The uterine wall responds to internal pressure the way a balloon does when you blow it up: at first it gives easily, then it grows more and more stubborn. But while a balloon simply stretches, the uterus resists thanks to a network of collagen fibers that first unfolds and then pulls taut, refusing to give any further. To understand how remodeling of this network changes the organ's function, researchers took segments of mouse uterus and inflated them with a tiny balloon inside an X-ray micro-CT scanner. Each inflation step was imaged at a resolution of 10 micrometers, that is, 10 times finer than a human hair.
Next, half the samples were soaked in a glutaraldehyde solution, a substance that cross-links proteins to one another the way glue joins threads into a rigid mesh. After this treatment, collagen can no longer slide and unfold, and the tissue should become noticeably stiffer. That is exactly what happened: the treated segments required higher pressure at every inflation level, and the work spent on stretching increased. To turn these observations into numbers, the scientists built a computer model of the uterine wall as a homogeneous material with two families of fibers. The model parameters were tuned so that the calculated contours matched those measured in the images. The fitting error did not exceed 3 percent.
Previously, to see changes in collagen, researchers did histology: a thin slice of tissue under a microscope. But a slice shows composition, not how the tissue will behave under load. Now, inflation data make it possible to quantify the mechanical consequences of remodeling, that is, of tissue restructuring. This opens the way to studying aging, disease, and toxic exposures in which collagen changes may be far smaller than those caused by glutaraldehyde treatment. The next step will be to test the smallest mechanical changes the method can detect, and to account for the layered structure of the wall, active muscle contraction, and viscosity, in order to bring the model closer to living tissue. For now, though, a balloon catheter the diameter of a pencil lead keeps coaxing the uterus to reveal 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.