Biological applications of second harmonic imaging

Spheroids as a 3D in vitro model to study bone and bone mineralization

Traumas, fractures, and diseases can severely influence bone tissue. Insight into bone mineralization is essential for the development of therapies and new strategies to enhance bone regeneration. 3D cell culture systems, in particular cellular spheroids, have gained a lot of interest as they can recapitulate crucial aspects of the in vivo tissue microenvironment, such as the extensive cell-cell and cell-extracellular matrix (ECM) interactions found in tissue. The potential of combining spheroids and various classes of biomaterials opens also new opportunities for research within bone tissue engineering. Characterizing cellular organization, ECM structure, and ECM mineralization is a fundamental step for understanding the biological processes involved in bone tissue formation in a spheroid-based model system. Still, many experimental techniques used in this field of research are optimized for use with monolayer cell cultures. There is thus a need to develop new and improving existing experimental techniques, for applications in 3D cell culture systems. In this review, bone composition and spheroids properties are described. This is followed by an insight into the techniques that are currently used in bone spheroids research and how these can be used to study bone mineralization. We discuss the application of staining techniques used with optical and confocal fluorescence microscopy, molecular biology techniques, second harmonic imaging microscopy, Raman spectroscopy and microscopy, as well as electron microscopy-based techniques, to evaluate osteogenic differentiation, collagen production and mineral deposition. Challenges in the applications of these methods in bone regeneration and bone tissue engineering are described. STATEMENT OF SIGNIFICANCE: 3D cell cultures have gained a lot of interest in the last decades as a possible technique that can be used to recreate in vitro in vivo biological process. The importance of 3D environment during bone mineralization led scientists to use this cell culture to study this biological process, to obtain a better understanding of the events involved. New and improved techniques are also required for a proper analysis of this cell model and the process under investigation. This review summarizes the state of the art of the techniques used to study bone mineralization and how 3D cell cultures, in particular spheroids, are tested and analysed to obtain better resolved results related to this complex biological process.

Multiphoton microscopy is a nondestructive label-free approach to investigate the 3D structure of gas cell walls in bread dough

During the different steps of bread-making, changes in the microstructure of the dough, particularly in the gas cell walls (GCW), have a major influence on the final bread crumb texture. Investigation of the spatial conformation of GCWs is still a challenge because it requires both high resolutions and 3D depth imaging. The originality of the present work lies in the use of label-free non-destructive multiphoton microscopy (NLOM) to image the 3D structure of GCWs, shedding light on their behavior and organization in wheat bread dough. We demonstrated that second and third harmonic generation (SHG, THG) allow imaging, respectively, of starch granules and interfaces in bread dough, while the gluten matrix was detected via two-photon excitation fluorescence (TPEF). Last, a distinction between the gluten network and starch granules was achieved using gluten endogenous fluorescence (EF) imaging, while the position, size, and 3D orientation of starch granules in GCWs were determined from harmonic imaging, made possible by the acquisition of backward and forward SHG with linear polarization. These innovative experiments highlight the strengths of NLOM for a label-free characterization of bread dough microstructure for the first time, in order to understand the role of starch granules in dough stabilization.

Estimation of crossbridge-state during cardiomyocyte beating using second harmonic generation

Estimation of dynamic change of crossbridge formation in living cardiomyocytes is expected to provide crucial information for elucidating cardiomyopathy mechanisms, efficacy of an intervention, and others. Here, we established an assay system to dynamically measure second harmonic generation (SHG) anisotropy derived from myosin filaments depended on their crossbridge status in pulsating cardiomyocytes. Experiments utilizing an inheritable mutation that induces excessive myosin-actin interactions revealed that the correlation between sarcomere length and SHG anisotropy represents crossbridge formation ratio during pulsation. Furthermore, the present method found that ultraviolet irradiation induced an increased population of attached crossbridges that lost the force-generating ability upon myocardial differentiation. Taking an advantage of infrared two-photon excitation in SHG microscopy, myocardial dysfunction could be intravitally evaluated in a Drosophila disease model. Thus, we successfully demonstrated the applicability and effectiveness of the present method to evaluate the actomyosin activity of a drug or genetic defect on cardiomyocytes. Because genomic inspection alone may not catch the risk of cardiomyopathy in some cases, our study demonstrated herein would be of help in the risk assessment of future heart failure.

