These images are from pig mitral valves using Second-Harmonic Generation (SHG) imaging, which highlights collagen fibrils. The mitral valve is responsible for directing blood flow in your heart. Without it, your heart will not be able to pump blood to the rest of your body. Mitral valves are composed largely of collagen and elastin fibrils with some fibroblasts interspersed. Collagen and elastin provide structural support as the valve undergoes large variations in pressure as your heart beats.
What is really interesting about collagen (to me, as an optics enthusiast) is that they have an intrinsic optical contrast due to its chemical structure. This particular chemical feature (the lack of inversion symmetry) gives rise to a unique optical phenomena called second harmonic generation. In short, when you shine ultrafast laser pulses (sub-nanosecond) at a sample that lacks inversion symmetry at the correct angle, the sample will generate a new wavelength of light at double the frequency of light you send in. In these images, we sent in 900-nm light (~150 femtosecond pulse width, 80MHz) and detected 450-nm light. This means you don’t really need dyes to see it, which come with a variety of benefits associated with many dyes, such as toxicity and homogeneous levels of staining. .
More commonly, SHG is a phenomenon typically used to seed lasing media for a variety of laser systems, among other things. But it turns out it’s a useful mode of contrast for studying biology. If you are interested in learning more on SHG imaging of collagen, check out this Nature Protocols paper from a prominent SHG research group.