Higher harmonic generation microscopy of in vitro cultured mammal oocytes and embryos

How great thou ART: biomechanical properties of oocytes and embryos as indicators of quality in assisted reproductive technologies

Assisted Reproductive Technologies (ART) have revolutionized infertility treatment and animal breeding, but their success largely depends on selecting high-quality oocytes for fertilization and embryos for transfer. During preimplantation development, embryos undergo complex morphogenetic processes, such as compaction and cavitation, driven by cellular forces dependent on cytoskeletal dynamics and cell-cell interactions. These processes are pivotal in dictating an embryo's capacity to implant and progress to full-term development. Hence, a comprehensive grasp of the biomechanical attributes characterizing healthy oocytes and embryos is essential for selecting those with higher developmental potential. Various noninvasive techniques have emerged as valuable tools for assessing biomechanical properties without disturbing the oocyte or embryo physiological state, including morphokinetics, analysis of cytoplasmic movement velocity, or quantification of cortical tension and elasticity using microaspiration. By shedding light on the cytoskeletal processes involved in chromosome segregation, cytokinesis, cellular trafficking, and cell adhesion, underlying oogenesis, and embryonic development, this review explores the significance of embryo biomechanics in ART and its potential implications for improving clinical IVF outcomes, offering valuable insights and research directions to enhance oocyte and embryo selection procedures.

Optical coherence microscopy allows for quality assessment of immature mouse oocytes

In brief

Optical coherence microscopy is a label-free and non-invasive imaging technique capable of 3D subcellular structure visualization. Here we show that this method allows for quality assessment of immature mouse oocytes based on their chromatin conformation and can be a valuable addition to the toolkit used in assisted reproduction procedures.

Abstract

The success of assisted reproductive technologies, and particularly in vitro maturation, is tightly linked to the quality of oocytes. Therefore, there is a need for robust, reliable, and easy-to-assess biomarkers of oocyte developmental competence. Microscopy techniques visualizing oocyte intracellular structure could provide such biomarkers. However, fluorescence imaging methods, applied frequently in biology and allowing for detailed structural and dynamic studies of single cells, require fluorescent tags to visualize cellular architecture and may cause short- and long-term photo-damage. On the other hand, traditional light microscopy, although relatively non-invasive, does not provide detailed structural information. Optical coherence microscopy (OCM) is a promising alternative, as it does not require sample pre-processing or labelling and can provide 3D images of intracellular structures. Here we applied OCM to assess the chromatin conformation of immature mouse oocytes, a feature that corresponds with their transcriptional status and developmental competence and cannot be examined by traditional light microscopy. We showed that OCM distinguished oocytes with so-called non-surrounded nucleoli (NSN) and surrounded nucleoli (SN) chromatin conformation with very high sensitivity and specificity and that OCM scanning did not decrease the quality of oocytes. Finally, we cross-referenced OCM data with the oocyte ability to undergo normal nuclear and cytoplasmic maturation and proven that indeed oocytes scored with OCM as NSN mature less effectively than oocytes scored as SN. Our results suggest that OCM may be a valuable addition to the imaging toolkit used in assisted reproduction procedures.

Metabolic state of human blastocysts measured by fluorescence lifetime imaging microscopy

STUDY QUESTION

Can non-invasive metabolic imaging via fluorescence lifetime imaging microscopy (FLIM) detect variations in metabolic profiles between discarded human blastocysts?

SUMMARY ANSWER

FLIM revealed extensive variations in the metabolic state of discarded human blastocysts associated with blastocyst development over 36 h, the day after fertilization and blastocyst developmental stage, as well as metabolic heterogeneity within individual blastocysts.

WHAT IS KNOWN ALREADY

Mammalian embryos undergo large changes in metabolism over the course of preimplantation development. Embryo metabolism has long been linked to embryo viability, suggesting its potential utility in ART to aid in selecting high quality embryos. However, the metabolism of human embryos remains poorly characterized due to a lack of non-invasive methods to measure their metabolic state.

STUDY DESIGN, SIZE, DURATION

We conducted a prospective observational study. We used 215 morphologically normal human embryos from 137 patients that were discarded and donated for research under an approved institutional review board protocol. These embryos were imaged using metabolic imaging via FLIM to measure the autofluorescence of two central coenzymes, nicotinamide adenine (phosphate) dinucleotide (NAD(P)H) and flavine adenine dinucleotide (FAD+), which are essential for cellular respiration and glycolysis.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Here, we used non-invasive FLIM to measure the metabolic state of human blastocysts. We first studied spatial patterns in the metabolic state within human blastocysts and the association of the metabolic state of the whole blastocysts with stage of expansion, day of development since fertilization and morphology. We explored the sensitivity of this technique in detecting metabolic variations between blastocysts from the same patient and between patients. Next, we explored whether FLIM can quantitatively measure metabolic changes through human blastocyst expansion and hatching via time-lapse imaging. For all test conditions, the level of significance was set at P < 0.05 after correction for multiple comparisons using Benjamini-Hochberg's false discovery rate.

MAIN RESULTS AND THE ROLE OF CHANCE

We found that FLIM is sensitive enough to detect significant metabolic differences between blastocysts. We found that metabolic variations between blastocyst are partially explained by both the time since fertilization and their developmental expansion stage (P < 0.05), but not their morphological grade. Substantial metabolic variations between blastocysts from the same patients remain, even after controlling for these factors. We also observe significant metabolic heterogeneity within individual blastocysts, including between the inner cell mass and the trophectoderm, and between the portions of hatching blastocysts within and without the zona pellucida (P < 0.05). And finally, we observed that the metabolic state of human blastocysts continuously varies over time.