Second harmonic generation microscopy: a powerful tool for bio-imaging

Second harmonic generation (SHG) microscopy is an important optical imaging technique in a variety of applications. This article describes the history and physical principles of SHG microscopy and its more advanced variants, as well as their strengths and weaknesses in biomedical applications. It also provides an overview of SHG and advanced SHG imaging in neuroscience and microtubule imaging and how these methods can aid in understanding microtubule formation, structuration, and involvement in neuronal function. Finally, we offer a perspective on the future of these methods and how technological advancements can help make SHG microscopy a more widely adopted imaging technique.

Biophysical Reviews: Turning the page from 2022 to 2023

This Editorial (vol. 15 issue 1-Regular Issue featuring an Issue Focus on the "100th Anniversary of Har Gobind Khorana") first describes the issue contents before providing both, a look back at some journal highlights from 2022, and a look forward to what we can expect from 2023. The Editorial closes with a roundup of new journal access features and an acknowledgement of those supporting the journal.

Neurophotonic tools for microscopic measurements and manipulation: status report

Neurophotonics was launched in 2014 coinciding with the launch of the BRAIN Initiative focused on development of technologies for advancement of neuroscience. For the last seven years, Neurophotonics' agenda has been well aligned with this focus on neurotechnologies featuring new optical methods and tools applicable to brain studies. While the BRAIN Initiative 2.0 is pivoting towards applications of these novel tools in the quest to understand the brain, this status report reviews an extensive and diverse toolkit of novel methods to explore brain function that have emerged from the BRAIN Initiative and related large-scale efforts for measurement and manipulation of brain structure and function. Here, we focus on neurophotonic tools mostly applicable to animal studies. A companion report, scheduled to appear later this year, will cover diffuse optical imaging methods applicable to noninvasive human studies. For each domain, we outline the current state-of-the-art of the respective technologies, identify the areas where innovation is needed, and provide an outlook for the future directions.

Second harmonic generation theory for a helical macromolecule with high sensitivity to structural disorder

Microscopic theory for the second harmonic generation in a helical molecular system is developed in the minimal coupling representation including non-local interaction effects. At the second order to the field we find a compact expression which combines dipolar, quadrupolar and magnetic contributions. A detailed derivation of the response is performed to specifically isolate the quadratic coupling terms, which we denote as the K coupling. Applying the theory to a helical macromolecule we find that the dipolar and quadrupolar contributions reflect the symmetry properties of the system and its homogeneity, while the K coupling contribution reveals inhomogeneities of the system.

Creative interior design by Plasmodium falciparum: Lipid metabolism and the parasite's secret chamber

Malaria parasites conceal themselves within host erythrocytes and establish a necessary logistics system through the three-membrane layered structures of these cells. To establish this system, lipid metabolism is needed for the de novo synthesis of lipids and the recycling of extracellular lipids and erythrocyte lipid components. Cholesterol supply depends on its uptake from the extracellular environment and erythrocyte cytoplasm, but phospholipids can be synthesized on their own. This differential production of lipid species creates unique modifications in the lipid profile of parasitized erythrocytes, which in turn may influence the biophysical and/or mechanical properties of organelles and vesicles and communication among them. Variations in local membrane properties possibly influence the transportation of various molecules such as parasite-derived proteins, because efficiencies in secretion, vesicle fusion and budding are partly determined by the lipid profiles. Comprehensive understanding of the parasite's lipid metabolism and the biophysics of lipid membranes provides fundamental knowledge about these pathogenic organisms and could lead to new anti-malarials.

Tumor extracellular matrix: lessons from the second-harmonic generation microscopy

Extracellular matrix (ECM) represents more than a mere intercellular cement. It is physiologically active in cell communication, adhesion and proliferation. Collagen is the most abundant protein, making up to 90% of ECM, and 30% of total protein weight in humans. Second-harmonic generation (SHG) microscopy represents an important tool to study collagen organization of ECM in freshly unfixed tissues and paraffin-embedded tissue samples. This manuscript aims to review some of the applications of SHG microscopy in Oncologic Pathology, mainly in the study of ECM of epithelial tumors. It is shown how collagen parameters measured by this technique can aid in the differential diagnosis and in prognostic stratification. There is a tendency to associate higher amount, lower organization and higher linearity of collagen fibers with tumor progression and metastasizing. These represent complex processes, in which matrix remodeling plays a central role, together with cancer cell genetic modifications. Integration of studies on cancer cell biology and ECM are highly advantageous to give us a more complete picture of these processes. As microscopic techniques provide topographic information allied with biologic characteristics of tissue components, they represent important tools for a more complete understanding of cancer progression. In this context, SHG has provided significant insights in human tumor specimens, readily available for Pathologists.

