Comparison of the structural features between chicken quadratomandibular joint and rat temporomandibular joint with...

The new jaw joint of mammals evolved between the squamosal and the dentary. We investigated the structural features of the chicken quadratomandibular joint (QMJ) (primary jaw joint) and the rat temporomandibular joint (TMJ) (new jaw joint) to determine whether these structures reflect their putative movements from macroscopic and histological perspectives.

Chicken QMJ and rat TMJ were dissected, and their joint apparatuses, including ligaments and meniscus/disc, were analyzed macroscopically. Microscopic analysis, incorporating immunohistochemistry and in situ hybridization, was conducted to investigate protein localization and gene expression within the extracellular matrix of cartilage and bone.

The chicken QMJ contained the jugomandibular ligament, running laterally and posteriorly to the QMJ, tightly bound to the articular meniscus. These features suggest a role in supporting and controlling quadrate bone movements. The rat TMJ lacked a lateral ligament, and the mandibular head had an elongated oval shape along the anterior-posterior axis. This morphology indicates that lateral mandibular movements are rare, with jaw motion predominantly occurring in the anterior-posterior direction. The chicken QMJ meniscus contained distinct cartilaginous tissues, whereas the rat TMJ articular disc acquired cartilaginous characteristics after occlusion was established (5 weeks postnatally), suggesting that the meniscus/disc properties are closely associated with mastication.

The structural features of chicken and rat jaw joints, including their joint apparatuses, appropriately reflect their respective functions, such as movement dynamics and resistance to mastication pressure.