Two-photon confocal microscopy: a nondestructive method for studying wound healing

Mapping the 3D remodeling of the extracellular matrix in human hypertrophic scar by multi-parametric multiphoton imaging using endogenous contrast

The hypertrophic scar is an aberrant form of wound healing process, whose clinical efficacy is limited by a lack of understanding of its pathophysiology. Remodeling of collagen and elastin fibers in the extracellular matrix (ECM) is closely associated with scar progression. Herein, we perform label-free multiphoton microscopy (MPM) of both fiber components from human skin specimens and propose a multi-fiber metrics (MFM) analysis model for mapping the structural remodeling of the ECM in hypertrophic scars in a highly-sensitive, three-dimensional (3D) manner. We find that both fiber components become wavier and more disorganized in scar tissues, while content accumulation is observed from elastin fibers only. The 3D MFM analysis can effectively distinguish normal and scar tissues with better than 95% in accuracy and 0.999 in the area under the curve value of the receiver operating characteristic curve. Further, unique organizational features with orderly alignment of both fibers are observed in scar-normal adjacent regions, and an optimized combination of features from 3D MFM analysis enables successful identification of all the boundaries. This imaging and analysis system uncovers the 3D architecture of the ECM in hypertrophic scars and exhibits great translational potential for evaluating scars in vivo and identifying individualized treatment targets.

Photoacoustic microscopy of vascular adaptation and tissue oxygen metabolism during cutaneous wound healing

Cutaneous wounds affect millions of people every year. Vascularization and blood oxygen delivery are critical bottlenecks in wound healing, and understanding the spatiotemporal dynamics of these processes may lead to more effective therapeutic strategies to accelerate wound healing. In this work, we applied multi-parametric photoacoustic microscopy (PAM) to study vascular adaptation and the associated changes in blood oxygen delivery and tissue oxygen metabolism throughout the hemostasis, inflammatory, proliferation, and early remodeling phases of wound healing in mice with skin puncture wounds. Multifaceted changes in the vascular structure, function, and tissue oxygen metabolism were observed during the 14-day monitoring of wound healing. On the entire wound area, significant elevations of the arterial blood flow and tissue oxygen metabolism were observed right after wounding and remained well above the baseline over the 14-day period. On the healing front, biphasic changes in the vascular density and blood flow were observed, both of which peaked on day 1, remained elevated in the first week, and returned to the baselines by day 14. Along with the wound closure and thickening, tissue oxygen metabolism in the healing front remained elevated even after structural and functional changes in the vasculature were stabilized. On the newly formed tissue, significantly higher blood oxygenation, flow, and tissue metabolism were observed compared to those before wounding. Blood oxygenation and flow in the new tissue appeared to be independent of when it was formed, but instead showed noticeable dependence on the phase of wound healing. This PAM study provides new insights into the structural, functional, and metabolic changes associated with vascular adaptation during wound healing and suggests that the timing and target of vascular treatments for wound healing may affect the outcomes.

Diagnostic and Prognostic Utility of Non-Invasive Multimodal Imaging in Chronic Wound Monitoring: a Systematic Review

Monitoring chronic wound [CW] healing is a challenging issue for clinicians across the world. Moreover, the health and cost burden of CW are escalating at a disturbing rate due to a global rise in population of elderly and diabetic cases. The conventional approach includes visual contour, sketches, or more rarely tracings. However, such conventional techniques bring forth infection, pain, allergies. Furthermore, these methods are subjective as well as time-consuming. As such, nowadays, non-touching and non-invasive CW monitoring system based on imaging techniques are gaining importance. They not only reduce patients' discomfort but also provide rapid wound diagnosis and prognosis. This review provides a survey of different types of CW characteristics, their healing mechanism and the multimodal non-invasive imaging methods that have been used for their diagnosis and prognosis. Current clinical practices as well as personal health systems [m-health and e-health] for CW monitoring have been discussed.

Three-dimensional visualisation of soft biological structures by X-ray computed micro-tomography

Whereas the two-dimensional (2D) visualisation of biological samples is routine, three-dimensional (3D) imaging remains a time-consuming and relatively specialised pursuit. Current commonly adopted techniques for characterising the 3D structure of non-calcified tissues and biomaterials include optical and electron microscopy of serial sections and sectioned block faces, and the visualisation of intact samples by confocal microscopy or electron tomography. As an alternative to these approaches, X-ray computed micro-tomography (microCT) can both rapidly image the internal 3D structure of macroscopic volumes at sub-micron resolutions and visualise dynamic changes in living tissues at a microsecond scale. In this Commentary, we discuss the history and current capabilities of microCT. To that end, we present four case studies to illustrate the ability of microCT to visualise and quantify: (1) pressure-induced changes in the internal structure of unstained rat arteries, (2) the differential morphology of stained collagen fascicles in tendon and ligament, (3) the development of Vanessa cardui chrysalises, and (4) the distribution of cells within a tissue-engineering construct. Future developments in detector design and the use of synchrotron X-ray sources might enable real-time 3D imaging of dynamically remodelling biological samples.

