A Joint Less Ordinary: Intriguing Roles for Hedgehog Signalling in the Development of the Temporomandibular Synovial Joint

Abstract

This review highlights the essential role of Hedgehog (Hh) signalling in the developmental steps of temporomandibular joint (TMJ) formation. We review evidence for intra- and potentially inter-tissue Hh signaling as well as Glioma-Associated Oncogene Homolog (GLI) dependent and independent functions. Morphogenesis and maturation of the TMJ’s individual components and the general landscape of Hh signalling is also covered. Comparison of the appendicular knee and axial TMJ also reveals interesting differences and similarities in their mechanisms of development, chondrogenesis and reliance on Hh signalling.

The TMJ Disc Is a Common Ancestral Feature in All Mammals, as Evidenced by the Presence of a Rudimentary Disc During Monotreme Development

“…However, unlike in therian mammals, the monotreme fibrocartilage failed to separate from the condylar to form an articular disc in the TMJ. Interactions with musculature, both mechanical (Habib et al, 2007;Purcell et al, 2012;Jahan et al, 2014;Nickel et al, 2018) and molecular (Shibukawa et al, 2007;Gu et al, 2008;Purcell et al, 2009Purcell et al, , 2012Kinumatsu et al, 2011;Michikami et al, 2012;Yasuda et al, 2012;Kubiak et al, 2016), have been suggested to be responsible for the proper formation of the TMJ disc. Lack of mechanical force or changes in signaling from the muscle in monotremes might therefore result in the disc remaining attached to the condylar.…”

The TMJ disc is a common ancestral feature in all mammals, as evidenced by the presence of a rudimentary disc during monotreme development

“…However, unlike in therian mammals, the monotreme fibrocartilage failed to separate from the condylar to form an articular disc in the TMJ. Interactions with musculature, both mechanical [13,30–32] and molecular [12,13,15,33–37], have been suggested to be responsible for the proper formation of the TMJ disc. Lack of mechanical force in monotremes might therefore result in the disc remaining attached to the condylar.…”

Development of the temporomandibular joint in miniature pig embryos

“…The cell sources between the TMJ articular cartilage and long bone joint are different. The growth plate cartilage is hyaline cartilage that develops from the embryonic mesoderm and contains Type 2 collagen (Kubiak & Ditzel, 2016;Suzuki & Iwata, 2016). However, the TMJ condylar cartilage originates from the embryonic ectoderm and is a type of fibrous cartilage mainly composed of Type I collagen.…”