In vivo identification of arteries and veins using two-photon excitation elastin autofluorescence

Modified histological staining for the identification of arterial and venous segments of brain microvessels

BACKGROUND

This study aimed to develop a modified histochemical staining technique to successfully identify arterial and venous segments of brain microvessels.

NEW METHOD

Gelatin/red ink-alkaline phosphatase-oil red O (GIAO) staining was developed from the traditional gelatin-ink perfusion method. Oil red Chinese ink for brush writing and painting mixed with gelatin was used to label cerebral vascular lumens. Subsequently, alkaline phosphatase staining was used to label endothelial cells on the arterial segments of cerebral microvessels. Thereafter, the red ink color in vessel lumens was highlighted with oil red O staining.

RESULTS

The arterial segments of the brain microvessels exhibited red lumens surrounded by dark blue walls, while the venous segments were bright red following GIAO staining. Meanwhile, the nerve fiber bundles were stained brownish-yellow, and the nuclei appeared light green under light microscope. After cerebral infarction, we used GIAO staining to determine angiogenesis features and detected notable vein proliferation inside the infarct core. Moreover, GIAO staining in conjunction with hematoxylin staining was performed to assess the infiltration of foamy macrophages.

COMPARISON WITH EXISTING METHOD

Red Chinese ink enabled subsequent multiple color staining on brain section. Oil red O was introduced to improved the resolution and contrast between arterial and venous segments of microvessels.

CONCLUSION

With excellent resolution, GIAO staining effectively distinguished arterial and venous segments of microvessels in both normal and ischemic brain tissue. GIAO staining, as described in the present study, will be useful for histological investigations of microvascular bed alterations in a variety of brain disorders.

Label-Free Assessment of Key Biological Autofluorophores: Material Characteristics and Opportunities for Clinical Applications

Autofluorophores are endogenous fluorescent compounds that naturally occur in the intra and extracellular spaces of all tissues and organs. Most have vital biological functions - like the metabolic cofactors NAD(P)H and FAD+, as well as the structural protein collagen. Others are considered to be waste products - like lipofuscin and advanced glycation end products - which accumulate with age and are associated with cellular dysfunction. Due to their natural fluorescence, these materials have great utility for enabling non-invasive, label-free assays with direct ties to biological function. Numerous technologies, with different advantages and drawbacks, are applied to their assessment, including fluorescence lifetime imaging microscopy, hyperspectral microscopy, and flow cytometry. Here, the applications of label-free autofluorophore assessment are reviewed for clinical and health-research applications, with specific attention to biomaterials, disease detection, surgical guidance, treatment monitoring, and tissue assessment - fields that greatly benefit from non-invasive methodologies capable of continuous, in vivo characterization.

Non-invasive assessment of hair regeneration in androgenetic alopecia mice in vivo using two-photon and second harmonic generation imaging

The identification of crucial targets for hair regrowth in androgenetic alopecia (AGA) involves determining important characteristics and different stages during the process of hair follicle regeneration. Traditional methods for assessing key features and different stages of hair follicle primarily involve taking skin tissue samples and determining them through various staining or other methods. However, non-invasive assessment methods have been long sought. Therefore, in this study, endogenous fluorescence signals from skin keratin and second harmonic signals from skin collagen fibers were utilized as probes, two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) imaging techniques were employed to non-invasively assess hair shafts and collagen fibers in AGA mice in vivo. The TPEF imaging technique revealed that the alternation of new and old hair shafts and the different stages of the growth period in AGA mice were delayed. In addition, SHG imaging found testosterone reduced hair follicle area and miniaturized hair follicles. The non-invasive TPEF and SHG imaging techniques provided important methodologies for determining significant characteristics and different stages of the growth cycle in AGA mice, which will facilitate future non-invasive assessments on human scalps in vivo and reduce the use of animal testing.

Histological findings for the absorption of small and large liposomes - the basis of future drug delivery and contrast media systems

BACKGROUND AND OBJECTIVES

The purpose of our study was to obtain and characterize carrier systems in different sizes that can affect oral absorption, since the mechanisms of liposome absorption are not yet fully understood. From stomach to the small intestine, liposomes can be gradually destroyed. Understanding the factors that affect oral absorption leads to developing safe and effective nanosystems to improve the oral delivery of therapeutics.

