Alligator Sesamoid Anatomy

We are happy to present a new project authored by Henry P. Tsai and myself entitled “Ontogeny of the Alligator Cartilago Transiliens and Its Significance for Sauropsid Jaw Muscle Evolution” which is out in PLoS ONE this week. The link to the paper is here. 

The paper describes a nodular structure characteristic to crocodilian jaw muscles known as the cartilago transiliens. Despite its familiarity to morphologists, anatomists, and the like, few studies have focused much on it. We tested the hypothesis that the structure is actually a sesamoid, or an intramuscular nodule, linking two historically disparate muscles. Historically, its been treated as a special structure without any particular developmental history. By using imaging, dissection and histology methods on a sample of different-aged alligators, we found that, based on a number of criteria, indeed the cartilaginous nodule is likely a sesamoid. Not earth-shattering research but it holds significance for understanding how the jaw muscles function, how they develop,  how they evolved among reptiles and the nature of the pterygoid buttress system of crocodilians. I had stumbled on this idea about the sesamoid/cartilago transiliens back when we published the 2007 J Morph Archosaur Jaw Muscle Homology paper. So, it’s good to see another paper spawn out of that work.

Besides integrating classical dissection and histology, the project was fun because we got to employ a relatively new staining technique using Lugol’s Iodine (I2KI) and MicroCT to visualize the 3D anatomy of the jaw musculature. I hinted at some of this last year and it’s good to see our first shot make it to press. Its remarkable how different iodine-enhanced CT is compared to MRI, which is sometimes used for muscle anatomy…though it’s best for brain and nervous tissue.

Similar coronal sections through same Alligator specimen using MicroMRI (Left) and Iodine-enhance MicroCT (Right)

We have put together a 3D model and pdf of the dataset, which is featured extensively in the paper. We bumped into a couple of technical difficulties at the last minute this week, so we aren’t able to launch at the same time the paper comes out (darn) however, it’ll be up soon enough. We’ll be employing this technique often in the Holliday Lab as it’s proven essential to understanding the 3D anatomy of muscles and other soft tissues that were always a challenge to convey on 2D figures, and almost impossible to get at using standard CT scanning and even MRI. Had this technique been around during my dissertation, it would’ve been a completely different monster. So jaw muscles v2.0 here we come.

The real milestone here is that this paper marks the 1st paper published with my 1st graduate student. hoo-rah. Henry is keen on limb anatomy moreso than heads, but in our Integrative Anatomy program, we have graduate students perform research rotations within their advisor’s lab as well as in other neighboring labs in IA or in other departments. This let’s them learn several subjects and techniques while they develop ideas for dissertations, get to know labs, etc. We thought this project would be a great compromise on learning bread & butter techniques in the lab, connective tissue biology & cartilage while working on a small soft tissue anatomy project on an animal we’re both quite fond of. Henry rocked it and was key in seeing its publication basically one year after he started graduate school.

About Casey

I am an Associate Professor of Anatomy at University of Missouri-Columbia. I teach Anatomy for the Medical School. I conduct research on the evolutionary morphology of vertebrates, particularly the structure, function, and evolution of the feeding apparatus. Much of this involves studying the biology of bone, cartilage and muscle. of dinosaurs and fossil crocs. I have a great job.
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