An encapsulated fibrin-based bioartificial tissue construct with integrated macrovessels, microchannels and capillary tubes

Facilitating sufficient nutrient and oxygen supply in large-scale bioartificial constructs is a critical step in organ bioengineering. Immediate perfusion not only depends on a dense capillary network, but also requires integrated large-diameter vessels that allow vascular anastomoses during implantation. These requirements set high demands for matrix generation as well as for in vitro cultivation techniques and remain mostly unsolved challenges up until today. Additionally, bioartificial constructs must have sufficient biomechanical stability to withstand mechanical stresses during and after implantation. We developed a bioartificial tissue construct with a fibrin matrix containing human umbilical vein endothelial cells and adipose tissue-derived stem cells facilitating capillary-like network formation. This core matrix was surrounded by a dense acellular fibrin capsule providing biomechanical stability. Two fibrin-based macrovessels were integrated on each side of the construct and interconnected via four 1.2 mm thick microchannels penetrating the cellularized core matrix. After four days of perfusion in a custom-built bioreactor, homogenous capillary-like network formation throughout the core matrix was observed. The fibrin capsule stabilized the core matrix and facilitated the generation of a self-supporting construct. Thus, the encapsulated fibrin tissue construct could provide a universal pre-vascularized matrix for seeding with different cell types in various tissue engineering approaches. This article is protected by copyright. All rights reserved.

Chemically induced hypoxia by dimethyloxalylglycine (DMOG)-loaded nanoporous silica nanoparticles supports endothelial tube formation by sustained VEGF release from adipose tissue-derived stem cells

Inadequate vascularization leading to insufficient oxygen and nutrient supply in deeper layers of bioartificial tissues remains a limitation in current tissue engineering approaches to which pre-vascularization offers a promising solution. Hypoxia triggering pre-vascularization by enhanced vascular endothelial growth factor (VEGF) expression can be induced chemically by dimethyloxalylglycine (DMOG). Nanoporous silica nanoparticles (NPSNPs, or mesoporous silica nanoparticles, MSNs) enable sustained delivery of molecules and potentially release DMOG allowing a durable capillarization of a construct. Here we evaluated the effects of soluble DMOG and DMOG-loaded NPSNPs on VEGF secretion of adipose tissue-derived stem cells (ASC) and on tube formation by human umbilical vein endothelial cells (HUVEC)-ASC co-cultures. Repeated doses of 100 µM and 500 µM soluble DMOG on ASC resulted in 3- to 7-fold increased VEGF levels on day 9 (P < 0.0001). Same doses of DMOG-NPSNPs enhanced VEGF secretion 7.7-fold (P < 0.0001) which could be maintained until day 12 with 500 µM DMOG-NPSNPs. In fibrin-based tube formation assays, 100 µM DMOG-NPSNPs had inhibitory effects whereas 50 µM significantly increased tube length, area and number of junctions transiently for 4 days. Thus, DMOG-NPSNPs supported endothelial tube formation by upregulated VEGF secretion from ASC and thus display a promising tool for pre-vascularization of tissue-engineered constructs. Further studies will evaluate their effect in hydrogels under perfusion.

A 3-Layered Bioartificial Blood Vessel with Physiological Wall Architecture Generated by Mechanical Stimulation

The generation of cellularized bioartificial blood vessels resembling all three layers of the natural vessel wall with physiological morphology and cell alignment is a long pursued goal in vascular tissue engineering. Simultaneous culture of all three layers under physiological mechanical conditions requires highly sophisticated perfusion techniques and still today remains a key challenge. Here, three-layered bioartificial vessels based on fibrin matrices were generated using a stepwise molding technique. Adipose-derived stem cells (ASC) were differentiated to smooth muscle cells (SMC) and integrated in a compacted tubular fibrin matrix to resemble the tunica media. The tunica adventitia-equivalent containing human umbilical vein endothelial cells (HUVEC) and ASC in a low concentration fibrin matrix was molded around it. Luminal seeding with HUVEC resembled the tunica intima. Subsequently, constructs were exposed to physiological mechanical stimulation in a pulsatile bioreactor for 72 h. Compared to statically incubated controls, mechanical stimulation induced physiological cell alignment in each layer: Luminal endothelial cells showed longitudinal alignment, cells in the media-layer were aligned circumferentially and expressed characteristic SMC marker proteins. HUVEC in the adventitia-layer formed longitudinally aligned microvascular tubes resembling vasa vasorum capillaries. Thus, physiologically organized three-layered bioartificial vessels were successfully manufactured by stepwise fibrin molding with subsequent mechanical stimulation.

