Visualizing intra-medulla lipids in human hair using ultra-multiplex CARS, SHG, and THG microscopy

Exploring liquid-liquid phase separation in vitro and in vivo using multimodal nonlinear optical imaging

Liquid-liquid phase separation leads to the formation of liquid droplets (LqDs) such as P granules in Caenorhabditis elegans (C. elegans). In this study, we demonstrate the label-free visualization of LqDs using multimodal nonlinear optical imaging both in vitro and in vivo. In vitro measurements with polymerized adenine [poly(A)], we found significantly higher poly(A) concentrations in LqDs compared to surrounding solutions, with the limit of detection (LoD) of 32 mg/mL. In vivo measurements, we performed label-free imaging of C. elegans. Despite efforts to detect P granules within P lineage cells in both wild-type C. elegans and green fluorescent protein (GFP)-tagged strains, no clear RNA-specific signals were observed. This indicates that the RNA concentration in P granules is lower than anticipated and falls below our in vitro LoD. These results underscore the challenges of label-free RNA detection in P granules.

Spectral histology of hair and hair follicle using infrared microspectroscopy

OBJECTIVE

Today, there is only limited knowledge of the spatial organization of hair chemistry. Infrared microspectroscopy is a well-established tool to provide such information and has significantly contributed to this field. In this study, we present new results combining multiple infrared microspectroscopy methods at different length scales to create a better chemical histology of human hair, including the hair follicle, hair shaft, hair medulla and hair cuticle.

METHODS

We used hyperspectral IR imaging & spectroscopy (HIRIS) and synchrotron-radiation FTIR microspectroscopy (SR-μFTIR) to measure transversal hair sections and SR-μFTIR to obtain high-resolution maps of longitudinal sections from the hair shaft and from the hair follicle. We used optical photothermal IR microspectroscopy (OPTIR) to analyse the cuticle surface of intact hairs.

RESULTS

By mapping longitudinal sections of the human hair follicle with confocal SR-μFTIR, we report the first demonstration of glycogen presence in the outer root sheath of the hair follicle by spectroscopy, and its quantification at the micron scale. Spectral maps, combined with machine learning-based analysis, enabled us to differentiate the various layers of the hair follicle and provided insights into the chemical changes that occur during hair formation in the follicle. Using HIRIS and SR-μFTIR to analyse the hair medulla in transversal sections of human hairs, we report here, for the first time by vibrational spectroscopy methods, the detection of unsaturated lipids at very low concentrations in the medulla. By analysing longitudinal sections of the hair shaft with SR-μFTIR, we found that calcium carboxylates are present in large regions of the hair cuticle, and not just in small focal areas as previously thought. We then use OPTIR to analyse the hair cuticle of intact hairs at submicron resolution without sectioning and report the distribution of calcium carboxylates at the surface of intact hair for the first time.

CONCLUSION

These new findings illustrate the potential of infrared microspectroscopy for imaging the chemical composition of human hair and may have implications for biomedical research or cosmetology.

Super-resolution infrared microspectroscopy reveals heterogeneous distribution of photosensitive lipids in human hair medulla

Human hair medulla chemical composition appears mostly homogenous when mapped by FTIR microspectroscopy even when using a synchrotron radiation source (SR-μFTIR) but it is expected to be heterogeneous. We performed sub-micron chemical mapping of hair cortex and medullas using Optical Photothermal Infrared microspectroscopy (OPTIR) and a mid-infrared Quantum Cascade Laser (QCL) source covering the fingerprint and the CH stretching region. Photodamages were observed in the hair cortex at mild laser power and occurred in the hair medulla even at the lowest power settings of the IR QCL pulsed at 100 kHz rate (4 μW/μm² average power density) and visible probe laser (200 μw/μm² average power density). Photoconversion of calcium carboxylates in other molecules, possibly sodium carboxylates, was observed. Attenuation of the IR QCL power by 40% using ZnSe filter and/or high-speed measurements (1000 cm^-1/s) succeeded in almost completely eliminating the photodamages and photoconversion. OPTIR maps and images showed that the medullas were highly heterogeneous at the submicron scale. We found calcium carboxylates, aliphatic lipids and wax esters in small units, hundreds of nanometers in size. The 1470 cm^-1 CO sym stretching peak of calcium carboxylates and the CH₂asym stretching peak from aliphatic lipids proved to be the most efficient peaks to track the distribution of these molecules. OPTIR had enough sensitivity to map accurately only the strongest peaks from lipids and calcium carboxylates, weaker peaks such as the ester CO and sulfoxide SO bands were not accurately detected by OPTIR even when they were shown to be present by SR-μFTIR. Quantification of the medulla components by OPTIR is difficult due to several factors: discontinuous QCL emission, and noise. The weaker peaks such as CH₃, CO, SO are often underestimated or not detected. We demonstrate here that OPTIR can be used to measure, map and image dark, photosensitive samples using very low IR power.