Imaging microphysiological systems: a review

Microphysiological systems (MPS), often referred to as "organ-on-chips," are microfluidic-based in vitro models that aim to recapitulate the dynamic chemical and mechanical microenvironment of living organs. MPS promise to bridge the gap between in vitro and in vivo models and ultimately improve the translation from preclinical animal studies to clinical trials. However, despite the explosion of interest in this area in recent years, and the obvious rewards for such models that could improve R&D efficiency and reduce drug attrition in the clinic, the pharmaceutical industry has been slow to fully adopt this technology. The ability to extract robust, quantitative information from MPS at scale is a key requirement if these models are to impact drug discovery and the subsequent drug development process. Microscopy imaging remains a core technology that enables the capture of information at the single-cell level and with subcellular resolution. Furthermore, such imaging techniques can be automated, increasing throughput and enabling compound screening. In this review, we discuss a range of imaging techniques that have been applied to MPS of varying focus, such as organoids and organ-chip-type models. We outline the opportunities these technologies can bring in terms of understanding mechanistic biology, but also how they could be used in higher-throughput screens, widening the scope of their impact in drug discovery. We discuss the associated challenges of imaging these complex models and the steps required to enable full exploitation. Finally, we discuss the requirements for MPS, if they are to be applied at a scale necessary to support drug discovery projects.

Intensity-dependent speckle contrast of second harmonic light in a nonlinear disordered medium

Coherent light propagation and scattering in diffuse media realize an interference effect that manifests as a random spatial distribution of bright and dark spots called speckle. Laser speckle contrast is a crucial parameter that quantifies light behavior in diffuse media, often of help in biomedical imaging applications. With several ultra-intense lasers coming onto the scene in recent years, the probability of nonlinearly generating harmonic light has considerably increased. To our knowledge, there are no studies on the simultaneous measurement of local speckle contrast (LSC) at fundamental and harmonic wavelengths. In this work, we experimentally characterize the intensity-dependent global and local speckle contrast at λ=1064nm (IR) and λ=532nm [second-harmonic generation (SHG)] in a diffuse sample made of potassium dihydrogen phosphate. Under pulsed excitation with IR light, second-harmonic light is generated and undergoes scattering, leading to speckle at both wavelengths. The radiation pressure on the sample induces displacement of the particles of the medium, which leads to the temporal averaging and hence reduction in speckle contrast of IR. The decrease in speckle contrast with input power is consistent with the theoretically predicted behavior. Next, the probability distribution of LSC for IR and SHG are observed to follow log-normal distributions with distinctive parameters, measured for co-polarized and counter-polarized channels. Parameter evolution with box-size is fit to the established global model for speckle contrast and also another alternative model. The fit parameters, illustrated as a function of input power, indicate possibilities of widening the global model to second-harmonic light.

Applications of second harmonic generation (SHG)/sum-frequency generation (SFG) imaging for biophysical characterization of the plasma membrane

The plasma membrane is a lipid bilayer of < 10 nm width that separates intra- and extra-cellular environments and serves as the site of cell-cell communication, as well as communication between cells and the extracellular environment. As such, biophysical phenomena at and around the plasma membrane play key roles in determining cellular physiology and pathophysiology. Thus, the selective visualization and characterization of the plasma membrane are crucial aspects of research in wide areas of biology and medicine. However, the specific characterization of the plasma membrane has been a challenge using conventional imaging techniques, which are unable to effectively distinguish between signals arising from the plasma membrane and those from intracellular lipid structures. In this regard, interface-specific second harmonic generation (SHG) and sum-frequency generation (SFG) imaging demonstrate great potential. When combined with exogenous SHG/SFG active dyes, SHG/SFG can specifically highlight the plasma membrane as the most prominent interface associated with cells. Furthermore, SHG/SFG imaging can be readily extended to multimodal multiphoton microscopy with simultaneous occurrence of other multiphoton phenomena, including multiphoton excitation and coherent Raman scattering, which shed light on the biophysical properties of the plasma membrane from different perspectives. Here, we review traditional and current applications, as well as the prospects of long-known but unexplored SHG/SFG imaging techniques in biophysics, with special focus on their use in the biophysical characterization of the plasma membrane.

