Integrative Anatomy, Department of Pathology and Anatomical Sciences, University of Missouri, Columbia, Missouri, United States of America.
PLoS One. 2011;6(9):e24935. doi: 10.1371/journal.pone.0024935. Epub 2011 Sep 16.
The cartilago transiliens is a fibrocartilaginous structure within the jaw muscles of crocodylians. The cartilago transiliens slides between the pterygoid buttress and coronoid region of the lower jaw and connects two muscles historically identified as m. pseudotemporalis superficialis and m. intramandibularis. However, the position of cartilago transiliens, and its anatomical similarities to tendon organs suggest the structure may be a sesamoid linking a single muscle. Incompressible sesamoids often form inside tendons that wrap around bone. However, such structures rarely ossify in reptiles and have thus far received scant attention. We tested the hypothesis that the cartilago transiliens is a sesamoid developed within in one muscle by investigating its structure in an ontogenetic series of Alligator mississippiensis using dissection, 3D imaging, and polarizing and standard light microscopy. In all animals studied, the cartilago transiliens receives collagen fibers and tendon insertions from its two main muscular attachments. However, whereas collagen fibers were continuous within the cartilaginous nodule of younger animals, such continuity decreased in older animals, where the fibrocartilaginous core grew to displace the fibrous region. Whereas several neighboring muscles attached to the fibrous capsule in older individuals, only two muscles had significant contributions to the structure in young animals. Our results indicate that the cartilago transiliens is likely a sesamoid formed within a single muscle (i.e., m. pseudotemporalis superficialis) as it wraps around the pterygoid buttress. This tendon organ is ubiquitous among fossil crocodyliforms indicating it is a relatively ancient, conserved structure associated with the development of the large pterygoid flanges in this clade. Finally, these findings indicate that similar tendon organs exist among potentially homologous muscle groups in birds and turtles, thus impacting inferences of jaw muscle homology and evolution in sauropsids in general.
三叉间软骨(transiliens cartilage)是鳄形目动物颌肌中的一种纤维软骨结构。三叉间软骨在翼突支柱和下颌的冠状区域之间滑动，并连接两个历史上被识别为 m. pseudotemporalis superficialis 和 m. intramandibularis 的肌肉。然而，三叉间软骨的位置及其与肌腱器官的解剖相似性表明，该结构可能是连接单个肌肉的籽骨。不可压缩的籽骨通常形成于环绕骨骼的肌腱内。然而，这种结构在爬行动物中很少发生骨化，因此迄今为止受到的关注甚少。我们通过在密西西比鳄的个体发育系列中进行解剖、3D 成像、偏光和标准光显微镜检查，测试了三叉间软骨是在单个肌肉内发育的籽骨的假设。在所有研究的动物中，三叉间软骨接收来自其两个主要肌肉附着点的胶原纤维和肌腱插入物。然而，在年轻动物的软骨结节中，胶原纤维是连续的，而在年龄较大的动物中，纤维软骨核心生长会取代纤维区域，这种连续性会降低。尽管在老年个体中，有几个相邻的肌肉附着在纤维囊上，但只有两个肌肉在年轻动物中对该结构有重要贡献。我们的结果表明，三叉间软骨很可能是在单个肌肉（即 m. pseudotemporalis superficialis）内形成的籽骨，因为它环绕着翼突支柱。这种肌腱器官在化石鳄形目动物中普遍存在，表明它是一种相对古老的、保守的结构，与这个类群中大型翼突翼板的发育有关。最后，这些发现表明，类似的肌腱器官存在于鸟类和海龟中潜在同源的肌肉群中，从而影响了对恐龙类恐龙颌肌同源性和进化的推断。
