If you inflate a balloon, at first it stretches easily, and then with more and more difficulty. The wall of the uterus behaves the same way, because inside it there is a network of collagen fibers. These fibers first straighten out, like a crumpled net, and then tighten and keep the wall from stretching further. Researchers took small pieces of a mouse uterus and inflated them with a tiny balloon inside an X-ray tomograph. Each inflation step was imaged to see how the shape of the wall changed.
Half of the samples were kept in a solution of glutaraldehyde, a substance that cross-links proteins to one another, the way glue joins threads. After that, collagen stops sliding, and the tissue becomes harder. That is exactly what happened: the treated pieces required higher pressure during inflation. The scientists built a computer model of the uterine wall and fitted its parameters so that the calculated shape matched the one seen in the images. It turned out that after cross-linking, the ground substance between the fibers became 9.2 times stiffer, the fibers themselves 2 times stiffer, and the sharpness of the transition from compliance to stubbornness increased 2.7 times. Previously, such changes could only be seen under a microscope on a tissue section, but a section does not show how the tissue will behave under load. The new method makes it possible to measure this from inflation data. It will be useful for studying aging, diseases, and toxic exposures, where collagen changes may be smaller than those caused by glue treatment. A balloon catheter the diameter of a pencil lead continues to coax the uterus into revealing its secrets.
The balloon catheter used to inflate the mouse uterus is 1 mm in diameter, roughly like a pencil lead.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.