Minimally invasive visualization of follicles in the ovaries of adolescent and young adult (AYA) patients with cancer is a promising technique for maintaining fertility by cryopreserving follicle-rich ovarian tissues. Optical coherence tomography (OCT) using near-infrared light, which is less invasive to the follicles, is a promising imaging technique for quantifying follicle density before cryopreservation. A spectral-domain OCT system equipped with a superluminescent diode with a central wavelength of 1300 nm was applied to visualize primordial follicles with a diameter of about 20 μm, which are often found at depths of 100–1000 μm in ovarian tissue. We used a 20x immersion objective to improve lateral image resolution and reduce ghost images due to surface reflections. OCT images of 30 μm diameter glass beads mimicking primordial follicles embedded in a polyacrylamide gel containing scattering particles were visualized at the depth required for imaging the ovaries. Ovarian tissues of 3-day-old and 12-day-old mice were fixed between an originally designed hydrophobic silicone plate and a cover glass. The distribution of primordial follicles and the structure of mature follicles were visualized in ovaries of 3-day-old and 12-day-old mice, respectively. These results indicate that near-infrared OCT at a central wavelength of 1300 nm with an extended depth of light penetration can effectively visualize the structure of follicles and quantify their density at depths where follicles are abundant in ovarian tissues. Further improvements in spatial resolution and image processing are expected to accelerate the contribution to reproductive medicine in the future.
Significance: Photobiomodulation is a well-established therapeutic modality. However, the mechanism of action is poorly understood, due to lack of research in the causal relationship between the near-infrared (NIR) light irradiation and its specific biological effects, hindering broader applications of this technology.
Aim: Since biological chromophores typically show several absorption peaks, we determined whether specific effects of photobiomodulation are induced with a combination of two wavelengths at a certain range of irradiance only, rather than a single wavelength of NIR light.
Approach: In order to analyze a wide array of combinations of multispectral NIR light at various irradiances efficiently, we developed a new optical platform equipped with two distinct wavelengths of NIR lasers by high-throughput multiple dosing for single-cell live imaging. Two wavelengths of 1064 and 1270 nm were selected based on their photobiomodulatory effects reported in the literature.
Results: A specific combination of wavelengths at low irradiances (250 to 400 mW / cm2 for 1064 nm and 55 to 65 mW / cm2 for 1270 nm) modulates mitochondrial retrograde signaling, including intracellular calcium and reactive oxygen species in T cells. The time-dependent density functional theory computation of binding of nitric oxide (NO) to cytochrome c oxidase indicates that the illumination with NIR light could result in the NO release, which might be involved in these changes.
Conclusions: This optical platform is a powerful tool to study causal relationship between a specific parameter of NIR light and its biological effects. Such a platform is useful for a further mechanistic study on not only photobiomodulation but also other modalities in photomedicine.
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