Multiphoton microscopy analysis of extracellular collagen I network formation by mesenchymal stem cells

Multiphoton Imaging of Maturation in Tissue Engineering

Donor cell-specific tissue-engineered (TE) implants are a promising therapy for personalized treatment of cardiovascular diseases, but current development protocols lack a stable longitudinal assessment of tissue development at subcellular resolution. As a first step toward such an assessment approach, in this study we establish a generalized labeling and imaging protocol to obtain quantified maturation parameters of TE constructs in three dimensions (3D) without the need of histological slicing, thus leaving the tissue intact. Focusing on intracellular matrix (ICM) and extracellular matrix (ECM) networks, multiphoton laser scanning microscopy (MPLSM) was used to investigate TE patches of different conditioning durations of up to 21 days. We show here that with a straightforward labeling procedure of whole-mount samples (so without slicing into thin histological sections), followed by an easy-to-use multiphoton imaging process, we obtained high-quality images of the tissue in 3D at various time points during development. The stacks of images could then be further analyzed to visualize and quantify the volume of cell coverage as well as the volume fraction and network of structural proteins. We showed that collagen and alpha-smooth muscle actin (α-SMA) volume fractions increased as normalized to full tissue volume and proportional to the cell count, with a converging trend to the final density of (4.0% ± 0.6%) and (7.6% ± 0.7%), respectively. The image analysis of ICM and ECM revealed a developing and widely branched interconnected matrix. We are currently working on the second step, that is, to integrate MPLSM endoscopy into a dynamic bioreactor system to monitor the maturation of intact TE constructs over time, thus without the need to take them out.

Mesoscopic structural analysis via deep learning processing, with a special reference to in vitro alteration in collagen fibre induced by a gap junction inhibitor

Dense connective tissue, including the ligament, tendon, fascia and cornea, is formed by regularly arranged collagen fibres synthesized by fibroblasts (Fbs). The mechanism by which fibre orientation is determined remains unclear. Periodontal ligament Fbs consistently communicate with their surroundings via gap junctions (GJs), leading to the formation of a wide cellular network. A method to culture Fb-synthesized collagen fibres was previously reported by Schafer et al. ('Ascorbic acid deficiency in cultured human fibroblasts'. J. Cell Biol. 34: 83-95, 1967). This method has been applied to investigate the ability and activity of Fb collagen synthesis/phagocytosis using conventional electron microscopy (EM). However, the three-dimensional mesoscopic architecture of collagen fibres and the influence of GJ inhibitors on collagen fibre formation in vitro are poorly understood. In this study, three-dimensional mesoscopic analysis was used to elucidate the mechanism of directional fibre formation. We investigated the influence of GJ inhibitors on collagen formation driven by periodontal ligament Fbs in vitro, histomorphometrically, and the structural properties of in vitro collagen fibre on a mesoscale quantitatively, using correlative light and EM optimized for picrosirius red staining and focused ion beam-scanning EM tomography. Our results indicate that under culture conditions, in the presence of a GJ inhibitor, the orientation of collagen fibres becomes more disordered than that in the control group. This suggests that the GJ might be involved in determining fibre orientation during collagen fibre formation. Elucidation of this mechanism may help develop novel treatment strategies for connective tissue orientation disorders. Graphical Abstract.

An investigation of a self-assembled cell-extracellular complex and its potentials in improving wound healing

BACKGROUND

To maintain and enhance the wound healing effects of mesenchymal stem cells (MSCs), a scaffold for hosting MSCs is needed, which ought to be completely biocompatible, durable, producible, and of human source.

OBJECTIVE

To build a cell-extracellular matrix (ECM) complex assembled by human umbilical cord mesenchymal stem cells (HuMSCs) and to investigate its clinical potentials in promoting wound healing.

METHOD

HuMSCs were isolated and expanded. When the cells of third passage reached confluency, ascorbic acid was added to stimulate the cells to deposit ECM where the cells grew in. Four weeks later, a cells-loaded ECM sheet was formed. The cell-ECM complex was observed under the scanning electron microscopy (SEM) and subjected to histological studies. The supernatants were collected and the cell-ECM complex was harvested at different time points and processed for enzyme-linked immune sorbent assay (ELISA) and mRNA analysis. The in vivo experiments were performed by means of implanting the cell-ECM complex on the mice back for up to 6 months and the specimens were collected for histological studies.

RESULTS

After 4 weeks of cultivation with ascorbic stimulation, a sheet was formed which is mainly composed with HuMSCs, collagen and hyaluronic acid. The cell-ECM complex can sustain to certain tensile force. The mRNA and protein levels of vascular endothelial growth factor-α (VEGF-α), hepatocyte growth factor (HGF), keratinocyte growth factor (KGF), and transforming growth factor-β1 (TGF-β1) were remarkably increased compared to monolayer-cultured cells. The implanted cell-ECM complex on mice was still noticeable with host cells infiltration and vascularization on 6 months.