Perfusion promotes endothelialized pore formation in high concentration fibrin gels otherwise unsuitable for tube development

Vascularization of tissue engineered implants is crucial for their survival and integration in the recipient's body. Pre-vascularized, fibrin-based implants offer a solution since low concentration fibrin hydrogels (1 mg/mL) have been shown to promote tube formation of endothelial cells in co-culture with adipogenic stem cells. However, higher fibrinogen concentrations (> 20 mg/mL) enabling the fabrication of stable implants are necessary.We here characterized fibrin gels of 1-30 mg/mL for their rheological properties and whether they support tube formation of endothelial cell-adipogenic stem cell co-cultures for up to 7 days. Moreover, 20 mg/mL gels containing preformed channels and endothelial cell-adipogenic stem cell co-culture were perfused continuously in a customized flow chamber with 3.9 dyn/cm² for 12 days and analyzed for capillary formation.Rheology of fibrin gels showed increasing stability proportional to fibrinogen concentration with 20 mg/mL gels having a storage module of 465 Pa. Complex tube networks stable for 7 days were observed at 1-5 mg/mL gels whereas higher concentrations showed initial sprouting only. However, perfusion of 20 mg/mL fibrin gels resulted in endothelialized pore formation in several layers of the gel with endothelial cell-adipogenic stem cell co-culture.Thus, perfusion supports the formation of capillary-like structures in fibrin gels that are too dense for spontaneous tube formation under static conditions. Future studies are necessary to further increase pore density and to investigate proper nutrition of tissue-specific target cells in the scaffold.

Biochemical Myogenic Differentiation of Adipogenic Stem Cells Is Donor Dependent and Requires Sound Characterization

IMPACT STATEMENT

We here showed that even under optimized conditions for biochemical differentiation of adipose-derived stem cells (with respect to a pronounced marker protein expression for a reasonable period of time) it was not possible to obtain functional smooth muscle cells from all donors. Moreover, an underestimated role may play the effect of the scaffold material on smooth muscle cell functionality. Both aspects are crucial for the successful tissue engineering of the vascular medial layer combining autologous cells with a suitable scaffold material and thus should be thoroughly addressed in each individualized therapeutic approach.

Complete Myogenic Differentiation of Adipogenic Stem Cells Requires Both Biochemical and Mechanical Stimulation

Vascular tissue engineering of the middle layer of natural arteries requires contractile smooth muscle cells (SMC) which can be differentiated from adipose-derived mesenchymal stem cells (ASC) by treatment with transforming growth factor-β, sphingosylphosphorylcholine and bone morphogenetic protein-4 (TSB). Since mechanical stimulation may support or replace TSB-driven differentiation, we investigated its effect plus TSB-treatment on SMC orientation and contractile protein expression. Tubular fibrin scaffolds with incorporated ASC or pre-differentiated SMC were exposed to pulsatile perfusion for 10 days with or without TSB. Statically incubated scaffolds served as controls. Pulsatile incubation resulted in collagen-I expression and orientation of either cell type circumferentially around the lumen as shown by alpha smooth muscle actin (αSMA), calponin and smoothelin staining as early, intermediate and late marker proteins. Semi-quantitative Westernblot analyses revealed strongly increased αSMA and calponin expression by either pulsatile (12.48-fold; p < 0.01 and 38.15-fold; p = 0.07) or static incubation plus TSB pre-treatment (8.91-fold; p < 0.05 and 37.69-fold; p < 0.05). In contrast, contractility and smoothelin expression required both mechanical and TSB stimulation since it was 2.57-fold increased (p < 0.05) only by combining pulsatile perfusion and TSB. Moreover, pre-differentiation of ASC prior to pulsatile perfusion was not necessary since it could not further increase the expression level of any marker.