LIMITATIONS, REASONS FOR CAUTION

Although we observed significant variations in metabolic parameters, our data are taken from human blastocysts that were discarded and donated for research and we do not know their clinical outcome. Moreover, the embryos used in this study are a mixture of aneuploid, euploid and embryos of unknown ploidy.

WIDER IMPLICATIONS OF THE FINDINGS

This work reveals novel aspects of the metabolism of human blastocysts and suggests that FLIM is a promising approach to assess embryo viability through non-invasive, quantitative measurements of their metabolism. These results further demonstrate that FLIM can provide biologically relevant information that may be valuable for the assessment of embryo quality.

STUDY FUNDING/COMPETING INTEREST(S)

Supported by the Blavatnik Biomedical Accelerator Grant at Harvard University. Becker and Hickl GmbH and Boston Electronics sponsored research with the loaning of equipment for FLIM. D.J.N. is an inventor on patent US20170039415A1.

TRIAL REGISTRATION NUMBER

N/A.

From Zygote to Blastocyst: Application of Ultrashort Lasers in the Field of Assisted Reproduction and Developmental Biology

Although the use of lasers in medical diagnosis and therapies, as well as in fundamental biomedical research is now almost routine, advanced laser sources and new laser-based methods continue to emerge. Due to the unique ability of ultrashort laser pulses to deposit energy into a microscopic volume in the bulk of a transparent material without disrupting the surrounding tissues, the ultrashort laser-based microsurgery of cells and subcellular components within structurally complex and fragile specimens such as embryos is becoming an important tool in developmental biology and reproductive medicine. In this review, we discuss the mechanisms of ultrashort laser pulse interaction with the matter, advantages of their application for oocyte and preimplantation embryo microsurgery (e.g., for oocyte/blastomere enucleation and embryonic cell fusion), as well as for nonlinear optical microscopy for studying the dynamics of embryonic development and embryo quality assessment. Moreover, we focus on ultrashort laser-based approaches and techniques that are increasingly being applied in the fundamental research and have the potential for successful translation into the IVF (in vitro fertilization) clinics, such as laser-mediated individual embryo labelling and controlled laser-assisted hatching.

Slide-free clinical imaging of melanin with absolute quantities using label-free third-harmonic-generation enhancement-ratio microscopy

The capability to image the 3D distribution of melanin in human skin in vivo with absolute quantities and microscopic details will not only enable noninvasive histopathological diagnosis of melanin-related cutaneous disorders, but also make long term treatment assessment possible. In this paper, we demonstrate clinical in vivo imaging of the melanin distribution in human skin with absolute quantities on mass density and with microscopic details by using label-free third-harmonic-generation (THG) enhancement-ratio microscopy. As the dominant absorber in skin, melanin provides the strongest THG nonlinearity in human skin due to resonance enhancement. We show that the THG-enhancement-ratio (erTHG) parameter can be calibrated in vivo and can indicate the melanin mass density. With an unprecedented clinical imaging resolution, our study revealed erTHG-microscopy's unique capability for long-term treatment assessment and direct clinical observation of melanin's micro-distribution to shed light into the unknown pathway and regulation mechanism of melanosome transfer and translocation.

Label-free optical imaging in developmental biology [Invited]

Application of optical imaging in developmental biology marks an exciting frontier in biomedical optics. Optical resolution and imaging depth allow for investigation of growing embryos at subcellular, cellular, and whole organism levels, while the complexity and variety of embryonic processes set multiple challenges stimulating the development of various live dynamic embryonic imaging approaches. Among other optical methods, label-free optical techniques attract an increasing interest as they allow investigation of developmental mechanisms without application of exogenous markers or fluorescent reporters. There has been a boost in development of label-free optical imaging techniques for studying embryonic development in animal models over the last decade, which revealed new information about early development and created new areas for investigation. Here, we review the recent progress in label-free optical embryonic imaging, discuss specific applications, and comment on future developments at the interface of photonics, engineering, and developmental biology.

Raman spectroscopy-based approach to study the female gamete

In the last years, an increasing interest has emerged on the development of new non-invasive methods for the assessment of oocyte quality in order to improve outcomes of assisted reproductive technologies (ARTs) either in medical or veterinary fields. Raman microspectroscopy (RMS) has been proposed as a promising tool for the examination of the mammalian female gamete and identification of markers of its developmental competence. This technique provides a unique spectral fingerprint indicative of molecular composition of the cell and allows probing subcellular compartments. Studies have been carried out analysing by RMS fixed or living oocytes derived from different animal models. RMS imaging has been successfully applied to discriminate the biochemical changes of the global molecular architecture of mouse oocytes at different stages of maturation and those occurring in different conditions of maturation and oocyte aging. RMS can also detect modifications of specific structural components, including the oocyte zona pellucida and F-actin subcortical cytoskeleton in fresh sheep oocytes and those underwent to vitrification procedures. Finally, the recent application of Coherent anti-Stokes Raman scattering (CARS) microscopy for examination of oocyte lipid component will be briefly discussed. CARS overcomes some limits of RMS providing vibrational and spectral information with higher sensitivity, spatial resolution which is ideal to study living oocytes. This review summarizes the research on RMS approaches for oocyte evaluation showing the high potential use, current limitations and new improvements.

Combined noninvasive metabolic and spindle imaging as potential tools for embryo and oocyte assessment

STUDY QUESTION

Is the combined use of fluorescence lifetime imaging microscopy (FLIM)-based metabolic imaging and second harmonic generation (SHG) spindle imaging a feasible and safe approach for noninvasive embryo assessment?