CONCLUSION

Our studies suggested that HuMSCs can be multi-cultivated through adding ascorbic stimulation and ECM containing collagen and hyaluronic acid were enriched around the cells which self-assembly formed a cell-ECM complex. Cell-ECM complex can improve growth factors secretion remarkably which means it may promote wound healing by paracrine.

Myofibrillar Lattice Remodeling Is a Structural Cytoskeletal Predictor of Diaphragm Muscle Weakness in a Fibrotic mdx (mdx Cmah-/-) Model

Duchenne muscular dystrophy (DMD) is a degenerative genetic myopathy characterized by complete absence of dystrophin. Although the mdx mouse lacks dystrophin, its phenotype is milder compared to DMD patients. The incorporation of a null mutation in the Cmah gene led to a more DMD-like phenotype (i.e., more fibrosis). Although fibrosis is thought to be the major determinant of ‘structural weakness’, intracellular remodeling of myofibrillar geometry was shown to be a major cellular determinant thereof. To dissect the respective contribution to muscle weakness, we assessed biomechanics and extra- and intracellular architecture of whole muscle and single fibers from extensor digitorum longus (EDL) and diaphragm. Despite increased collagen contents in both muscles, passive stiffness in mdx Cmah−/− diaphragm was similar to wt mice (EDL muscles were twice as stiff). Isometric twitch and tetanic stresses were 50% reduced in mdx Cmah−/− diaphragm (15% in EDL). Myofibrillar architecture was severely compromised in mdx Cmah−/− single fibers of both muscle types, but more pronounced in diaphragm. Our results show that the mdx Cmah−/− genotype reproduces DMD-like fibrosis but is not associated with changes in passive visco-elastic muscle stiffness. Furthermore, detriments in active isometric force are compatible with the pronounced myofibrillar disarray of the dystrophic background.

Quantitative and qualitative analysis of pulmonary arterial hypertension fibrosis using wide-field second harmonic generation microscopy

We demonstrated that wide-field second harmonic generation (SHG) microscopy of lung tissue in combination with quantitative analysis of SHG images is a powerful tool for fast and label-free visualization of the fibrosis pathogenesis in pulmonary arterial hypertension (PAH). Statistical analysis of the SHG images revealed changes of the collagen content and morphology in the lung tissue during the monocrotaline-induced PAH progression in rats. First order statistics disclosed the dependence of the collagen overproduction on time, the second order statistics indicated tightening of collagen fiber network around blood vessels and their spreading into the alveolar region. Fourier analysis revealed that enhancement of the fiber orientation in the collagen network with PAH progression was followed with its subsequent reduction at the terminating phase of the disease. Proposed approach has potential for assessing pulmonary fibrosis in interstitial lung disease, after lung(s) transplantation, cancer, etc.

Impact of Endothelial Progenitor Cells in the Vascularization of Osteogenic Scaffolds

The microvascular endothelial network plays an important role in osteogenesis, bone regeneration and bone tissue engineering. Endothelial progenitor cells (EPCs) display a high angiogenic and vasculogenic potential. The endothelialization of scaffolds with endothelial progenitor cells supports vascularization and tissue formation. In addition, EPCs enhance the osteogenic differentiation and bone formation of mesenchymal stem cells (MSCs). This study aimed to investigate the impact of EPCs on vascularization and bone formation of a hydroxyapatite (HA) and beta-tricalcium phosphate (ß-TCP)–fibrin scaffold. Three groups were designed: a scaffold-only group (A), a scaffold and EPC group (B), and a scaffold and EPC/MSC group (C). The HA/ß–TCP–fibrin scaffolds were placed in a porous titanium chamber permitting extrinsic vascularization from the surrounding tissue. Additionally, intrinsic vascularization was achieved by means of an arteriovenous loop (AV loop). After 12 weeks, the specimens were explanted and investigated by histology and CT. We were able to prove a strong scaffold vascularization in all groups. No differences regarding the vessel number and density were detected between the groups. Moreover, we were able to prove bone formation in the coimplantation group. Taken together, the AV loop is a powerful tool for vascularization which is independent from scaffold cellularization with endothelial progenitor cells’ prior implantation.