Characterization of thymine microcrystals by CARS and SHG microscopy

Identification of chemically homologous microcrystals in a polycrystal sample is a big challenge and requires developing specific highly sensitive tools. Second harmonic (SHG) and coherent anti-Stokes Raman scattering (CARS) spectroscopy can be used to reveal arrangement of thymine molecules, one of the DNA bases, in microcrystalline sample. Strong dependence of CARS and SHG intensity on the orientation of the linear polarization of the excitation light allows to obtain high resolution images of thymine microcrystals by additionally utilizing the scanning microscopy technique. Experimental findings and theoretical interpretation of the results are compared. Presented experimental data together with quantum chemistry-based theoretical interpretation allowed us to determine the most probable organization of the thymine molecules.

Nonlinear multispectral imaging for tumor delineation

SIGNIFICANCE

In breast-preserving tumor surgery, the inspection of the excised tissue boundaries for tumor residue is too slow to provide feedback during the surgery. The discovery of positive margins requires a new surgery which is difficult and associated with low success. If the re-excision could be done immediately this is believed to improve the success rate considerably.

AIM

Our aim is for a fast microscopic analysis that can be done directly on the excised tissue in or near the operating theatre.

APPROACH

We demonstrate the combination of three nonlinear imaging techniques at selected wavelengths to delineate tumor boundaries. We use hyperspectral coherent anti-Stokes Raman scattering (CARS), second harmonic generation (SHG), and two-photon excited fluorescence (TPF) on excised patient tissue.

RESULTS

We show the discriminatory power of each of the signals and demonstrate a sensitivity of 0.87 and a specificity of 0.95 using four CARS wavelengths in combination with SHG and TPF. We verify that the information is independent of sample treatment.

CONCLUSIONS

Nonlinear multispectral imaging can be used to accurately determine tumor boundaries. This demonstration using microscopy in the epi-direction directly on thick tissue slices brings this technology one step closer to clinical implementation.

Advanced microscopy techniques for revealing molecular structure of starch granules

Starch is a major source of our daily diet and it is important to understand the molecular structure that plays a significant role in its wide number of applications. In this review article, microscopic structures of starch granules from potato, corn, rice canna, tania, wheat, sweet potato, and cassava are revealed using advanced microscopic techniques. Optical microscopy depicts the size and shape, polarization microscopy shows the anisotropy properties of starch granules, scanning electron microscopy (SEM) displays surface topography, and confocal microscopy is used to observe the three-dimensional internal structure of starch granules. The crystallinity of starch granules is revealed by second harmonic generation (SHG) microscopy and atomic force microscopy (AFM) provides mechanical properties including strength, texture, and elasticity. These properties play an important role in understanding the stability of starch granules under various processing conditions like heating, enzyme degradation, and hydration and determining its applications in various industries such as food packaging and textile industries.

Second harmonic generation polarization microscopy as a tool for protein structure analysis

Cryo-electron microscopy and X-ray crystallography have been the major tools of protein structure analysis for decades and will certainly continue to be essential in the future. Moreover, nuclear magnetic resonance or Förster resonance energy transfer can measure structural dynamics. Here, we propose to add optical second-harmonic generation (SHG), which is a nonlinear optical scattering process sensitive to molecular structures in illuminated materials, to the tool-kit of structural analysis methodologies. SHG can be expected to probe the structural changes of proteins in the physiological condition, and thus link protein structure and biological function. We demonstrate that a conformational change as well as its dynamics in protein macromolecular assemblies can be detected by means of SHG polarization measurement. To prove the capability of SHG polarization measurement with regard to protein structure analysis, we developed an SHG polarization microscope to analyze microtubules in solution. The difference in conformation between microtubules with different binding molecules was successfully observed as polarization dependence of SHG intensity. We also succeeded in capturing the temporal variation of structure in a photo-switchable protein crystal in both activation and inactivation processes. These results illustrate the potential of this method for protein structure analysis in physiological solutions at room temperature without any labeling.

Molecular understanding of label-free second harmonic imaging of microtubules

Microtubules are a vital component of the cell’s cytoskeleton and their organization is crucial for healthy cell functioning. The use of label-free SH imaging of microtubules remains limited, as sensitive detection is required and the true molecular origin and main determinants required to generate SH from microtubules are not fully understood. Using advanced correlative imaging techniques, we identified the determinants of the microtubule-dependent SH signal. Microtubule polarity, number and organization determine SH signal intensity in biological samples. At the molecular level, we show that the GTP-bound tubulin dimer conformation is fundamental for microtubules to generate detectable SH signals. We show that SH imaging can be used to study the effects of microtubule-targeting drugs and proteins and to detect changes in tubulin conformations during neuronal maturation. Our data provide a means to interpret and use SH imaging to monitor changes in the microtubule network in a label-free manner.

