Towards organ printing: engineering an intra-organ branched vascular tree

Abstract

Importance of the field: Effective vascularization of thick three-dimensional engineered tissue constructs is a problem in tissue engineering. As in native organs, a tissue-engineered intra-organ vascular tree must be comprised of a network of hierarchically branched vascular segments. Despite this requirement, current tissue-engineering efforts are still focused predominantly on engineering either large-diameter macrovessels or microvascular networks.

Areas covered in this review: We present the emerging concept of organ printing or robotic additive biofabrication of an intra-organ branched vascular tree, based on the ability of vascular tissue spheroids to undergo self-assembly.

What the reader will gain: The feasibility and challenges of this robotic biofabrication approach to intra-organ vascularization for tissue engineering based on organ-printing technology using self-assembling vascular tissue spheroids including clinically relevantly vascular cell sources are analyzed.

Take home message: It is not possible to engineer 3D thick tissue or organ constructs without effective vascularization. An effective intra-organ vascular system cannot be built by the simple connection of large-diameter vessels and microvessels. Successful engineering of functional human organs suitable for surgical implantation will require concomitant engineering of a 'built in' intra-organ branched vascular system. Organ printing enables biofabrication of human organ constructs with a 'built in' intra-organ branched vascular tree.

Figure 1. Design of vascular tree

A. The complexity of branching pattern of the intraorgan vascular tree is illustrated by this view of the kidney vasculature [30]. B. Classification of branching patterns of a vascular tree. From left to right: symmetrical, asymmetrical and mixed. C. Scheme for designing a patient-specific ‘blueprint’ of a vascular tree: (top, left to right) image acquisition; skeletonization; skeletonized model, (bottom, left to right) enlargement of the skeletonized model using the coefficient of tissue retraction; ‘blueprint’ model; final printed segment of vascular tree.

Figure 2. Postprinting evolution of printed vascular constructs

A. Biofabricated vascular ring formed from 10 vascular tissue spheroids comprised of human vascular smooth muscle cells. Fifty percent of spheroids were fabricated from cells labeled with red vital fluorescent stain and fifty percent were fabricated from cells labeled with green vital fluorescent stain. There is no apparent cell mixing during tissue spheroid fusion. B. Sequential steps in changes to morphology and geometry during tissue fusion in a ring-like tissue construct biofabricated from tissue spheroids comprised of human smooth muscle cells. A dramatic reduction of internal ring diameter is evident while no change in the diameter of the individual tissue spheroids was observed. C. There is a nonlinear relationship between the coefficient of retraction and number of cell aggregates (tissue spheroids). D. Scheme demonstrating calculation of changes in morphology of closely placed tissue spheroids in a ring-like tissue construct into torus-like tissue constructs. The diameter of tissue spheroids is constant during the tissue fusion process while the internal diameter of the ring construct is dramatically reduced. E. Computer simulation (using ‘Surface Evolver’ software) of morphological changes in a tube-like vascular tissue construct during tissue fusion. The initial tube-like vascular tissue construct becomes shorter and narrower. The hexagonal pattern of tissue spheroid packing is clearly demonstrated.

Figure 3. Scheme for bioprinting an organ construct with a ‘built in’ intra-organ branched vascular tree

A. Solid vascular tissue spheroids (red = endothelial cells; green = smooth muscle/pericytic cells). B. Uni-lumenal vascular tissue spheroids with a single lumen (red = endothelial cells; green = smooth muscle/pericytic cells). C. Tissue-specific spheroids with internal microvascular networks (yellow = tissue-specific cells; red = microvasculature). D. Bioprinting of a large-diameter vascular segment using solid vascular tissue spheroids. E. Bioprinting of an intermediate-diameter vascular segment using uni-lumenal vascular tissue spheroids with a single lumen. F. Bioprinting of an organ segment using tissue-specific spheroids with internal microvascular networks G. Bioprinting of a vascularized organ segment using all three types of tissue spheroids.