mesenchymal condensation may give rise to the fossa, and that the fossa possesses multiple skeletal origins. Third, hyperactivated Fgf signaling may alter the fate of mesenchymal cells. The specific fate of an individual neural crest cell is determined by the signals they receive from the surrounding tissues (for review, see Trainor, 2010
). The glenoid fossa and cranial muscles derive partly from cranial neural crest cells (Gu et al., 2008
; Tzahor, 2009
), and hyperactivation of Fgfr4
might favor muscle rather than bone differentiation, thereby generating temporalis muscle instead of fossa.
In summary, we showed that Fgfr4
expression is restricted to the cranial muscles and mimics the expression of Spry1, Spry2
, and Spry4
, suggesting that sprouty genes modulate signaling downstream of FGFR4 in the TMJ. Moreover, FGF6 is a key ligand of FGFR4 and has been reported to play a crucial role in myogenesis (reviewed in Armand et al., 2006
). In the absence of Spry1
, Fgf signaling via
FGFR4 may be hyperactivated in the muscle, increasing myoblast proliferation, evidenced by increased cell proliferation in the mutants. Therefore, we suggest that condensations of the temporal bone that give rise to the glenoid fossa are able to form in the absence of Spry1
, but the overgrown temporalis muscle impedes the fusion of these two elements to form the fossa ().
Figure 4. Model showing the requirement of sprouty genes in TMJ formation. Spry1, Spry2, Spry4, Fgfr4, and Fgf6 are expressed in the lateral pterygoid (lp) and temporalis (tm) muscle around the TMJ during embryonic development. Conditional inactivation of Spry1 (more ...)
Future studies will need to investigate the relationship between sprouty genes and other signals involved in the growth and differentiation of muscle cells, as well as how these affect the formation of the fossa. Further molecular understanding of TMJ organogenesis is essential to improve diagnoses and develop novel therapeutic approaches for TMJ disorders.