Microtubules (MTs) are well-studied cytoskeleton components, but have primarily been investigated using fixation or invasive techniques. Here, the authors use label-free second harmonic (SH) fluorescence and correlative light electron microscopy to pinpoint determinants required for SH from MTs.

Optical molecular imaging of corpora amylacea in human brain tissue

Label-free multiphoton imaging constitutes a promising technique for clinical diagnosis and therapeutic monitoring. Corpora amylacea (CoA) are starch-like structures often found in the diseased brain, whose origin and role in nervous pathologies are still a matter of debate. Recently, CoA in the diseased human hippocampus were found to be second harmonic generation (SHG) active. Here, we show that CoA formed in other parts of the diseased brain and in brain neoplasms display a similar SHG activity. The SHG pattern of CoA depended on laser polarization, indicating that a radial structure is responsible for their nonlinear activity. Vibrational spectroscopy was used to study the biochemistry underlying the SHG activity. Infrared (IR) and Raman spectroscopy showed that CoA contain polyglucosans that are biochemically similar to glycogen, but with an unusual structure that is similar to amylopectin, which justifies the nonlinear activity of CoA. Our findings explain the SHG activity of CoA and demonstrate that CoA in the pathological brain are amenable to label-free multiphoton imaging. Further research will clarify whether intraoperative assessment of CoA can be diagnostically exploited.

Label-Free Fried Starchy Matrix: Investigation by Harmonic Generation Microscopy

An innovative methodology based on non-destructive observation by using harmonic generation microscopy is proposed for detection and location of starch granules and oil in a fried starchy matrix and topography analysis of food products. Specific fluorescent probes were used to label the main biochemical components of the starchy fried matrix, namely starch and oil. Fluorescence of starch and oil respectively stained with Safranin O and Nile red was observed from non-linear microscopy. By using sequential scanning and specific emission filters, it was possible to merge fluorescence and harmonic generation signals. Second harmonic generation (SHG) generated by starch granules was superposed with safranin fluorescence, whereas third harmonic generation (THG), not restricted to the superposition with Nile red fluorescent signal, was used to investigate the topography of the fried product. By these experiments, starch granule mapping and topography of the starchy fried product were obtained without any destructive preparation of the sample. This label-free approach using harmonic generation microscopy is a very promising methodology for microstructure investigation of a large panel of starchy food products.

Bone regeneration after traumatic skull injury in Xenopus tropicalis

The main purpose of regenerative biology is to improve human health by exploiting cellular and molecular mechanisms favoring tissue repair. In recent years, non-mammalian vertebrates have emerged as powerful model organisms to tackle the problem of tissue regeneration. Here, we analyze the process of bone repair in metamorphosing Xenopus tropicalis tadpoles subjected to traumatic skull injury. Five days after skull perforation, a dense and highly vascularized mesenchymal is apparent over the injury site. Using an in vivo bone staining procedure based on independent pulses of Alizarin red and Calcein green, we show that the deposition of new bone matrix completely closes the wound in 15 days. The absence of cartilage implies that bone repair follows an intramembranous ossification route. Collagen second harmonic imaging reveals that while a well-organized lamellar type of bone is deposited during development, a woven type of bone is produced during the early-phase of the regeneration process. Osteoblasts lying against the regenerating bone robustly express fibrillar collagen 1a1, SPARC and Dlx5. These analyses establish Xenopus tropicalis as a new model system to improve traumatic skull injury recovery.

PhD AM, Burstein R. Activation of pial and dural macrophages and dendritic cells by cortical spreading depression

OBJECTIVE

Cortical spreading depression (CSD) has long been implicated in migraine attacks with aura. The process by which CSD, a cortical event that occurs within the blood-brain barrier (BBB), results in nociceptor activation outside the BBB is likely mediated by multiple molecules and cells. The objective of this study was to determine whether CSD activates immune cells inside the BBB (pia), outside the BBB (dura), or in both, and if so, when.

METHODS

Investigating cellular events in the meninges shortly after CSD, we used in vivo two-photon imaging to identify changes in macrophages and dendritic cells (DCs) that reside in the pia, arachnoid, and dura and their anatomical relationship to TRPV1 axons.

