Biomedical subjects

Alvin T Yeh

Publications and source records attributed to Alvin T Yeh.

11 recordsLinked to original sources

PubMed2006

Ex vivo characterization of sub-10-fs pulses.

An all-mirror dispersion-compensation setup is used to correct for quadratic and cubic phase distortions induced within a custom nonlinear optical microscope. Mouse tail tendon is used to characterize sub-10-fs pulses by interferometric autocorrelation. This is an ideal method for characterizing dispersion from the optical system, immersion medium, and wet biological sample. The generation of very short autocorrelations demonstrates the ability to compensate for phase distortions within the imaging system and efficient second-harmonic upconversion of the ultrashort pulse spectrum within collagen. Compensated autocorrelation traces are presented for biologically relevant objective lenses.

Animals↗

PubMed2006

Influence of glycerol on the mechanical reversibility and thermal damage susceptibility of collagenous tissues.

Clinical procedures wherein supraphysiologic temperatures must be achieved in deep layers of tissue via light are often compromised by optical scattering and absorption. Optical clearing of tissue superficial to the target improves the efficacy of such procedures. Glycerol is an attractive chemical agent for achieving dramatic reductions in tissue turbidity, but its net effects on healthy tissue are not fully understood. In this paper, we investigate possible alterations of biaxial mechanical properties in a model collagenous tissue, bovine epicardium, induced by glycerol. Furthermore, we examine the effects of glycerol on the biaxial thermomechanical properties of epicardium constrained at near-physiologic length. It is seen that mechanical changes induced by glycerol are fully reversed upon rehydration in normal saline. Moreover, glycerol protects cleared tissue by increasing its thermal stability and minimizing thermal alterations of mechanical properties.

Animals↗

PubMed2005

Photodynamic therapy on keloid fibroblasts in tissue-engineered keratinocyte-fibroblast co-culture.

BACKGROUND AND OBJECTIVES: Keloids are disfiguring, proliferative scars that are a pathologic response to cutaneous injury. An organotypic tissue culture system (the Raft model 1-10) was used to investigate the feasibility of using photodynamic therapy (PDT) as an adjunctive therapy to treat keloids following surgical excision. The Raft co-culture system mimics skin by layering keratinocytes on top of fibroblasts embedded in a collagen matrix. PDT uses drugs that produce singlet oxygen in situ when irradiated by light, and may lead to a number of effects in living tissues varying from the modulation of growth to apoptosis. PDT is already used to treat several benign and malignant diseases in organs such as the skin, retina, and esophagus. STUDY DESIGN/MATERIALS AND METHODS: Normal adult, neonatal, and keloid fibroblasts and keratinocytes were isolated from skin obtained from patients undergoing elective procedures and used to construct the Rafts. Mature Rafts (after 4 days) were incubated with 5-amino levulinic acid (5-ALA), a photosensitizer, for 3 hours and were laser-irradiated (635 nm) for total energy delivery of 5 J/cm2, 10 J/cm2, or 20 J/cm2. Rafts were examined 24 hours and 14 days later. Cell viability was determined using confocal imaging combined with live-dead fluorescent dyes. Multi-photon microscope (MPM) imaged collagen structure and density. As Rafts contract over time, surface area was measured using optical micrometry daily. RESULTS: At 10 and 20 J/cm2, near-total cell death was observed in all constructs, while at 5 J/cm2 cell viability was comparable to controls. Cell viability in keloid and neonatal Rafts was greater than that observed in normal adult Rafts. Treated Rafts contracted less over the 14-day period compared to controls. Contraction and collagen density were greatest in keloid and neonatal Rafts. CONCLUSIONS: A PDT dosimetry range was established, which reduces tissue contraction and collagen density while minimizing injury to fibroblasts.

Adult↗

PubMed2005

Nonlinear optical microscopy of articular cartilage.