SUMMARY ANSWER

Metabolic imaging can sensitively detect meaningful metabolic changes in embryos, SHG produces high-quality images of spindles and the methods do not significantly impair embryo viability.

WHAT IS KNOWN ALREADY

Proper metabolism is essential for embryo viability. Metabolic imaging is a well-tested method for measuring metabolism of cells and tissues, but it is unclear if it is sensitive enough and safe enough for use in embryo assessment.

STUDY DESIGN, SIZE, DURATION

This study consisted of time-course experiments and control versus treatment experiments. We monitored the metabolism of 25 mouse oocytes with a noninvasive metabolic imaging system while exposing them to oxamate (cytoplasmic lactate dehydrogenase inhibitor) and rotenone (mitochondrial oxidative phosphorylation inhibitor) in series. Mouse embryos (n = 39) were measured every 2 h from the one-cell stage to blastocyst in order to characterize metabolic changes occurring during pre-implantation development. To assess the safety of FLIM illumination, n = 144 illuminated embryos were implanted into n = 12 mice, and n = 108 nonilluminated embryos were implanted into n = 9 mice.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Experiments were performed in mouse embryos and oocytes. Samples were monitored with noninvasive, FLIM-based metabolic imaging of nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) autofluorescence. Between NADH cytoplasm, NADH mitochondria and FAD mitochondria, a single metabolic measurement produces up to 12 quantitative parameters for characterizing the metabolic state of an embryo. For safety experiments, live birth rates and pup weights (mean ± SEM) were used as endpoints. For all test conditions, the level of significance was set at P < 0.05.

MAIN RESULTS AND THE ROLE OF CHANCE

Measured FLIM parameters were highly sensitive to metabolic changes due to both metabolic perturbations and embryo development. For oocytes, metabolic parameter values were compared before and after exposure to oxamate and rotenone. The metabolic measurements provided a basis for complete separation of the data sets. For embryos, metabolic parameter values were compared between the first division and morula stages, morula and blastocyst and first division and blastocyst. The metabolic measurements again completely separated the data sets. Exposure of embryos to excessive illumination dosages (24 measurements) had no significant effect on live birth rate (5.1 ± 0.94 pups/mouse for illuminated group; 5.7 ± 1.74 pups/mouse for control group) or pup weights (1.88 ± 0.10 g for illuminated group; 1.89 ± 0.11 g for control group).

LIMITATIONS, REASONS FOR CAUTION

The study was performed using a mouse model, so conclusions concerning sensitivity and safety may not generalize to human embryos. A limitation of the live birth data is also that although cages were routinely monitored, we could not preclude that some runt pups may have been eaten.

WIDER IMPLICATIONS OF THE FINDINGS

Promising proof-of-concept results demonstrate that FLIM with SHG provide detailed biological information that may be valuable for the assessment of embryo and oocyte quality. Live birth experiments support the method's safety, arguing for further studies of the clinical utility of these techniques.

STUDY FUNDING/COMPETING INTEREST(S)

Supported by the Blavatnik Biomedical Accelerator Grant at Harvard University and by the Harvard Catalyst/The Harvard Clinical and Translational Science Center (National Institutes of Health Award UL1 TR001102), by NSF grants DMR-0820484 and PFI-TT-1827309 and by NIH grant R01HD092550-01. T.S. was supported by a National Science Foundation Postdoctoral Research Fellowship in Biology grant (1308878). S.F. and S.A. were supported by NSF MRSEC DMR-1420382. Becker and Hickl GmbH sponsored the research with the loaning of equipment for FLIM. T.S. and D.N. are cofounders and shareholders of LuminOva, Inc., and co-hold patents (US20150346100A1 and US20170039415A1) for metabolic imaging methods. D.S. is on the scientific advisory board for Cooper Surgical and has stock options with LuminOva, Inc.

Study on melanin enhanced third harmonic generation in a live cell model

Melanin dominates the endogenous contrasts of in vivo third harmonic generation (THG) imaging of human skin. A recent study investigated the THG in melanin solution and a linear relationship between melanin concentration and THG intensity was reported, in contrast to the expected nonlinear relationship. Since melanin hydrocolloid solution is very different from the skin tissue, here we report our study on the origin of the melanin-enhanced THG by using a live cell model. Different from the previous conclusion, our live cell study has indicated an initial nonlinear process where the THG intensity was enhanced according to the 3.5th power of melanin mass density (MMD). When the MMD is higher than 11 mg/ml, a transition from the resonance-enhanced THG to the high-order hyper-Rayleigh scattering process occurs. This saturation phenomenon of the virtual-transition-based THG nonlinear process is attributed to the multi-melanosome-induced scattering within the sub-femtoliter focal volume.

Raman spectroscopy on live mouse early embryo while it continues to develop into blastocyst in vitro

Laser based spectroscopic methods can be versatile tools in investigating early stage mammalian embryo structure and biochemical processes in live oocytes and embryos. The limiting factor for using the laser methods in embryological studies is the effect of laser irradiation on the ova. The aim of this work is to explore the optimal parameters of the laser exposure in Raman spectroscopic measurements applicable for studying live early embryos in vitro without impacting their developmental capability. Raman spectra from different areas of mouse oocytes and 2-cells embryos were measured and analyzed. The laser power and exposure time were varied and further embryo development was evaluated to select optimal conditions of the measurements. This work demonstrates safe laser irradiation parameters can be selected, which allow acquisition of Raman spectra suitable for further analysis without affecting the early mouse embryo development in vitro up to morphologically normal blastocyst. The estimation of living embryo state is demonstrated via analysis and comparison of the spectra from fertilized embryo with the spectra from unfertilized oocytes or embryos subjected to UV laser irradiation. These results demonstrate the possibility of investigating preimplantation mammalian embryo development and estimating its state/quality. It will have potential in developing prognosis of mammalian embryos in assisted reproductive technologies.