A systematic review on the lipid composition of human hair

Hair lipid composition varies by ethnic hair type and by hair layer. Lipids in the cuticle, cortex, and medulla of the hair shaft provide a protective barrier to environmental and chemical damage, prevent hair breakage and desorption, and affect the elastic and tensile properties of hair. The aim of this systematic review is to provide an overview of the lipid composition and ethnic differences of human hair, effects of external damage on lipid content and properties, and changes in hair lipid composition associated with disease states. PubMed/MEDLINE was searched up to March 2021 according to PRISMA guidelines for articles discussing the lipid content of human hair and effects of physical, chemical, or environmental damage, and disease. Fifty-nine articles investigating the lipid content of hair were included for review. Lipids affect fluid permeability, hydration, strength, and texture of ethnic hair fibers. Lipid loss is accelerated by hair-damaging treatments such as bleach, dye, perm, straightening, and surfactant use, and sun and aging processes, leading to dehydrated, breakable, disordered, and dull hair. Diseases including acne, alopecia, and breast, gastric, prostate, lung, and rectal cancers display elevated hair lipid levels. Lipids are vital in protection against damage and maintenance of healthy hair. Further studies are needed to investigate the effects of lipids on the structural properties of ethnic hair, and changes in hair lipid composition with various dermatologic and systemic diseases.

Giant Enhancement of Second-Harmonic Generation in Hybrid Metasurface Coupled MoS2 with Fano-Resonance Effect

Plasmonic nanostructures have been regarded as potential candidates for boosting the nonlinear up-conversion rate at the nanoscale level due to their strong near-field enhancement and inherent high design freedom. Here, we design a hybrid metasurface to realize the moderate interaction of Fano resonance and create the dual-resonant mode-matching condition to facilitate the nonlinear process of second harmonic generation (SHG). The hybrid metasurface presents dipolar and octupolar plasmonic modes near the fundamental and doubled-frequency wavelengths, respectively, further utilized to enhance the SHG of low-dimensional MoS2 semiconductors. The maximum intensity of SHG in hybrid metasurface coupled MoS2 is more than ten thousand times larger than that of other structure-units coupled MoS2. The conversion efficiency is reported to be as high as 3.27 × 10-7. This work paves the way to optimize nonlinear light-matter interactions in low-dimensional structures coupled with semiconductors.

Coherent anti-Stokes Raman scattering cell imaging and segmentation with unsupervised data analysis

Coherent Raman imaging has been extensively applied to live-cell imaging in the last 2 decades, allowing to probe the intracellular lipid, protein, nucleic acid, and water content with a high-acquisition rate and sensitivity. In this context, multiplex coherent anti-Stokes Raman scattering (MCARS) microspectroscopy using sub-nanosecond laser pulses is now recognized as a mature and straightforward technology for label-free bioimaging, offering the high spectral resolution of conventional Raman spectroscopy with reduced acquisition time. Here, we introduce the combination of the MCARS imaging technique with unsupervised data analysis based on multivariate curve resolution (MCR). The MCR process is implemented under the classical signal non-negativity constraint and, even more originally, under a new spatial constraint based on cell segmentation. We thus introduce a new methodology for hyperspectral cell imaging and segmentation, based on a simple, unsupervised workflow without any spectrum-to-spectrum phase retrieval computation. We first assess the robustness of our approach by considering cells of different types, namely, from the human HEK293 and murine C2C12 lines. To evaluate its applicability over a broader range, we then study HEK293 cells in different physiological states and experimental situations. Specifically, we compare an interphasic cell with a mitotic (prophase) one. We also present a comparison between a fixed cell and a living cell, in order to visualize the potential changes induced by the fixation protocol in cellular architecture. Next, with the aim of assessing more precisely the sensitivity of our approach, we study HEK293 living cells overexpressing tropomyosin-related kinase B (TrkB), a cancer-related membrane receptor, depending on the presence of its ligand, brain-derived neurotrophic factor (BDNF). Finally, the segmentation capability of the approach is evaluated in the case of a single cell and also by considering cell clusters of various sizes.

Spectroscopic second and third harmonic generation microscopy using a femtosecond laser source in the third near-infrared (NIR-III) optical window

In this study, second harmonic generation (SHG) and third harmonic generation (THG) spectroscopic imaging were performed on biological samples using a femtosecond laser source in the third near-infrared (NIR) optical window (NIR-III). Using a visible-NIR spectrometer, the SHG and THG signals were simultaneously detected and were extracted using spectral analysis. Visualization of biological samples such as cultured cells (HEK293 T), mouse brain slices, and the nematode Caenorhabditis elegans was performed in a label-free manner. In particular, in an SHG image of an entire coronal brain section (8 × 6 mm2), we observed mesh-like and filamentous structures in the arachnoid mater and wall of the cerebral ventricle, probably corresponding to the collagen fibers, cilia, and rootlet. Moreover, the THG images clearly depicted the densely packed axons in the white matter and cell nuclei at the cortex of the mouse brain slice sample and lipid-rich granules such as lipid droplets inside the nematode. The observations and conclusions drawn from this technique confirm that it can be utilized for various biological applications, including in vivo label-free imaging of living animals.