MATERIALS AND METHODS

We determined the efficiency of the absorption of small and large liposomes at the level of gingival mucosa, heart, liver, testicles, kidneys, and lungs, using frozen-section fluorescence microscopy, on rat tissues after liposomes administration. A number of 36 male rats were divided in four groups: control groups, A and C, consisted of six rats each and did not receive liposomes; two other groups, B and D, were the experimental ones, and consisted of 12 male rats each. The animals received small liposomes (75-76 nm) and large liposomes (80-87 nm), respectively, administered either by endogastric tube or intraperitoneal injection. After 24 hours, the animals were sacrificed, and we harvested the organs. We performed frozen sections and analyzed them with fluorescence microscopy.

RESULTS

The frozen sections obtained from all organs revealed a higher absorption level of small liposomes in the testicles, liver, and gum, while the large liposomes had a greater affinity for the liver, with variations dependent on the route of administration.

CONCLUSIONS

Frozen-section fluorescence microscopy is a reliable technique for visualization of liposome absorption. Based on the size of these nanosystems, we revealed significant absorption for small liposomes in testicles, liver, heart, and gum, and for large liposomes mainly in the liver, compared with the control groups. The study advocates for the usage of liposomes for medical purposes, based on their absorption proprieties.

A Pleomorphic Puzzle: Heterogeneous Pulmonary Vascular Occlusions in Patients with COVID-19

Vascular occlusions in patients with coronavirus diseases 2019 (COVID-19) have been frequently reported in severe outcomes mainly due to a dysregulation of neutrophils mediating neutrophil extracellular trap (NET) formation. Lung specimens from patients with COVID-19 have previously shown a dynamic morphology, categorized into three types of pleomorphic occurrence based on histological findings in this study. These vascular occlusions in lung specimens were also detected using native endogenous fluorescence or NEF in a label-free method. The three types of vascular occlusions exhibit morphology of DNA rich neutrophil elastase (NE) poor (type I), NE rich DNA poor (type II), and DNA and NE rich (type III) cohort of eleven patients with six males and five females. Age and gender have been presented in this study as influencing variables linking the occurrence of several occlusions with pleomorphic contents within a patient specimen and amongst them. This study reports the categorization of pleomorphic occlusions in patients with COVID-19 and the detection of these occlusions in a label-free method utilizing NEF.

Two-Photon Fluorescence Microscopy and Applications in Angiogenesis and Related Molecular Events

The role of angiogenesis in health and disease have gained considerable momentum in recent years. Visualizing angiogenic patterns and associated events of surrounding vascular beds in response to therapeutic and laboratory-grade biomolecules have become a commonplace in regenerative medicine and the biosciences. To aid imaging investigations in angiogenesis, the two-photon excitation fluorescence microscopy (2PEF), or multiphoton fluorescence microscopy is increasingly utilized in scientific investigations. The 2PEF microscope confers several distinct imaging advantages over other fluorescence excitation microscopy techniques - for the observation of in-depth, three-dimensional vascularity in a variety of tissue formats, including fixed tissue specimens and in vivo vasculature in live specimens. Understanding morphological and subcellular changes that occur in cells and tissues during angiogenesis will provide insights to behavioral responses in diseased states, advance the engineering of physiologically-relevant tissue models and provide biochemical clues for the design of therapeutic strategies. We review the applicability and limitations of the 2PEF microscope on the biophysical and molecular-level signatures of angiogenesis in various tissue models. Imaging techniques and strategies for best practices in 2PEF microscopy will be reviewed.

Multimodal microscopy for the simultaneous visualization of five different imaging modalities using a single light source

Optical microscopy has been widely used in biomedical research as it provides photophysical and photochemical information of the target in subcellular spatial resolution without requiring physical contact with the specimen. To obtain a deeper understanding of biological phenomena, several efforts have been expended to combine such optical imaging modalities into a single microscope system. However, the use of multiple light sources and detectors through separated beam paths renders previous systems extremely complicated or slow for in vivo imaging. Herein, we propose a novel high-speed multimodal optical microscope system that simultaneously visualizes five different microscopic contrasts, i.e., two-photon excitation, second-harmonic generation, backscattered light, near-infrared fluorescence, and fluorescence lifetime, using a single femtosecond pulsed laser. Our proposed system can visualize five modal images with a frame rate of 3.7 fps in real-time, thereby providing complementary optical information that enhances both structural and functional contrasts. This highly photon-efficient multimodal microscope system enables various properties of biological tissues to be assessed.