Multimodal imaging platform for optical virtual skin biopsy enabled by a fiber-based two-color ultrafast laser source

We demonstrate multimodal label-free nonlinear optical microscopy in human skin enabled by a fiber-based two-color ultrafast source. Energetic femtosecond pulses at 775 nm and 1250 nm are simultaneously generated by an Er-fiber laser source employing frequency doubling and self-phase modulation enabled spectral selection. The integrated nonlinear optical microscope driven by such a two-color femtosecond source enables the excitation of endogenous two-photon excitation fluorescence, second-harmonic generation, and third-harmonic generation in human skin. Such a 3-channel imaging platform constitutes a powerful tool for clinical application and optical virtual skin biopsy.

l-Carnitine affects preimplantation embryo development toward infertility in mice

l-Carnitine (l-Cn), despite the beneficial role as energy-generating substance delivering long-chain fatty acids to the β-oxidation pathway in mitochondria, has been accused to cause an endometriosis-like state to BALB/c mice manifested by increased inflammatory cytokines in serum and peritoneal fluid, accumulation of immune cells in the peritoneal cavity and uterine walls and most importantly, correlating to infertility. Exploring this type of infertility, the effect of l-Cn on preimplantation embryo development, ovarian integrity and systemic maternal immunity was studied. Using nonlinear microscopy analysis, which was shown to be a powerful tool for determining embryo quality by quantitatively estimating the lipid body (LB) content of the cells, it was shown that in vitro and in vivo administration of l-Cn significantly decreased LB mean area in zygotes. Daily intraperitoneal administration of 2.5mg l-Cn for 3, 4 and 7days to mice significantly decreased the percent of normal zygotes. However, only the 7-day treatment persisted by affecting 2- and 8-cell stage embryos, while almost abolishing blastocyst development. Such effects were accompanied by abnormal ovarian histology, showing increased numbers of corpora luteus and elevated progesterone concentration in the serum. In addition, it was shown that the 7-day l-Cn treatment pushed maternal systemic immunity toward inflammation and immunosuppression by increasing CD11b-, CD25- and CD11bGr1-positive cells in spleen, which opposed the necessity for immunostimulation at these early stages of pregnancy. In conclusion, the results presented here demonstrated that elevated doses of l-Cn affect early stages of embryo development, leading to infertility.

Live-cell quantification and comparison of mammalian oocyte cytosolic lipid content between species, during development, and in relation to body composition using nonlinear vibrational microscopy

Cytosolic lipids participate in the growth, development, and overall health of mammalian oocytes including many roles in cellular homeostasis. Significant emphasis has been placed on the study of lipids as a dynamic organelle, which in turn requires the development of tools and techniques to quantitate and compare how lipid content relates to cellular structure, function, and normalcy. Objectives of this study were to determine if nonlinear vibrational microscopy (e.g., coherent anti-Stokes Raman scattering or CARS microscopy) could be used for live-cell imaging to quantify and compare lipid content in mammalian oocytes during development and in relation to body composition; and compare its efficacy to methods involving cellular fixation and staining protocols. Results of this study demonstrate that CARS is able to identify lipids in live mammalian oocytes, and there exists quantifiable and consistent differences in percent lipid composition across ooctyes of different species, developmental stages, and in relation to body composition. Such a method of live-cell lipid quantification has (i) experimental power in basic cell biology, (ii) practical utility for identifying developmental predictive biomarkers while advancing biology-based oocyte/embryo selection, and (iii) ability to yield rationally supporting technology for decision-making in rodents, domestic species, and human assisted reproduction and/or fertility preservation.

Detection of malformations in sea urchin plutei exposed to mercuric chloride using different fluorescent techniques

Embryos of Mediterranean sea urchin Paracentrotus lividus and subtropical Echinometra mathaei were exposed to 5,10, 15 and 20µgL(-1), and to 1, 2, 3 and 4µgL(-1) mercuric chloride (HgCl2), respectively. The effective concentration (EC50) inducing malformation in 50% of 4-arm pluteus stage (P4) was 16.14µgL(-1) for P. lividus and 2.41µgL(-1) for E. mathaei. Two-photon (TP), second (SHG) and third harmonic generation (THG) microscopy techniques, TUNEL staining, propidium iodide (PI) and Hoechst 33342 probes were used to detect light signals or to stain apoptotic and necrotic cells in fixed and alive plutei. Signals were detected differently in the two species: TP fluorescence, commonly associated with apoptotic cells, did not increase with increasing HgCl2 concentrations in P. lividus and in fact, the TUNEL did not reveal induction of apoptosis. PI fluorescence increased in P. lividus in a dose-dependent manner, suggesting a loss of cell permeability. In E. mathaei plutei TP fluorescence increased at increasing HgCl2 concentrations. THG microscopy revealed skeletal rods in both species. Different fluorescent techniques, used in this study, are proposed as early-warning systems to visualize malformations and physiological responses in sea urchin plutei.

