Sharma, Neelima, Yara Haridy, and Neil Shubin. "Synovial joints were present in the common ancestor of jawed fish but lacking in jawless fish." PLoS biology 23, no. 2 (2025): e3002990.

 

What They Did

The researchers carried out imaging studies on jaw and pelvis joints of embryonic and juvenile skates (which are cartilaginous fish) and skull joints of juvenile and adult lampreys (which are jawless fish). They found that skate joints have cartilage skeleton components shaped so as to fit together, but with a gap between them. These joints appear to function similarly to the joints of humans and other tetrapods, where the bones are lined with cartilage at the joint and separated by a gap filled with lubricating fluid. The lamprey skull joints, however, do not show a similar gap, nor are the components shaped to fit together.

They also found that one type of collagen is found throughout the cartilaginous skeleton, while another kind is only at the edges of joints. Both skates and lampreys produced the same type of lubricating proteins around the joints, and the skate embryos produced signaling proteins around the developing joints. These proteins are also involved in the development and function of tetrapod joints.

In tetrapod embryos, the muscles have to contract for the joints to develop. The researchers added a paralytic to the tank water of some of the skate embryos and found that their joints did not develop normally, suggesting that muscle contraction is necessary for their joint development as well. Finally, they examined the fossil of a placoderm, an extinct fish that developed after jawless fish but before modern cartilaginous fish, and found that it had joints similar to those of skates and tetrapods.

Further Exploration

I learned a bit about bones and joint development while trying to understand this article. All the places where bones are fused together, such as in the skull, are also joints (see https://my.clevelandclinic.org/health/body/25137-joints). The type of joints I usually think of (knees, shoulders, etc.) are called synovial joints, with the synovial membrane providing cushioning between them. Those are the types of joints the researchers looked for in their fish studies.

Most human joints develop from a region of embryonic cells differentiating into two sections of bone-precursor cells, leaving with cells between them that develop into the joint tissues. Those in-between cells pinch apart, leaving a space between them, so you end up with two bones forming a joint from that one region of cells (see https://wisc.pb.unizin.org/mindmotionanatomy/chapter/limb-development/). The jaw joint is different because the jaw bones develop separately from two regions of cells, and then the joint tissues develop from the cells between the bones. I think of the more common joints as developing “from one into two” and the jaw joint as developing “from two into one” – though of course they all consist of two bones forming one joint.

The bones that make up the more common joints also go through a stage of being made of cartilage first, with the bone developing from the cartilage framework. The bones in the skull don’t go through the cartilage stage. I wonder if that difference in development is related to the particular problems associated with TMJ, but that’s a rabbit hole for another day!

Image credit: Andy Martinez / NOAA

https://commons.wikimedia.org/wiki/File:Leucoraja_erinacea.jpg