Vascularization in tissue engineering: fundamentals and state-of-art

Abstract

Vascularization is among the top challenges that impede the clinical application of engineered tissues. This challenge has spurred tremendous research endeavor, defined as vascular tissue engineering (VTE) in this article, to establish a pre-existing vascular network inside the tissue engineered graft prior to implantation. Ideally, the engineered vasculature can be integrated into the host vasculature via anastomosis to supply nutrient to all cells instantaneously after surgery. Moreover, sufficient vascularization is of great significance in regenerative medicine from many other perspectives. Due to the critical role of vascularization in successful tissue engineering, we aim to provide an up-to-date overview of the fundamentals and VTE strategies in this article, including angiogenic cells, biomaterial/bio-scaffold design and bio-fabrication approaches, along with the reported utility of vascularized tissue complex in regenerative medicine. We will also share our opinion on the future perspective of this field.

Keywords: advanced biofabrication; biomaterial; regenerative medicine; stem cell; vascular tissue engineering.

Figure 1.

Schematic of physiological development of mammalian vasculature. (A) Mesodermal-derived angioblasts in early mammalian embryos give rise to dorsal aorta, cardinal vein and various local primary vascular plexus. In the meantime, endothelial precursor cells in the yolk sac aggregate into blood islands and generate primary vascular plexus. The primary vessels then remodel and mature to form a hierarchical, functional vasculature. (B) When an existing blood vessel initiates expansion, some endothelial cells are activated to adopt a tip cell phenotype that can sprout and invade the surrounding basement membrane. Adjacent stalk cells follow the tip cells, proliferate to support sprout elongation and lumenize. Stalk cells also deposit basement membrane and recruit mural cells to stabilize newly formed vessels.

Figure 2.

Summary of cell sources for VTE. (A) Human embryonic stem cell line can assume a vascular smooth muscle cell phenotype that express SMA, Myosin IIB, and SM22a. (B) Human mesenchymal stem cells, either derived from induced pluripotent stem cells (left) or adult tissue (right), can serve as mural cells to stabilize engineered blood vessel via differentiation, cell–cell contact, or paracrine effect. (C) Schematic of cellular composition of a blood vessel. (D) Endothelial cells can be derived from patient-specific induced pluripotent stem cells (upper left), uniformly expressing CD31 (upper right), forming capillary-like network on Matrigel (lower left, green) and uptaking AC-LDL (lower right, green). (E) EPCs are another promising source of ECs. They are aligned to laminar shear stress (upper right). These cells can undergo sub-lineage specification in response to different degree of shear stress, expressing arterial (Cx-40) and veinous markers (COUP-TFII), respectively. (A) is adapted from [56] Copyright 2015. With permission of Springer. (B), (D) and (E) are unpublished data acquired in our lab.

Figure 3.

Biophysical cues to be involved in VTE. (A) When hMSCs on PDMS micro-textured substrates were implanted subcutaneously in mice, histological analysis revealed increased vascular invasion on micro-textured substrates. Reprinted from [125], Copyright 2015, with permission from Elsevier. (B) ECs were elongated along the direction of aligned collagen nanofibrils in vitro. Reprinted from [127], Copyright 2013, with permission from Elsevier. (C) Curvature in scale larger than single cell was found to significantly influence the angiogenesis behavior of ECs [128]. (D) Laminar, unidirectional shear stress regulated arterial-veinous specification of hPSC-derived ECs. Reprinted from [142], Copyright 2015, with permission from Elsevier. All pictures are adapted from published data with permission.

Figure 4.

Overview of present strategies to engineer vasculature. (A) Hollow microchannels can be created within 3D hydrogel after removal of the fugitive material pre-patterned by casting or 3D printing. The ECs are then seeded onto the inner wall of the microchannels and thereby patterned by the pre-designed channel geometry and shape. (B) Nano/microfibrillar scaffolds prepared by electrospinning and shear-extrusion can render fibril alignment that regulates EC orientation and augment angiogenic behaviors. (C) The cells and biomaterials can be patterned concurrently by an extrusion-based printer featuring core-shell coaxial nozzles to print long, hollow microfibers with a cell-laden alginate wall to resemble the architecture of a native blood vessel. (D) The scaffold free, 3D EC spheroids may act as individual vascular units to vascularize internally, as well as pre-vascularize the surrounding tissue, and thus enhance the survival, homing and functionality of other impregnated cells within the construct. (E) Scaffolds that present angiogenic factors or resembles blood vessel morphology can be pre-implanted in a vascular site to be conditioned/vascularized in vivo through vessel ingrowth or encapsulation by host tissue, and then harvested and transplanted to the site of injury.