Enabling the detection of UV signal in multimodal nonlinear microscopy with catalogue lens components

Using an optical system made from fused silica catalogue optical components, third-order nonlinear microscopy has been enabled on conventional Ti:sapphire laser-based multiphoton microscopy setups. The optical system is designed using two lens groups with straightforward adaptation to other microscope stands when one of the lens groups is exchanged. Within the theoretical design, the optical system collects and transmits light with wavelengths between the near ultraviolet and the near infrared from an object field of at least 1 mm in diameter within a resulting numerical aperture of up to 0.56. The numerical aperture can be controlled with a variable aperture stop between the two lens groups of the condenser. We demonstrate this new detection capability in third harmonic generation imaging experiments at the harmonic wavelength of ∼300 nm and in multimodal nonlinear optical imaging experiments using third-order sum frequency generation and coherent anti-Stokes Raman scattering microscopy so that the wavelengths of the detected signals range from ∼300 nm to ∼660 nm.

Third-harmonic generation microscopy reveals dental anatomy in ancient fossils

Fossil teeth are primary tools in the study of vertebrate evolution, but standard imaging modalities have not been capable of providing high-quality images in dentin, the main component of teeth, owing to small refractive index differences in the fossilized dentin. Our first attempt to use third-harmonic generation (THG) microscopy in fossil teeth has yielded significant submicrometer level anatomy, with an unexpectedly strong signal contrasting fossilized tubules from the surrounding dentin. Comparison between fossilized and extant teeth of crocodilians reveals a consistent evolutionary signature through time, indicating the great significance of THG microscopy in the evolutionary studies of dental anatomy in fossil teeth.

Measuring microtubule polarity in spindles with second-harmonic generation

The spatial organization of microtubule polarity, and the interplay between microtubule polarity and protein localization, is thought to be crucial for spindle assembly, anaphase, and cytokinesis, but these phenomena remain poorly understood, in part due to the difficulty of measuring microtubule polarity in spindles. We develop and implement a method to nonperturbatively and quantitatively measure microtubule polarity throughout spindles using a combination of second-harmonic generation and two-photon fluorescence. We validate this method using computer simulations and by comparison to structural data on spindles obtained from electron tomography and laser ablation. This method should provide a powerful tool for studying spindle organization and function, and may be applicable for investigating microtubule polarity in other systems.

Mitigating phototoxicity during multiphoton microscopy of live Drosophila embryos in the 1.0-1.2 µm wavelength range

Light-induced toxicity is a fundamental bottleneck in microscopic imaging of live embryos. In this article, after a review of photodamage mechanisms in cells and tissues, we assess photo-perturbation under illumination conditions relevant for point-scanning multiphoton imaging of live Drosophila embryos. We use third-harmonic generation (THG) imaging of developmental processes in embryos excited by pulsed near-infrared light in the 1.0-1.2 µm range. We study the influence of imaging rate, wavelength, and pulse duration on the short-term and long-term perturbation of development and define criteria for safe imaging. We show that under illumination conditions typical for multiphoton imaging, photodamage in this system arises through 2- and/or 3-photon absorption processes and in a cumulative manner. Based on this analysis, we derive general guidelines for improving the signal-to-damage ratio in two-photon (2PEF/SHG) or THG imaging by adjusting the pulse duration and/or the imaging rate. Finally, we report label-free time-lapse 3D THG imaging of gastrulating Drosophila embryos with sampling appropriate for the visualisation of morphogenetic movements in wild-type and mutant embryos, and long-term multiharmonic (THG-SHG) imaging of development until hatching.

Tunable third-harmonic generation in a chalcogenide-tellurite hybrid optical fiber with high refractive index difference

A chalcogenide-tellurite hybrid optical fiber with a step-index structure is fabricated by the rod-in-tube drawing technique. The core is made of 15Ge-3Ga-12Sb-70S (mol. %) glass, and the cladding is made of 78TeO₂-5ZnO-12Li₂O-5Bi₂O₃ (mol. %) glass. The refractive index difference Δn=0.24. Tunable third-harmonic generation from 568 to 869 nm is observed when the optical fiber is pumped by an optical parametric oscillator with the pump wavelength changing from 1700 to 2600 nm.

Advances in noninvasive functional imaging of bone

The demand for functional imaging in clinical medicine is comprehensive. Although the gold standard for the functional imaging of human bones in clinical settings is still radionuclide-based imaging modalities, nonionizing noninvasive imaging technology in small animals has greatly advanced in recent decades, especially the diffuse optical imaging to which Britton Chance made tremendous contributions. The evolution of imaging probes, instruments, and computation has facilitated exploration in the complicated biomedical research field by allowing longitudinal observation of molecular events in live cells and animals. These research-imaging tools are being used for clinical applications in various specialties, such as oncology, neuroscience, and dermatology. The Bone, a deeply located mineralized tissue, presents a challenge for noninvasive functional imaging in humans. Using nanoparticles (NP) with multiple favorable properties as bioimaging probes has provided orthopedics an opportunity to benefit from these noninvasive bone-imaging techniques. This review highlights the historical evolution of radionuclide-based imaging, computed tomography, positron emission tomography, and magnetic resonance imaging, diffuse optics-enabled in vivo technologies, vibrational spectroscopic imaging, and a greater potential for using NPs for biomedical imaging.

Determination of chronological aging parameters in epidermal keratinocytes by in vivo harmonic generation microscopy

Skin aging is an important issue in geriatric and cosmetic dermatology. To quantitatively analyze changes in keratinocytes related to intrinsic aging, we exploited a 1230 nm-based in vivo harmonic generation microscopy, combining second- and third-harmonic generation modalities. 52 individuals (21 men and 31 women, age range 19-79) were examined on the sun-protected volar forearm. Through quantitative analysis by the standard algorithm provided, we found that the cellular and nuclear size of basal keratinocytes, but not that of granular cells, was significantly increased with advancing age. The cellular and nuclear areas, which have an increase of 0.51 μm(2) and 0.15 μm(2) per year, respectively, can serve as scoring indices for intrinsic skin aging.