High-resolution imaging of the osteogenic and angiogenic interface at the site of murine cranial bone defect repair via multiphoton microscopy

The spatiotemporal blood vessel formation and specification at the osteogenic and angiogenic interface of murine cranial bone defect repair were examined utilizing a high-resolution multiphoton-based imaging platform in conjunction with advanced optical techniques that allow interrogation of the oxygen microenvironment and cellular energy metabolism in living animals. Our study demonstrates the dynamic changes of vessel types, that is, arterial, venous, and capillary vessel networks at the superior and dura periosteum of cranial bone defect, suggesting a differential coupling of the vessel type with osteoblast expansion and bone tissue deposition/remodeling during repair. Employing transgenic reporter mouse models that label distinct types of vessels at the site of repair, we further show that oxygen distributions in capillary vessels at the healing site are heterogeneous as well as time- and location-dependent. The endothelial cells coupling to osteoblasts prefer glycolysis and are less sensitive to microenvironmental oxygen changes than osteoblasts. In comparison, osteoblasts utilize relatively more OxPhos and potentially consume more oxygen at the site of repair. Taken together, our study highlights the dynamics and functional significance of blood vessel types at the site of defect repair, opening up opportunities for further delineating the oxygen and metabolic microenvironment at the interface of bone tissue regeneration.

Actually Seeing What Is Going on - Intravital Microscopy in Tissue Engineering

Intravital microscopy (IVM) study approach offers several advantages over in vitro, ex vivo, and 3D models. IVM provides real-time imaging of cellular events, which provides us a comprehensive picture of dynamic processes. Rapid improvement in microscopy techniques has permitted deep tissue imaging at a higher resolution. Advances in fluorescence tagging methods enable tracking of specific cell types. Moreover, IVM can serve as an important tool to study different stages of tissue regeneration processes. Furthermore, the compatibility of different tissue engineered constructs can be analyzed. IVM is also a promising approach to investigate host reactions on implanted biomaterials. IVM can provide instant feedback for improvising tissue engineering strategies. In this review, we aim to provide an overview of the requirements and applications of different IVM approaches. First, we will discuss the history of IVM development, and then we will provide an overview of available optical modalities including the pros and cons. Later, we will summarize different fluorescence labeling methods. In the final section, we will discuss well-established chronic and acute IVM models for different organs.

Co-culture systems of osteoblasts and osteoclasts: Simulating in vitro bone remodeling in regenerative approaches

Bone is an extremely dynamic tissue, undergoing continuous remodeling for its whole lifetime, but its regeneration or augmentation due to bone loss or defects are not always easy to obtain. Bone tissue engineering (BTE) is a promising approach, and its success often relies on a "smart" scaffold, as a support to host and guide bone formation through bone cell precursors. Bone homeostasis is maintained by osteoblasts (OBs) and osteoclasts (OCs) within the basic multicellular unit, in a consecutive cycle of resorption and formation. Therefore, a functional scaffold should allow the best possible OB/OC cooperation for bone remodeling, as happens within the bone extracellular matrix in the body. In the present work OB/OC co-culture models, with and without scaffolds, are reviewed. These experimental systems are intended for different targets, including bone remodeling simulation, drug testing and the assessment of biomaterials and 3D scaffolds for BTE. As a consequence, several parameters, such as cell type, cell ratio, culture medium and inducers, culture times and setpoints, assay methods, etc. vary greatly. This review identifies and systematically reports the in vitro methods explored up to now, which, as they allow cellular communication, more closely resemble bone remodeling and/or the regeneration process in the framework of BTE. STATEMENT OF SIGNIFICANCE: Bone is a dynamic tissue under continuous remodeling, but spontaneous healing may fail in the case of excessive bone loss which often requires valid alternatives to conventional treatments to restore bone integrity, like bone tissue engineering (BTE). Pre-clinical evaluation of scaffolds for BTE requires in vitro testing where co-cultures combining innovative materials with osteoblasts (OBs) and osteoclasts (OCs) closely mimic the in vivo repair process. This review considers the direct and indirect OB/OC co-cultures relevant to BTE, from the early mouse-cell models to the recent bone regenerative systems. The co-culture modeling of bone microenvironment provides reliable information on bone cell cross-talk. Starting from improved knowledge on bone remodeling, bone disease mechanisms may be understood and new BTE solutions are designed.

Serum-Free Culture of Human Mesenchymal Stem Cell Aggregates in Suspension Bioreactors for Tissue Engineering Applications

Mesenchymal stem cells (MSCs) have the capacity to differentiate towards bone, fat, and cartilage lineages. The most widely used culture and differentiation protocols for MSCs are currently limited by their use of serum-containing media and small-scale static culture vessels. Suspension bioreactors have multiple advantages over static culture vessels (e.g., scalability, control, and mechanical forces). This study sought to compare the formation and culture of 3D aggregates of human synovial fluid MSCs within suspension bioreactors and static microwell plates. It also sought to elucidate the benefits of these techniques in terms of productivity, cell number, and ability to generate aggregates containing extracellular matrix deposition. MSCs in serum-free medium were either (1) inoculated as single cells into suspension bioreactors, (2) aggregated using static microwell plates prior to being inoculated in the bioreactor environment, or (3) aggregated using microwell plates and kept in the static environment. Preformed aggregates that were size-controlled at inoculation had a greater tendency to form large, irregular super aggregates after a few days of suspension culture. The single MSCs inoculated into suspension bioreactors formed a more uniform population of smaller aggregates after a definite culture period of 8 days. Both techniques showed initial deposition of extracellular matrix within the aggregates. When the relationship between aggregate size and ECM deposition was investigated in static culture, midsized aggregates (100-300 cells/aggregate) were found to most consistently maximize sGAG and collagen productivity. Thus, this study presents a 3D tissue culture method, which avoids the clinical drawbacks of serum-containing medium that can easily be scaled for tissue culture applications.

