Microvascular development in the rat arteriovenous loop model in vivo-A step by step intravital microscopy analysis

The arteriovenous (AV) loop model is a key technique to solve one of the major problems of tissue engineering-providing adequate vascular support for a tissue construct of significant size. However, the molecular and cellular mechanisms of vascularization and factors influencing the generation of new tissue in the AV loop are still poorly understood. We previously established a novel intravital microscopy approach to study these events. In this study, we implanted our observation chamber filled with two types of hydrogels such as fibrin and methacrylate gelatin (GelMA) and performed intravital microscopy (IVM) on days 7, 14, and 21. Initial microvessel formation was observed in GelMA on day 14, while the vessel network showed clear indicators of network rearrangement and maturation on day 21. No visible microvessels were observed in fibrin. The chambers were explanted on day 21. Histological examination revealed higher numbers of microvessels in GelMA compared to fibrin, while the AV loop was thrombosed in all fibrin constructs, possibly due to matrix degradation. GelMA proved to be an ideal matrix for IVM studies in the AV loop model due to its slow degradation and transparency. This IVM model can be employed as a novel tool for live and thus faster comprehension of crucial events in the tissue regeneration process, which can improve tissue engineering application.

Vessel grafts for tissue engineering revisited-Vessel segments show location-specific vascularization patterns in ex vivo ring assay

OBJECTIVE

Transplantation of prefabricated tissue-engineered flaps can be a potential alternative for healing large tissue defects. Providing adequate vascular supply for an engineered tissue construct is one of the key points in establishing successful tissue engineering-based treatment approaches. In tissue engineering-based vascularization techniques like the arteriovenous loop, vascular grafts with high angiogenic potential can help to enhance neovascularization and tissue formation. Therefore, our study aimed to compare the angiogenic potential of vascular grafts from different locations in the rat.

METHODS

The angiogenic activity was investigated by an ex vivo vessel outgrowth ring assay using 1-mm height vascular segments embedded in fibrin for 2 weeks.

RESULTS

Maximum vessel outgrowth was observed on Days 10-12. Upper extremity vessels exhibited stronger outgrowth than lower extremity vessels. Moreover, arterial vessels demonstrated higher angiogenic potential compared with venous vessels.

CONCLUSION

Collectively, our ex vivo findings suggest that upper extremity arterial vessels have a higher angiogenic capacity, which could be used to improve neovascularization and tissue formation in tissue engineering.

Watching the Vessels Grow: Establishment of Intravital Microscopy in the Arteriovenous Loop Rat Model

Tissue engineering in reconstructive surgery seeks to generate bioartificial tissue substitutes. The arteriovenous (AV) loop allows the generation of axially vascularized tissue constructs. Cellular mechanisms of this vascularization process are largely unclear. In this study, we developed two different chamber models for intravital microscopy and imaging of the AV loop in the rat. Multiple design variations were implanted and the stability of the chamber and AV loop patency was tested in vivo. Our novel chamber facilitates repetitive observation of the AV loop using fluorescence-enhanced intravital microscopy. This technique can be used for daily evaluation of leukocyte-endothelial cell interactions, vascularization, and tissue formation in the AV loop model on 14 consecutive days. Therefore, our newly developed model for intravital microscopy will provide better understanding of cellular and molecular processes in tissue engineering in the AV loop. Moreover, it supports initiation of the novel approaches for therapeutic applications. Impact statement In the Arteriovenous (AV) loop, axially vascularized tissue can be generated and modified using different tissue engineering approaches. Cellular mechanisms of this vascularization process are largely unclear. We managed to develop an intravital microscopy model for long-term observation of intravascular and perivascular events in the AV loop. Leukocyte-endothelial cell interactions, vascularization, and tissue formation in the AV loop can now be evaluated on a day-to-day basis. This will provide better understanding of cellular and molecular processes happening during tissue engineering within the AV loop.

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.

Diagnosis and treatment of Guillain-Barré Syndrome in childhood and adolescence: An evidence- and consensus-based guideline

This evidence- and consensus-based practical guideline for the diagnosis and treatment of Guillain-Barré Syndrome (GBS) in childhood and adolescence has been developed by a group of delegates from relevant specialist societies and organisations; it is the result of an initiative by the German-Speaking Society of Neuropediatrics (GNP), and is supported by the Association of Scientific Medical Societies (AWMF, Arbeitsgemeinschaft der Wissenschaftlichen Medizinischen Fachgesellschaften). A systematic analysis of the literature revealed that only a few adequately-controlled studies exist for this particular age group, while none carries a low risk of bias. For this reason, the diagnostic and therapeutic recommendations largely rely on findings in adult patients with GBS, for which there are a higher number of suitable studies available. Consensus was established using a written, multi-step Delphi process. A high level of consensus could be reached for the crucial steps in diagnosis and treatment. We recommend basing the diagnostic approach on the clinical criteria of GBS and deriving support from CSF and electrophysiological findings. Repetition of invasive procedures that yield ambiguous results is only recommended if the diagnosis cannot be ascertained from the other criteria. For severe or persistently-progressive GBS treatment with intravenous immunoglobulin (IVIG) is recommended, whereas in cases of IVIG intolerance or inefficacy we recommended treatment with plasmapheresis. Corticosteroids are ineffective for GBS but can be considered when acute onset chronic inflammatory demyelinating polyneuropathy (A-CIDP) is suspected due to a prolonged disease course. The full German version of the Guideline is available on the AWMF website (https://www.awmf.org/leitlinien/detail/ll/022-008.html).

Tissue plasminogen activator promotes postischemic neutrophil recruitment via its proteolytic and nonproteolytic properties

OBJECTIVE

Neutrophil infiltration of the postischemic tissue considerably contributes to organ dysfunction on ischemia/reperfusion injury. Beyond its established role in fibrinolysis, tissue-type plasminogen activator (tPA) has recently been implicated in nonfibrinolytic processes. The role of this serine protease in the recruitment process of neutrophils remains largely obscure.

APPROACH AND RESULTS

Using in vivo microscopy on the postischemic cremaster muscle, neutrophil recruitment and microvascular leakage, but not fibrinogen deposition at the vessel wall, were significantly diminished in tPA(-/-) mice. Using cell transfer techniques, leukocyte and nonleukocyte tPA were found to mediate ischemia/reperfusion-elicited neutrophil responses. Intrascrotal but not intra-arterial application of recombinant tPA induced a dose-dependent increase in the recruitment of neutrophils, which was significantly higher compared with stimulation with a tPA mutant lacking catalytic activity. Whereas tPA-dependent transmigration of neutrophils was selectively reduced on the inhibition of plasmin or gelatinases, neutrophil intravascular adherence was significantly diminished on the blockade of mast cell activation or lipid mediator synthesis. Moreover, stimulation with tPA caused a significant elevation in the leakage of fluorescein isothiocyanate dextran to the perivascular tissue, which was completely abolished on neutrophil depletion. In vitro, tPA-elicited macromolecular leakage of endothelial cell layers was abrogated on the inhibition of its proteolytic activity.

CONCLUSIONS

Endogenously released tPA promotes neutrophil transmigration to reperfused tissue via proteolytic activation of plasmin and gelatinases. As a consequence, tPA on transmigrating neutrophils disrupts endothelial junctions allowing circulating tPA to extravasate to the perivascular tissue, which, in turn, amplifies neutrophil recruitment through the activation of mast cells and release of lipid mediators.