Characterization of oral squamous cell carcinoma based on higher-harmonic generation microscopy

In vivo higher-harmonic generation microscopy (HGM) performed on healthy human oral mucosa not only provides images with a <500 nm lateral resolution at a 280 μm penetration depth, but also leaves no photodamages in the tissues. These advantages suggest that HGM could serve as an ideal virtual biopsy tool for in vivo, in situ, and immediate histopathological diagnosis of oral cancer. However, translation of such mechanism for clinical cancer diagnosis requires evidence based algorithm capable to differentiate cancerous tissues from normal. It is thus critical to investigate if the endogenous contrast provided by the HGM would be high enough to differentiate cancerous versus normal tissues in human oral mucosa. In this report, ex vivo HGM study was performed on the cancerous mucosa from 10 patients with oral squamous cell carcinoma. Compared with histology, HGM revealed histopathological features including the cytological abnormalities, loss of differentiation, interruption of basement membrane, and irregular epithelial stratification in all 10 specimens. In addition, distinct patterns of collagen fibers and increased distribution area of actin filaments in tumor cells were noted. These results indicate HGM holds great potential for the optical biopsy screening of oral cancer lesions.

Femtosecond laser nanosurgery of sub-cellular structures in HeLa cells by employing Third Harmonic Generation imaging modality as diagnostic tool

Femtosecond laser assisted nanosurgery of microscopic biological specimens is a relatively new technique which allows the selective disruption of sub-cellular structures without causing any undesirable damage to the surrounding regions. The targeted structures have to be stained in order to be clearly visualized for the nanosurgery procedure. However, the validation of the final nanosurgery result is difficult, since the targeted structure could be simply photobleached rather than selectively destroyed. This fact comprises a main drawback of this technique. In our study we employed a multimodal system which integrates non-linear imaging modalities with nanosurgery capabilities, for the selective disruption of sub-cellular structures in HeLa cancer cells. Third Harmonic Generation (THG) imaging modality was used as a tool for the identification of structures that were subjected to nanosurgery experiments. No staining of the biological samples was required, since THG is an intrinsic property of matter. Furthermore, cells' viability after nanosurgery processing was verified via Two Photon Excitation Fluorescence (TPEF) measurements.

When multiphoton microscopy sees near infrared

The need for quantification and real time visualization of developmental processes has called for increasingly sophisticated imaging techniques. Among them, multiphoton microscopy reveals itself to be an extremely versatile tool owing to its unique ability to combine fluorescent imaging, laser ablation, and higher harmonic generation. Furthermore, recent advances in femtosecond lasers and optical parametric oscillators (OPO) are now opening doors for imaging at unprecedented wavelengths centered in the tissue transparency window. This Review describes promising multiphoton approaches using OPO and the growing number of useful applications of non-linear microscopy in the field of developmental biology. Basic characteristics associated with these techniques are described along with the main experimental challenges when applied to embryo imaging.

In vivo optical virtual biopsy of human oral mucosa with harmonic generation microscopy

Recent clinical studies on human skin indicated that in vivo multi-harmonic generation microscopy (HGM) can achieve sub-micron resolution for histopathological analysis with a high penetration depth and leave no energy or photodamages in the interacted tissues. It is thus highly desired to apply HGM for in vivo mucosa histopathological diagnosis. In this paper, the first in vivo optical virtual biopsy of human oral mucosa by using epi-HGM is demonstrated. We modified an upright microscope to rotate the angle of objective for in vivo observation. Our clinical study reveals the capability of HGM to in vivo image cell distributions in human oral mucosa, including epithelium and lamina propria with a high penetration depth greater than 280 μm and a high spatial resolution better than 500 nm. We also found that the third-harmonic-generation (THG) contrast on nucleus depends strongly on its thicknesses, in agreement with a numerical simulation. Besides, 4% acetic acid was found to be able to enhance the THG contrast of nucleus in oral mucosa, while such enhancement was found to decay due to the metabolic clearance of the contrast enhancer by the oral mucosa. Our clinical study indicated that, the combined epi-THG and epi-second-harmonic-generation (SHG) microscopy is a promising imaging tool for in vivo noninvasive optical virtual biopsy and disease diagnosis in human mucosa.

Long-term imaging of mouse embryos using adaptive harmonic generation microscopy

We present a detailed description of an adaptive harmonic generation (HG) microscope and culture techniques that permit long-term, three-dimensional imaging of mouse embryos. HG signal from both pre- and postimplantation stage (0.5-5.5 day-old) mouse embryos are fully characterized. The second HG images reveal central spindles during cytokinesis whereas third HG images show several features, such as lipid droplets, nucleoli, and plasma membranes. The embryos are found to develop normally during one-day-long discontinuous HG imaging, permitting the observation of several dynamic events, such as morula compaction and blastocyst formation.

Cell tracking in live Caenorhabditis elegans embryos via third harmonic generation imaging microscopy measurements

In this study, we demonstrate the potential of employing third harmonic generation (THG) imaging microscopy measurements for cell tracking studies in live Caenorhabditis elegans (C. elegans) embryos. A 1028-nm femtosecond laser was used for the excitation of unstained C. elegans samples. Different C. elegans embryonic stages (from two-cell to threefold) were imaged. Live biological specimens were irradiated for prolonged periods of time (up to 7 h), testifying to the nondestructive nature of this nonlinear imaging technique. Thus, THG image contrast modality is a powerful diagnostic tool for probing in vivo cell division during early embryogenesis.