OBJECTIVE: To assess the ability of nonlinear optical microscopy (NLOM) to image ex vivo healthy and degenerative bovine articular cartilage. METHOD: Fresh bovine femoral-tibial joints were obtained from an abattoir. Articular cartilage specimens were harvested from the tibial plateau. Normal and degenerative specimens were imaged by NLOM and subsequently fixed and processed for histological examination. RESULTS: NLOM provided high resolution images of articular cartilage at varying depths with high sensitivity to tissue morphology and high specificity to tissue components without fixing, sectioning or staining. Spectroscopic segmentation of nonlinear optical signals isolated the collagen matrix from the chondron (chondrocyte and non-collagen pericellular matrix). Images from the superficial zone were consistent with the presence of a matrix composed of both elastin-like and collagen fibers distributed in a depth-dependent morphological arrangement, whereas only collagen was demonstrated in the middle and deep zones. Alterations of collagen matrix associated with advanced degenerative joint disease (fibrocartilage) were observed with NLOM. Individual chondrocytes were imaged and demonstrated intracellular fluorescence consistent with the presence of products of intracellular biochemical processes. CONCLUSION: Thin images of living articular cartilage using NLOM may be obtained with (sub-)cellular resolution at varying depths without fixing, sectioning or staining. Extracellular matrical collagen and chondron may be imaged separately in native tissue using spectrally distinct, endogenous, nonlinear optical signals. NLOM was sensitive to macromolecular composition and pathologic changes in articular cartilage matrix. Advances in instrumentation may lead to the application of NLOM to study articular cartilage in vivo.

Animals↗

PubMed2004

Enhanced cytotoxic effects of 5-aminolevulinic acid-mediated photodynamic therapy by concurrent hyperthermia in glioma spheroids.

During photodynamic therapy (PDT) both normal and pathological brain tissue, in close proximity to the light source, can experience significant temperature increases. The purpose of this study was to investigate the anti-tumor effects of concurrent 5-aminolevulinic acid (ALA)-mediated PDT and hyperthermia (HT) in human and rat glioma spheroids. Human or rat glioma spheroids were subjected to PDT, HT, or a combination of the two treatments. Therapies were given concurrently to simulate the conditions that will occur during patient PDT. Predictions of diffusion theory suggest that brain tissue immediately adjacent to a spherical light applicator may experience temperature increases approaching 8 degrees C for laser input powers of 2 W. In the in vitro model employed here, HT had no effect on spheroid survival at temperatures below 49 degrees C, while sub-threshold fluence PDT results in only modest decrease in survival. HT (40-46 degrees C) and PDT interact in a synergistic manner if the two treatments are given concurrently. The degree of synergism increases with increasing temperature and light fluence. Apoptosis is the primary mode of cell death following both low-fluence rate PDT and combined HT + PDT.

Aminolevulinic Acid↗

PubMed2003

Reversible dissociation of collagen in tissues.

The turbidity of most biologic tissues hinders the use of lasers for diagnostic and therapeutic purposes. Hyperosmotic agents such as glycerol have been used to alter the optical scattering properties of turbid tissues. The mechanism of this effect, "optical clearing", however, remains incompletely understood. Multiphoton microscopy utilizing second harmonic generation can be used to monitor collagen structural changes in the presence of glycerol. This study suggests that the use of glycerol for tissue "optical clearing" screens noncovalent intermolecular interactions between low-order collagen structures, resulting in fiber disassembly. Dissociation of collagen fibers in native and engineered tissues in the presence of glycerol and reassociation with the application of saline are imaged dynamically. Collagen fiber reassembly is governed by the displacement of glycerol by saline in native and engineered skin. These results reveal the assembly process of high-order collagen structures and propose a molecular mechanism for the increase in tissue transparency observed after glycerol application.

Animals↗

PubMed2002

Effects of combined photodynamic therapy and ionizing radiation on human glioma spheroids.

The effects of combined photodynamic therapy (PDT) and ionizing radiation are studied in a human glioma spheroid model. The degree of interaction between the two modalities depends in a complex manner on factors such as PDT irradiation fluence, fluence rate and dose of ionizing radiation. It is shown that gamma radiation and PDT interact in a synergistic manner only if both light fluence and gamma radiation dose exceed approximately 25 J cm(-2) and 8 Gy, respectively. Synergistic interactions are observed only for the lower fluence rate (25 mW cm(-2)) investigated. The degree of interaction appears to be independent of both sequence and the PDT or ionizing radiation time intervals investigated (1 and 24 h). Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling assays show that low-fluence rate PDT is very efficient at inducing apoptotic cell death, whereas neither high-fluence rate PDT nor ionizing radiation produces significant apoptosis. Although the mechanisms remain to be elucidated, the data imply that the observed synergism is likely not due to gamma-induced cell cycle arrest or to PDT-induced inhibition of DNA repair.

Brain Neoplasms↗

PubMedDate not supplied

Modeling aberrant wound healing using tissue-engineered skin constructs and multiphoton microscopy.

