New muscle layer discovered in the jaw

The newly discovered muscle layer extends from the back of the cheekbone to the anterior muscle process of the lower jaw. (S= superficial layer, D= deep layer, C= coronoid layer). Credit: Jens. C. Türp, University of Basel / UZB

Human anatomy still has a few surprises in store for us: researchers from the University of Basel have discovered a previously unknown part of our jaw muscles and have described this layer in detail for the first time.

The masseter muscle is the most prominent of the jaw muscles. If you place your fingers on the back of your cheeks and press your teeth together, you will feel the muscle tighten. Anatomy textbooks usually describe the masseter as consisting of a superficial part and a deep part.

Now, researchers led by Dr. Szilvia Mezey from the Department of Biomedicine at the University of Basel and Prof. Jens Christoph Türp from the University Center for Dentistry Basel (UZB) have described the structure of the masseter muscle as consisting of a additional third, even deeper layer. In the scientific journal annals of anatomy, they propose to name this layer Musculus masseter pars coronidea – in other words, the coronoid section of the masseter – because the newly described muscular layer is attached to the muscular process (or “coronoid”) of the lower jaw.

The anatomical study was based on a detailed examination of formalin-fixed jaw musculature, computer tomograms and analysis of stained tissue sections from deceased people who had donated their bodies to science. This was in addition to MRI data from a living person.

As if a new animal species had been discovered

“This deep section of the masseter muscle is clearly distinct from the other two layers in terms of course and function,” says Mezey. The arrangement of muscle fibers, she says, suggests that this layer is involved in stabilizing the lower jaw. It also appears to be the only part of the masseter that can pull the lower jaw back, i.e. towards the ear.

A look at historical anatomy studies and textbooks reveals that the structure of the masseter muscle has raised questions in the past. In a previous edition of Grey’s Anatomy, from the year 1995, the editors also describe the masseter muscle as having three layers, although the cited studies are based on the jaw musculature of other species and partly contradict each other.

Other individual studies from the early 2000s also reported three layers, but they divided the superficial section of the masseter into two layers and agreed with standard work in their description of the deeper section.

“Given these conflicting descriptions, we wanted to examine the structure of the masseter muscle again in depth,” says Türp. “Although it is generally accepted that anatomical research over the past 100 years has left nothing unturned, our discovery is a lot like zoologists discovering a new species of vertebrate.”

Reference: “The human masseter muscle revisited: First description of its coronoid part” by Szilvia E. Mezey, Magdalena Müller-Gerbl, Mireille Toranelli and Jens Christoph Türp, December 2, 2021, Annals of Anatomy – Anatomischer Anzeiger.
DOI: 10.1016/j.aanat.2021.151879

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