Label-free live brain imaging and targeted patching with third-harmonic generation microscopy

The ability to visualize neurons inside living brain tissue is a fundamental requirement in neuroscience and neurosurgery. Especially the development of a noninvasive probe of brain morphology with micrometer-scale resolution is highly desirable, as it would provide a noninvasive approach to optical biopsies in diagnostic medicine. Two-photon laser-scanning microscopy (2PLSM) is a powerful tool in this regard, and has become the standard for minimally invasive high-resolution imaging of living biological samples. However, while 2PLSM-based optical methods provide sufficient resolution, they have been hampered by the requirement for fluorescent dyes to provide image contrast. Here we demonstrate high-contrast imaging of live brain tissue at cellular resolution, without the need for fluorescent probes, using optical third-harmonic generation (THG). We exploit the specific geometry and lipid content of brain tissue at the cellular level to achieve partial phase matching of THG, providing an alternative contrast mechanism to fluorescence. We find that THG brain imaging allows rapid, noninvasive label-free imaging of neurons, white-matter structures, and blood vessels simultaneously. Furthermore, we exploit THG-based imaging to guide micropipettes towards designated neurons inside live tissue. This work is a major step towards label-free microscopic live brain imaging, and opens up possibilities for the development of laser-guided microsurgery techniques in the living brain.

Regulation of mouse oocyte microtubule and organelle dynamics by PADI6 and the cytoplasmic lattices

Organelle positioning and movement in oocytes is largely mediated by microtubules (MTs) and their associated motor proteins. While yet to be studied in germ cells, cargo trafficking in somatic cells is also facilitated by specific recognition of acetylated MTs by motor proteins. We have previously shown that oocyte-restricted PADI6 is essential for formation of a novel oocyte-restricted fibrous structure, the cytoplasmic lattices (CPLs). Here, we show that α-tubulin appears to be associated with the PADI6/CPL complex. Next, we demonstrate that organelle positioning and redistribution is defective in PADI6-null oocytes and that alteration of MT polymerization or MT motor activity does not induce organelle redistribution in these oocytes. Finally, we report that levels of acetylated microtubules are dramatically suppressed in the cytoplasm of PADI6-null oocytes, suggesting that the observed organelle redistribution failure is due to defects in stable cytoplasmic MTs. These results demonstrate that the PADI6/CPL superstructure plays a key role in regulating MT-mediated organelle positioning and movement.

A sub-100 fs self-starting Cr:forsterite laser generating 1.4 W output power

Without cavity dumping or external amplification, we report a femtosecond Cr:forsterite laser with a 1.4 W output power and 2 W in continuous wave (CW) operated with a crystal temperature of 267 K. In the femtosecond regime, the oscillator generates Kerr-lens-mode-locked 84 fs pulses with a repetition rate of 85 MHz, corresponding to a high 16.5 nJ pulse energy directly from a single Cr:forsterite resonator. This intense femtosecond Cr:forsterite laser is ideal to pump varieties of high power fiber light sources and could be thus ideal for many biological and spectroscopy applications.

Multi-photon resonance enhancement of third harmonic generation in human oxyhemoglobin and deoxyhemoglobin

Cancer cells require excessive oxygen and nutrition to support their rapid growth, so angiogenesis and decrease of blood oxygen are often associated with areas of cancer development. Current technologies for blood oxygen measurement, however, do not possess high spatial resolution and therefore cannot be used to detect small tumors in their early stage. In this paper, we studied the third harmonic generation (THG) spectra of oxy- and deoxyhemoglobin in the 1170-1365 nm region, which is strongly influenced by the multi-photon resonance effect, especially around the Soret transition band. Our spectroscopic results thus indicate the high potential of THG spectroscopic microscopy for oxygen depletion level measurement of a single red blood cell in vivo.

Miniaturized video-rate epi-third-harmonic-generation fiber-microscope

With a micro-electro-mechanical system (MEMS) mirror, we successfully developed a miniaturized epi-third-harmonic-generation (epi-THG) fiber-microscope with a video frame rate (31 Hz), which was designed for in vivo optical biopsy of human skin. With a large-mode-area (LMA) photonic crystal fiber (PCF) and a regular microscopic objective, the nonlinear distortion of the ultrafast pulses delivery could be much reduced while still achieving a 0.4 microm lateral resolution for epi-THG signals. In vivo real time virtual biopsy of the Asian skin with a video rate (31 Hz) and a sub-micron resolution was obtained. The result indicates that this miniaturized system was compact enough for the least invasive hand-held clinical use.

Characterisation of the dynamic behaviour of lipid droplets in the early mouse embryo using adaptive harmonic generation microscopy

BACKGROUND

Lipid droplets (LD) are organelles with an important role in normal metabolism and disease. The lipid content of embryos has a major impact on viability and development. LD in Drosophila embryos and cultured cell lines have been shown to move and fuse in a microtubule dependent manner. Due to limitations in current imaging technology, little is known about the behaviour of LD in the mammalian embryo. Harmonic generation microscopy (HGM) allows one to image LD without the use of exogenous labels. Adaptive optics can be used to correct aberrations that would otherwise degrade the quality and information content of images.