RESULTS

We found that activated meningeal macrophages retract their processes and become circular, and that activated meningeal DCs stop migrating. We found that CSD activates pial macrophages instantaneously, pial, subarachnoid, and dural DCs 6-12 minutes later, and dural macrophages 20 minutes later. Dural macrophages and DCs can appear in close proximity to TRPV1-positive axons.

INTERPRETATION

The findings suggest that activation of pial macrophages may be more relevant to cases where aura and migraine begin simultaneously, that activation of dural macrophages may be more relevant to cases where headache begins 20 to 30 minutes after aura, and that activation of dural macrophages may be mediated by activation of migratory DCs in the subarachnoid space and dura. The anatomical relationship between TRPV1-positive meningeal nociceptors, and dural macrophages and DCs supports a role for these immune cells in the modulation of head pain. Ann Neurol 2018;83:508-521.

Label-Free Imaging of Umbilical Cord Tissue Morphology and Explant-Derived Cells

In situ detection of MSCs remains difficult and warrants additional methods to aid with their characterization in vivo. Two-photon confocal laser scanning microscopy (TPM) and second harmonic generation (SHG) could fill this gap. Both techniques enable the detection of cells and extracellular structures, based on intrinsic properties of the specific tissue and intracellular molecules under optical irradiation. TPM imaging and SHG imaging have been used for label-free monitoring of stem cells differentiation, assessment of their behavior in biocompatible scaffolds, and even cell tracking in vivo. In this study, we show that TPM and SHG can accurately depict the umbilical cord architecture and visualize individual cells both in situ and during culture initiation, without the use of exogenously applied labels. In combination with nuclear DNA staining, we observed a variance in fluorescent intensity in the vessel walls. In addition, antibody staining showed differences in Oct4, αSMA, vimentin, and ALDH1A1 expression in situ, indicating functional differences among the umbilical cord cell populations. In future research, marker-free imaging can be of great added value to the current antigen-based staining methods for describing tissue structures and for the identification of progenitor cells in their tissue of origin.

Deformation mechanisms of human amnion: Quantitative studies based on second harmonic generation microscopy

Multiphoton microscopy has proven to be a versatile tool to analyze the three-dimensional microstructure of the fetal membrane and the mechanisms of deformation on the length scale of cells and the collagen network. In the present contribution, dedicated microscopic tools for in situ mechanical characterization of tissue under applied mechanical loads and the related methods for data interpretation are presented with emphasis on new stepwise monotonic uniaxial experiments. The resulting microscopic parameters are consistent with previous ones quantified for cyclic and relaxation tests, underlining the reliability of these techniques. The thickness reduction and the substantial alignment of collagen fiber bundles in the compact and fibroblast layer starting at very small loads are highlighted, which challenges the definition of a reference configuration in terms of a force threshold. The findings presented in this paper intend to inform the development of models towards a better understanding of fetal membrane deformation and failure, and thus of related problems in obstetrics and other clinical conditions.

Second harmonic generation microscopy of fetal membranes under deformation: normal and altered morphology

INTRODUCTION

Insight into the microstructure of fetal membrane and its response to deformation is important for understanding causes of preterm premature rupture of the membrane. However, the microstructure of fetal membranes under deformation has not been visualized yet. Second harmonic generation microscopy, combined with an in-situ stretching device, can provide this valuable information.

METHODS

Eight fetal membranes were marked over the cervix with methylene blue during elective caesarean section. One sample per membrane of reflected tissue, between the placenta and the cervical region, was cyclically stretched with a custom built inflation device. Samples were mounted on an in-situ stretching device and imaged with a multiphoton microscope at different deformation levels. Microstructural parameters such as thickness and collagen orientation were determined. Image entropy was evaluated for the spongy layer.

RESULTS

The spongy layer consistently shows an altered collagen structure in the cervical and cycled tissue compared with the reflected membrane, corresponding to a significantly higher image entropy. An increased thickness of collagenous layers was found in cervical and stretched samples in comparison to the reflected tissue. Significant collagen fibre alignment was found to occur already at moderate deformation in all samples.

CONCLUSIONS

For the first time, second harmonic generation microscopy has been used to visualize the microstructure of fetal membranes. Repeated mechanical loading was shown to affect the integrity of the amnion-chorion interface which might indicate an increased risk of premature rupture of fetal membrane. Moreover, mechanical loading might contribute to morphological alterations of the fetal membrane over the cervical region.