Diffusion Limits of an in Vitro Thick Prevascularized Tissue

Abstract

Although tissue engineering promises to replace or restore lost function to nearly every tissue in the body, successful applications are currently limited to tissue less than 2 mm in thickness. in vivo capillary networks deliver oxygen and nutrients to thicker (> 2 mm) tissues, suggesting that introduction of a preformed in vitro vascular network may be a useful strategy for engineered tissues. This article describes a system for generating capillary-like networks within a thick fibrin matrix. Human umbilical vein endothelial cells, growing on the surface of microcarrier beads, were embedded in fibrin gels a known distance (Delta = 1.8-4.5 mm) from a monolayer of human dermal fibroblasts. The distance of the growth medium, which contained vascular endothelial growth factor and basic fibroblast growth factor, from the beads, C, was varied from 2.7 to 7.2 mm. Capillaries with visible lumens sprouted in 2-3 days, reaching lengths that exceeded 500 microm within 6-8 days. On day 7, capillary network formation was largely independent of C; however, a strong inverse correlation with Delta was observed, with the maximum network formation at Delta = 1.8 mm. Surprisingly, the thickness of the gel was not a limiting factor for oxygen diffusion as these tissue constructs retained a relatively high oxygen tension of > 125 mmHg. We conclude that diffusion of oxygen in vitro is not limiting, allowing the development of tissue constructs on the order of centimeters in thickness. In addition, diffusion of fibroblast-derived soluble mediators is necessary for stable capillary formation, but is significantly impeded relative to that of nutrients present in the medium.

Chimeric Vessel Tissue Engineering Driven by Endothelialized Modules in Immunosuppressed Sprague-Dawley Rats

“…Hence, only cells within a diffusion distance of 150-200 mm of the host blood vessels are adequately perfused and there can be significant cell loss at the core of implanted engineered tissues without an adequate vascular network. 1,2 Another challenge is the immune response and rejection of engineered tissues, which leads to most in vivo tissue engineering being done in immune-compromised animals to circumvent or at least minimize these issues. The use of immune-compromised models also decreases the inflammatory response that in part drives wound healing, angiogenesis, and tissue remodeling, critical aspects of the ultimate integration of the tissue construct with the host.…”

Whole-organ engineering with natural extracellular materials

“…As host-vessel ingrowth requires a finite time to penetrate into the depth of the implanted tissue, necrosis can occur prior to sufficient vascularization, resulting in implant failure. Many studies have explored means to facilitate bioengineered angiogenesis [106,107] and ongoing in vitro attempts to prevascularize engineered tissue may have a future in in vivo applications [104,[108][109][110]. Notably, one study has already demonstrated evidence of angiogenesis in vivo attributed to multi-cell type co-culture [111].…”

Computational modeling of the fluid mechanical environment of regular and irregular scaffolds

“…The cultivation of tissue constructs in vitro and static conditions, typically result in an outer shell of viable cells, while the inner core becomes necrotic due to the poor diffusive delivery of nutrients and the accumulation of wastes [6] . A bioreactor system has been developed to generate dynamic fluid perfusion within cell-seeded scaffolds, and it was found to have beneficial effects on cell function and the growth of bone tissue [7−11] .…”