Effects of Material-Tissue Interactions on Bone Regeneration Outcomes Using Baghdadite Implants in a Large Animal Model

Extensive bone loss due to trauma or disease leads to impaired healing. Current bone grafts and substitutes have major drawbacks that limit their effectiveness for treating large bone defects. A number of bone substitutes in development are undergoing preclinical testing, but few studies specifically investigate the in vivo material-tissue interactions that provide an important indicator to long-term implant safety and efficacy. This study is the first of its kind to specifically investigate in vivo material-tissue interactions at the bone-implant interface. Baghdadite scaffolds implanted in critical-sized segmental defects in sheep tibia for 26 weeks are analyzed by focused ion beam scanning electron microscopy, multiphoton microscopy, and histology. The scaffolds are seen to induce extensive bone formation that directly abut the implant surfaces with no evidence of chronic inflammation or fibrous capsule formation. Bone remodeling is influenced by slow in vivo degradation around and within the implant, causing portions of the implant to be incorporated into the newly formed bone. These findings have important implications for predicting the long-term effects of baghdadite ceramics in promoting defect healing, and support the translation of baghdadite scaffolds as a new generation of bone graft substitutes with improved properties for the repair of large bone defects.

Bacterial nanocellulose stimulates mesenchymal stem cell expansion and formation of stable collagen-I networks as a novel biomaterial in tissue engineering

Biomimetic scaffolds are of great interest to tissue engineering (TE) and tissue repair as they support important cell functions. Scaffold coating with soluble collagen-I has been used to achieve better tissue integration in orthopaedy, however, as collagen persistence was only temporary such efforts were limited. Adequate coverage with cell-derived ECM collagen-I would promise great success, in particular for TE of mechanically challenged tissues. Here, we have used label-free, non-invasive multiphoton microscopy (MPM) to characterise bacterial nanocellulose (BNC) - a promising biomaterial for bone TE - and their potency to stimulate collagen-I formation by mesenchymal stem cells (MSCs). BNC fleeces were investigated by Second Harmonic Generation (SHG) imaging and by their characteristic autofluorescence (AF) pattern, here described for the first time. Seeded MSCs adhered fast, tight and very stable, grew to multilayers and formed characteristic, wide-spread and long-lasting collagen-I. MSCs used micron-sized lacunae and cracks on the BNC surface as cell niches. Detailed analysis using a collagen-I specific binding protein revealed a highly ordered collagen network structure at the cell-material interface. In addition, we have evidence that BNC is able to stimulate MSCs towards osteogenic differentiation. These findings offer new options for the development of engineered tissue constructs based on BNC.

Multimodal imaging quality control of epithelia regenerated with cultured human donor corneal limbal epithelial stem cells

Current imaging techniques for the characterization of differentiated corneal limbal stem cells are destructive and cannot be used in eye bank for monitoring the regenerated epithelium in culture. We presented a minimally invasive, multimodal, marker-free imaging method for the investigation of epithelia regenerated with cultured human donor corneal limbal epithelial stem cells. Two-photon fluorescence and harmonic generation signals were collected from specimens in culture and used for evaluating the structure and morphology of epithelia cultured on two different bio-scaffolds; in addition, donor human corneal tissues were used as controls. The method provided reliable information on the organization of cellular and extracellular components of biomaterial substrates and was highly sensitive to determine differences between the density packing arrangement of epithelial cells of different biomaterials without relying on inferences from exogenous labels. The present minimally invasive standardized quality control methodology can be reliably translated to eye banks and used for monitoring harvested corneal limbal stem cells growth and differentiation in bioengineered materials.

Assessment of Population and ECM Production Using Multiphoton Microscopy as an Indicator of Cell Viability

Multiphoton microscopy allows continuous depth-resolved, nondestructive imaging of scaffold-seeded cells during cell or tissue culture. Spectrally separated images in high resolution can be provided while cells are conserved in their native state. Here we describe the seeding of mesenchymal stem cells to bacterial nanocellulose hydropolymer scaffolds followed by 2-channel imaging of cellular autofluorescence (AF) and collagen-I formation using second harmonic generation (SHG) signals. With this approach the simultaneous observation of the progression of cell morphology and production of extracellular matrix as hallmarks of viability and cell fitness is possible.