BACKGROUND: Keloids and hypertrophic scars result from aberrant wound healing and remain a potential complication of any surgical procedure or trauma. Investigation of aberrant wound healing has been limited to the study of growth factors, collagen precursors, and DNA synthesis in simple in vitro systems, which necessitate removal or destruction of cells or factors in the growth environment of cell cultures. Multiphoton microscopy (MPM) can use endogenous chromophores such as collagen and nicotinamide adenine dinucleotide hydrogenase to produce thin optical sections of thick living tissues without the use of dyes or stains. Endogenous second-harmonic-generation (SHG) signals in collagen can be collected to form an MPM image. OBJECTIVE: To present a novel wound-healing model used to investigate keloid-derived fibroblast activity and collagen production in the same intact tissue-engineered construct over time. METHODS: Artificial tissue constructs called RAFTs (produced by suspension of keloid or normal dermal fibroblasts in type I collagen gel with an overlying keratinocyte layer) were cultured at air-fluid interface. Multiphoton microscopy SHG images of collagen in the intact tissue constructs consisting of normal or keloid-derived fibroblasts were obtained. The constructs were then incised with a scalpel. Serial MPM and phase-contrast microscopy images were obtained to monitor changes in the extracellular matrix in response to wounding of the artificial skin construct over 8 days. RESULTS: The tissue-engineered constructs formed a bilayer resembling the dermis and epidermis of human skin. Phase-contrast microscopy revealed migration of keratinocytes into the defect created by scalpel wounding. The constructs were found to contract with time after wounding. The MPM SHG images showed collagen deposition in the tissue constructs after wounding. Tissue constructs with keloid-derived fibroblasts were found to deposit collagen at a higher rate than those with normal fibroblasts. CONCLUSIONS: The MPM model described herein permits serial observation of the same intact specimens without the need for fixation or cytotoxic stains. Furthermore, it demonstrates the biologic activity of RAFT artificial tissue constructs.

Cells, Cultured↗

PubMedDate not supplied

Imaging wound healing using optical coherence tomography and multiphoton microscopy in an in vitro skin-equivalent tissue model.

Laser thermal injury and subsequent wound healing in organotypic, skin-equivalent tissue models were monitored using optical coherence tomography (OCT), multiphoton microscopy (MPM), and histopathology. The in vitro skin-equivalent raft tissue model was composed of dermis with type I collagen and fibroblast cells and epidermis of differentiated keratinocytes. Noninvasive optical imaging techniques were used for time-dependent, serial measurements of matrix destruction and reconstruction and compared with histopathology. The region of laser thermal injury was clearly delineated in OCT images by low signal intensity. High resolution MPM imaging using second-harmonic generation revealed alterations in collagen microstructure organization with subsequent matrix reconstruction. Fibroblast cell migration in response to injury was monitored by MPM using two-photon excited fluorescence. This study illustrates the complementary features of linear and nonlinear light-tissue interaction in intrinsic signal optical imaging and their use for noninvasive, serial monitoring of wound healing processes in biological tissues.

Burns↗

PubMedDate not supplied

Molecular interactions of exogenous chemical agents with collagen--implications for tissue optical clearing.

Reduction of optical scattering in turbid biological tissues using nonreactive chemical agents has potential applications for light-based diagnostics and therapeutics. Optical clearing effects by exogenous chemical agents, in particular sugars and sugar alcohols, have been found to be temporary with tissue rehydration. Applications with dermatologic laser therapies are now being investigated, but suffer from the inability of studied agents to penetrate the superficial layers of human skin. Selection, design, and refinement of topically effective chemical agents are hindered by a lack of fundamental understanding of tissue clearing mechanisms. We present recent work, particularly from the biochemistry community, detailing molecular interactions between chemical agents and collagen. This body of work demonstrates the perturbative effects of sugars and sugar alcohols on collagen high-order structures at micro- and nanometer length scales by screening noncovalent bonding forces. In addition, these studies emphasize the nonreactive nature of agent-collagen interactions and the ability of noncovalent bonding forces to recover with agent removal and drive reassembly of destabilized collagen structures. A mechanism of tissue optical clearing is proposed based on agent destabilization of high-order collagen structures.

Animals↗

PubMedDate not supplied

Collagen solubility correlates with skin optical clearing.

Biomedical optics and photomedicine applications are challenged by the turbidity of most biological tissue systems. Nonreactive, biocompatible chemical agents can induce a reversible reduction in optical scattering of collagenous tissues such as human skin. Herein we show that a chemical agent's tissue optical clearing potential is directly related to its collagen solubility, providing a rational design basis for effective, percutaneous formulations.

Animals↗