Multiphoton microscopy study of the morphological and quantity changes of collagen and elastic fiber components in keloid disease

Multiphoton microscopy was used to study the extracellular matrix of keloid at the molecular level without tissue fixation and staining. Direct imaging of collagen and elastin was achieved by second harmonic generation and two-photon excited fluorescence, respectively. The morphology and quantity of collagen and elastin in keloid were characterized and quantitatively analyzed in comparison to normal skin. The study demonstrated that in keloid, collagen content increased in both the upper dermis and the deep dermis, while elastin mostly showed up in the deep dermis and its quantity is higher compared to normal skin. This suggests the possibility that abnormal fibroblasts synthesized an excessive amount of collagen and elastin at the beginning of keloid formation, corresponding to the observed deep dermis, while after a certain time point, the abnormal fibroblast produced mostly collagen, corresponding to the observed upper dermis. The morphology of collagen and elastin in keloid was disrupted and presented different variations. In the deep dermis, elastic fibers showed node structure, while collagen showed obviously regular gaps between adjacent bundles. In the upper dermis, collagen bundles aligned in a preferred direction, while elastin showed as sparse irregular granules. This new molecular information provided fresh insight about the development process of keloid.

Characteristics of scar margin dynamic with time based on multiphoton microscopy

Scar margins dynamic with time were quantitatively characterized using multiphoton microscopy (MPM). 2D large-area and 3D focused images of elastin and collagen at scar margins were obtained to extract quantitative parameters. An obvious boundary was observed at the scar margin, showing altered morphological patterns of elastin and collagen on both sides. Content alteration of elastin and collagen between the two sides of boundary were defined to characterize scar margins from different individuals. The statistical results from 15 normal scar samples strongly demonstrated that content alteration degree of elastin and collagen had decreasing tendency with the increase of patient age or scar duration, consistent with the fact of normal scars regressing spontaneously over time. It indicated that alteration degree can potentially serve as quantitative indicators to examine wound healing and scar progression over time. With the advent of clinical portable multiphoton endoscopes, the MPM technique can be applied in tracking scar formation and progression in vivo by examination of scar margin.

Optical spectroscopy and imaging for the noninvasive evaluation of engineered tissues

Optical spectroscopy and imaging approaches offer the potential to noninvasively assess different aspects of the cellular, extracellular matrix, and scaffold components of engineered tissues. In addition, the combination of multiple imaging modalities within a single instrument is highly feasible, allowing acquisition of complementary information related to the structure, organization, biochemistry, and physiology of the sample. The ability to characterize and monitor the dynamic interactions that take place as engineered tissues develop promises to enhance our understanding of the interdependence of processes that ultimately leads to functional tissue outcomes. It is expected that this information will impact significantly upon our abilities to optimize the design of biomaterial scaffolds, bioreactors, and cell systems. Here, we review the principles and performance characteristics of the main methodologies that have been exploited thus far, and we present examples of corresponding tissue engineering studies.

Application of nonlinear optical microscopy for imaging skin

Recent advances in the use of nonlinear optical microscopy (NLOM) in skin microscopy are presented. Nonresonant spectroscopies including second harmonic generation, coherent anti-Stokes Raman and two-photon absorption are described and applications to problems in skin biology are detailed. These nonlinear techniques have several advantages over traditional microscopy methods that rely on one-photon excitation: intrinsic 3D imaging with <1 microm spatial resolution, decreased photodamage to tissue samples and penetration depths up to 1,000 microm with the use of near-infrared lasers. Thanks to these advantages, nonlinear optical spectroscopy has become a powerful tool to study the physical and biochemical properties of the skin. Structural information can be obtained using the response of endogenous chemical species in the skin, such as collagen or lipids, indicating that optical biopsy may replace current invasive, time-consuming traditional histology methods. Insertion of specific probe molecules into the skin provides the opportunity to monitor specific biochemical processes such as skin transport, molecular penetration, barrier homeostasis and ultraviolet radiation-induced reactive oxygen species generation. While the field is quite new, it seems likely that the use of NLOM to probe structure and biochemistry of live skin samples will only continue to grow.

Nondestructive imaging of live human keloid and facial tissue using multiphoton microscopy

OBJECTIVES

To use multiphoton microscopy to image collagen fibers and matrix structure in nonfixed human keloid tissue and normal human facial skin obtained following surgery and to compare the findings to existing knowledge of normal skin and keloid morphology to determine if this technology is a suitable adjunct for conventional histology.

METHODS

Epidermis was removed to expose the fibroblast-rich dermal layer that was then imaged using a multiphoton confocal microscope (Zeiss-Meta 510; Carl Zeiss, Jena, Germany). An 800-nm tunable titanium/sapphire femtosecond laser (Mai-Tai; Newport Co Spectra-Physics, Mountain View, California) was used to excite the tissue; second harmonic generation between 397 and 408 nm and autofluorescent signals were collected. Images were obtained using a Plan-Neofluar x40 oil immersion objective lens and a Plan-Apochromat x63 oil immersion lens.

RESULTS

Compared with normal skin, keloids showed disorganized collagen fibers arranged in complex swirls and bundles 20 to 30 microm in diameter. Normal tissue showed collagen fibers as distinct, straight strands less than 10 microm in diameter. Differences between normal and keloid tissue were subtle but apparent.

CONCLUSIONS

The value of imaging living tissue is a significant benefit. Because keloids and hypertrophic scars result from altered collagen metabolism, the development of clinical multiphoton microscopy systems may allow examination of wound healing dynamics in vivo and potentially provides a means to monitor therapy without the need for biopsy or the risk of injury to tissue.