Divergent changes of flow through individual blood vessels upon localized heating

From Localized Mild Hyperthermia to Improved Tumor Oxygenation: Physiological Mechanisms Critically Involved in Oncologic Thermo-Radio-Immunotherapy

(1) Background: Mild hyperthermia (mHT, 39-42 °C) is a potent cancer treatment modality when delivered in conjunction with radiotherapy. mHT triggers a series of therapeutically relevant biological mechanisms, e.g., it can act as a radiosensitizer by improving tumor oxygenation, the latter generally believed to be the commensurate result of increased blood flow, and it can positively modulate protective anticancer immune responses. However, the extent and kinetics of tumor blood flow (TBF) changes and tumor oxygenation are variable during and after the application of mHT. The interpretation of these spatiotemporal heterogeneities is currently not yet fully clarified. (2) Aim and methods: We have undertaken a systematic literature review and herein provide a comprehensive insight into the potential impact of mHT on the clinical benefits of therapeutic modalities such as radio- and immuno-therapy. (3) Results: mHT-induced increases in TBF are multifactorial and differ both spatially and with time. In the short term, changes are preferentially caused by vasodilation of co-opted vessels and of upstream normal tissue vessels as well as by improved hemorheology. Sustained TBF increases are thought to result from a drastic reduction of interstitial pressure, thus restoring adequate perfusion pressures and/or HIF-1α- and VEGF-mediated activation of angiogenesis. The enhanced oxygenation is not only the result of mHT-increased TBF and, thus, oxygen availability but also of heat-induced higher O2 diffusivities, acidosis- and heat-related enhanced O2 unloading from red blood cells. (4) Conclusions: Enhancement of tumor oxygenation achieved by mHT cannot be fully explained by TBF changes alone. Instead, a series of additional, complexly linked physiological mechanisms are crucial for enhancing tumor oxygenation, almost doubling the initial O2 tensions in tumors.

Tissue-engineered bone formation using periosteal-derived cells and polydioxanone/pluronic F127 scaffold with pre-seeded adipose tissue-derived CD146 positive endothelial-like cells

The aim of this study was to generate tissue-engineered bone formation using periosteal-derived cells seeded into a polydioxanone/pluronic F127 (PDO/Pluronic F127) scaffold with adipose tissue-derived CD146 positive endothelial-like cells. Considering the hematopoietic and mesenchymal phenotypes of adipose tissue-derived cells cultured in EBM-2 medium, CD146 positive adipose tissue-derived cells was sorted to purify more endothelial cells in characterization. These sorted cells were referred to as adipose tissue-derived CD146 positive endothelial-like cells. Periosteum is a good source of osteogenic cells for tissue-engineered bone formation. Periosteal-derived cells were found to have good osteogenic capacity in a PDO/Pluronic F127 scaffold, which could provide a suitable environment for the osteoblastic differentiation of these cells. Through the investigation of capillary-like tube formation on matrigel and the cellular proliferation of adipose tissue-derived CD146 positive endothelial-like cells cultured in different media conditions, we examined these cells could be cultured in EBM-2 with osteogenic induction factors. We also observed that the osteogenic activity of periosteal-derived cells could be good in EBM-2 with osteogenic induction factors, in the early period of culture. The experimental results obtained in the miniature pig model suggest that tissue-engineered bone formation using periosteal-derived cells and PDO/Pluronic F127 scaffold with pre-seeded adipose tissue-derived CD146 positive endothelial-like cells can be used to restore the bony defects of the maxillofacial region when used in clinics.

Autogenous greater omentum, as a free nonvascularized graft, enhances bone healing: an experimental nonunion model

Reconstruction of vascularity is an early event in fracture healing and upregulation of angiogenesis may therefore promote the formation of bone. We have investigated the potentiality of autogenous free nonvascularized greater omentum to stimulate the formation of bone in an experimental hypertrophic nonunion model. Twelve dogs assigned into two identical groups underwent a standard nonunion operation. In the experimental group, this was followed by application of autogenous greater omentum as a free nonvascularized graft around the osteotomy gap. Radiographic assessments were conducted time-sequentially until euthanasia 16 weeks after surgery. Histological analysis was performed on the mid-radial diaphysis containing the 4-month-old osteotomy site. Radiological and histological properties of the group treated with free transplant of the greater omentum revealed complete union. In contrast, there was no evidence indicating union in the control group. Analyses of the radiological and histological scores confirmed that osteotomies treated with free transplant of the autogenous greater omentum had united, whereas the osteotomies of the control group failed to unite. Significant differences between the mean values for radiological and histological-grading score in the control and experimental groups were detected (p < 0.05). We showed that free graft of autogenous greater omentum could stimulate the formation of competent bone in an environment deprived of its normal vascularization. Hence, it could be recommended to enhance healing when the fractures are at risk of nonunion.

Sequential changes in vessel formation and micro-vascular function during bone repair

BACKGROUND

Angiogenesis, the process of new vessel formation from a pre-existing vascular network, is essential for bone development and repair. New vessel formation and microvascular functions are crucial during bone repair, not only for sufficient nutrient supply, transport of macromolecules and invading cells, but also because they govern the metabolic microenvironment. Despite its central role, very little is known about the initial processes of vessel formation and microvascular function during bone repair.

METHODS

To visualize and quantify the process of vessel formation and microvascular function during bone repair, we transplanted neonatal femora with a substantial defect into dorsal skin-fold chambers in severe combined immunodeficient (SCID) mice for continuous noninvasive in-vivo evaluation. We employed intravital microscopic techniques to monitor effective microvascular permeability, functional vascular density, blood flow rate and leukocyte flux repeatedly over 16 days. Oxytetracyclin and v. Kossa/v. Giesson staining was performed to quantify the calcification process in vivo and in vitro.

RESULTS

Development of a hematoma surrounding the defect area was the initial event, which was accompanied by a significant increase in microvascular permeability and blood flow rate. With absorption of the hematoma and vessel maturation, permeability decreased continuously, while vascular density and tissue perfusion increased. Histological evaluation revealed that the remodeling of the substantial defect prolonged the in-vivo monitored calcification process.

INTERPRETATION

The size of the initial substantial defect correlated positively with increased permeability, suggesting improved release of permeability-inducing cytokines. The unchanged permeability in the control group with boiled bones and a substantial defect corroborated these findings. The adaptation to increasing metabolic demands was initially mediated by increased blood flow rate, later with increasing vascular density through increased tissue perfusion rate. These insights into the sequence of microvascular alterations may assist in the development of targeted drug delivery therapies and caution against the use of permeability-altering drugs during bone healing.