Automated Detection and Diameter Estimation for Mouse Mesenteric Artery Using Semantic Segmentation

BACKGROUND

Pressurized myography is useful for the assessment of small artery structures and function. However, this procedure requires technical expertise for sample preparation and effort to choose an appropriate sized artery. In this study, we developed an automatic artery/vein differentiation and a size measurement system utilizing machine learning algorithms.

METHODS AND RESULTS

We used 654 independent mouse mesenteric artery images for model training. The model yielded an Intersection-over-Union of 0.744 ± 0.031 and a Dice coefficient of 0.881 ± 0.016. The vessel size and lumen size calculated from the predicted vessel contours demonstrated a strong linear correlation with manually determined vessel sizes (R = 0.722 ± 0.048, p < 0.001 for vessel size and R = 0.908 ± 0.027, p < 0.001 for lumen size). Last, we assessed the relation between the vessel size before and after dissection using a pressurized myography system. We observed a strong positive correlation between the wall/lumen ratio before dissection and the lumen expansion ratio (R = 0.832, p < 0.01). Using multivariate binary logistic regression, 2 models estimating whether the vessel met the size criteria (lumen size of 160-240 μm) were generated with an area under the receiver operating characteristic curve of 0.761 for the upper limit and 0.747 for the lower limit.

CONCLUSION

The U-Net-based image analysis method could streamline the experimental approach.

Widely-tunable synchronisation-free picosecond laser source for multimodal CARS, SHG, and two-photon microscopy

We demonstrate a continuous wave (CW) seeded synchronization-free optical parametric amplifier (OPA) pumped by a picosecond, 1 µm laser and show its performance when used as a simple yet powerful source for label-free coherent anti-Stokes Raman scattering (CARS), concurrent second harmonic generation (SHG), and two-photon fluorescence microscopy in an epi-detection geometry. The average power level of above 175 mW, spectral resolution of 8 cm^-1, and 2 ps pulse duration are well optimized for CARS microscopy in bio-science and bio-medical imaging systems. Our OPA is a much simpler setup than either the "gold-standard" laser and optical parametric oscillator (OPO) combination traditionally used for CARS imaging, or the more recently developed OPA systems pumped with femtosecond pulses [1]. Rapid and accurate tuning between resonances was achieved by changing the poled channels and temperature of the periodically-poled lithium niobate (PPLN) OPA crystal together with the OPA seed wavelength. The Pump-Stokes frequency detuning range fully covered the C-H stretching band used for the imaging of lipids. By enabling three multiphoton techniques using a compact, synchronization free laser source, our work paves the way for the translation of label-free multi-photon microscopy imaging from biomedical research to an imaging based diagnostic tool for use in the healthcare arena.

Super resolution measurement of collagen fibers in biological samples: Validation of a commercial solution for multiphoton microscopy

Multiphoton microscopy is a powerful, non-invasive technique to image biological specimens. One current limitation of multiphoton microscopy is resolution as many of the biological molecules and structures investigated by research groups are similar in size or smaller than the diffraction limit. To date, the combination of multiphoton and super-resolution imaging has proved technically challenging for biology focused laboratories to implement. Here we validate that the commercial super-resolution Airyscan detector from ZEISS, which is based on image scanning microscopy, can be integrated under warranty with a pulsed multi-photon laser to enable multiphoton microscopy with super-resolution. We demonstrate its biological application in two different imaging modalities, second harmonic generation (SHG) and two-photon excited fluorescence (TPEF), to measure the fibre thicknesses of collagen and elastin molecules surpassing the diffraction limit by a factor of 1.7±0.3x and 1.4±0.3x respectively, in human heart and lung tissues, and 3-dimensional in vitro models. We show that enhanced resolution and signal-to-noise of SHG using the Airyscan compared to traditional GaAs detectors allows for automated and precise measurement of collagen fibres using texture analysis in biological tissues.