RESULTS

We have built a harmonic generation microscope with adaptive optics to characterise early mouse embryogenesis. At fertilization, LD are small and uniformly distributed, but in the implanting blastocyst, LD are larger and enriched in the invading giant cells of the trophectoderm. Time-lapse studies reveal that LD move continuously and collide but do not fuse, instead forming aggregates that subsequently behave as single units. Using specific inhibitors, we show that the velocity and dynamic behaviour of LD is dependent not only on microtubules as in other systems, but also on microfilaments. We explore the limits within which HGM can be used to study living embryos without compromising viability and make the counterintuitive finding that 16 J of energy delivered continuously over a period of minutes can be less deleterious than an order of magnitude lower energy delivered dis-continuously over a period of hours.

CONCLUSIONS

LD in pre-implantation mouse embryos show a previously unappreciated complexity of behaviour that is dependent not only on microtubules, but also microfilaments. Unlike LD in other systems, LD in the mouse embryo do not fuse but form aggregates. This study establishes HGM with adaptive optics as a powerful tool for the study of LD biology and provides insights into the photo-toxic effects of imaging embryos.

Direct backward third-harmonic generation in nanostructures

Direct-backward third harmonic generation (DBTHG) has been regarded as negligible or even inexistent due to the large value of wave-vector mismatch. In the past, BTHG signals were often interpreted as back-reflected or back-scattered forward-THG (FTHG). In this paper, we theoretically and experimentally demonstrate that backward third harmonic waves can be directly generated, and that their magnitude can be comparable with FTHG in nanostructures. Experimental data of DBTHG from ZnO thin films, CdSe quantum dots and Fe(3)O(4) nanoparticles agree well with simulation results based on the Green's function. An integral equation was also derived for fast computation of DBTHG in nano films. Our investigation suggests that DBTHG can be a potentially powerful tool in nano-science research, especially when combined with FTHG measurements.

Second-harmonic generation imaging of collagen fibers in myocardium for atrial fibrillation diagnosis

Atrial fibrillation (AF) is the most common irregular heart rhythm and the mortality rate for patients with AF is approximately twice the mortality rate for patients with normal sinus rhythm (NSR). Some research has indicated that myocardial fibrosis plays an important role in predisposing patients to AF. Therefore, realizing the relationship between myocardial collagen fibrosis and AF is significant. Second-harmonic generation (SHG) is an optically nonlinear coherent process to image the collagen network. We perform SHG microscopic imaging of the collagen fibers in the human atrial myocardium. Utilizing the SHG images, we can identify the differences in morphology and the arrangement of collagen fibers between NSR and AF tissues. We also quantify the arrangement of the collagen fibers using Fourier transform images and calculating the values of angle entropy. We indicate that SHG imaging, a nondestructive and reproducible method to analyze the arrangement of collagen fibers, can provide explicit information about the relationship between myocardial fibrosis and AF.

In vivo harmonic generation biopsy of human skin

The ability to in vivo image deep tissues noninvasively with a high resolution is strongly required for optical virtual biopsy. Higher harmonic generation microscopy, combined with second- and third-harmonic generation microscopies, is applied to 17 Asian volunteers' forearm skin. After continuous observation for 30 min, no visible damage was found. Our study proves that harmonic generation biopsy (HGB) is able to satisfy the safety requirement and to provide high penetrability (approximately 300 microm) and submicron resolution all at the same time and is a promising tool for future virtual biopsy of skin diseases. In contrast to a previous study on fixed human skin specimens, a much improved penetrability and much reduced resolution-degradation versus depth are found in this in vivo examination.

Adaptive harmonic generation microscopy of mammalian embryos

Adaptive optics is implemented in a harmonic generation microscope using a wavefront sensorless correction scheme. Both the second- and third-harmonic intensity signals are used as the optimization metric. Aberration correction is performed to compensate both system- and specimen-induced aberrations by using an efficient optimization routine based upon Zernike polynomial modes. Images of live mouse embryos show an improved signal level and resolution.

Infrared-based third and second harmonic generation imaging of cornea

The cornea functions as an optical lens and plays an important role in vision. For corneal diagnosis and treatment such as refractive surgery, a microscopic imaging system with a 3-D cellular resolution and retinal safety is strongly desired. Recently, confocal and multiphoton microscopies have been applied to corneal imaging with visible to near-infrared light sources. To increase retinal safety, an infrared light source is be needed. In this work, an infrared-based third and second harmonic generation microscopic study of mouse eyes is reported with approximately 700-mum penetrability and high cellular resolution. This study provides a critical reference for future development of infrared-based corneal imaging.

Noninvasive in vitro and in vivo assessment of epidermal hyperkeratosis and dermal fibrosis in atopic dermatitis

Atopic dermatitis (AD) is characterized by hyperkeratosis of epidermis and fibrosis within dermis in chronic skin lesions. Thus far, the histology of skin lesions has been evaluated only by examination of excised specimens. A noninvasive in vivo tool is essential to evaluate the histopathological changes during the clinical course of AD. We used Cr:forsterite laser-based multimodality nonlinear microscopy to analyze the endogenous molecular signals, including third-harmonic generation (THG), second-harmonic generation (SHG), and two-photon fluorescence (TPF) from skin lesions in AD. Significant differences in thickness of epidermis and stratum corneum (SC), and modified degrees of fibrosis in dermis (measured by THG signals and SHG signals, respectively), are clearly demonstrated in in vitro studies. Increased TPF levels are positively associated with the levels of the THG signals from the SC. Our in vitro observations of histological changes are replicated in the in vivo studies. These findings were reproducible in skin lesions from human AD. For the first time, we demonstrate the feasibility of preclinical applications of Cr:forsterite laser-based nonlinear microscopy. Our findings suggest that the optical signatures of THG, TPF, and SHG can be used as molecular markers to assess the pathophysiological process of AD and the effects of local treatment.