Biodegradable scaffolds seeded with bone marrow mononuclear cells (BMCs) are the earliest tissue-engineered vascular grafts (TEVGs) to be used clinically. These TEVGs transform into living blood vessels in vivo, with an endothelial cell (EC) lining invested by smooth muscle cells (SMCs); however, the process by which this occurs is unclear. To test if the seeded BMCs differentiate into the mature vascular cells of the neovessel, we implanted an immunodeficient mouse recipient with human BMC (hBMC)-seeded scaffolds. As in humans, TEVGs implanted in a mouse host as venous interposition grafts gradually transformed into living blood vessels over a 6-month time course. Seeded hBMCs, however, were no longer detectable within a few days of implantation. Instead, scaffolds were initially repopulated by mouse monocytes and subsequently repopulated by mouse SMCs and ECs. Seeded BMCs secreted significant amounts of monocyte chemoattractant protein-1 and increased early monocyte recruitment. These findings suggest TEVGs transform into functional neovessels via an inflammatory process of vascular remodeling.bone marrow | monocyte chemoattractant protein-1 | tissue engineering | neovascularization C ongenital heart disease is a leading cause of infant mortality, often requiring early surgical intervention to correct fatal cardiovascular malformations. Prosthetic vascular grafts are widely used in these reconstructive operations, but revisions are often necessary because of their inability to grow or effectively remodel within a growing child (1-3). A strategy to address this issue is the use of living tissue-engineered vascular grafts (TEVGs). Constructed from biodegradable polyester tubes seeded with autologous bone marrow mononuclear cells (BMCs), these grafts undergo extensive remodeling in animal recipients and appear to transform into living blood vessels, similar in morphology and function to the native veins into which they are interposed (4, 5). Ongoing clinical studies evaluating BMC-seeded grafts as venous conduits for congenital heart surgery report excellent safety profiles and 100% patency rates at 1-3 years of follow-up (6-8). Additionally, these grafts demonstrate growth potential, suggesting they may be more effective for the pediatric patient population than currently available vascular grafts (8,9).Although the functional efficacy and clinical utility of TEVGs are promising, little is known about how these BMC-seeded polyester tubes transform into living blood vessels in host recipients. It has been proposed that stem cells within the seeded BMC population differentiate into the endothelial cells (ECs) and smooth muscle cells (SMCs) of the developing neovessel, ultimately replacing the degrading polyester tube (10). This hypothesis, however, has not been directly examined.We recently developed a method for constructing small-diameter biodegradable synthetic scaffolds suitable for use as vascular grafts in mice (11). These tubular scaffolds are composed of the same materials and design used in clinical TEVGs...

Tissue-engineered vascular grafts can be used as conduits in patients with single ventricle physiology. Graft stenosis is the primary mode of graft failure. Further follow-up and investigation for the mechanism of stenosis are warranted.

Biodegradable conduits or patches seeded with autologous bone marrow cells showed normal function (good patency to a maximum follow-up of 32 months). As living tissues, these vascular structures may have the potential for growth, repair, and remodeling. The tissue-engineering approach may provide an important alternative to the use of prosthetic materials in the field of pediatric cardiovascular surgery. Longer follow-up is necessary to confirm the durability of this approach.

The primary graft-related complication during the first clinical trial evaluating the use of tissue-engineered vascular grafts (TEVGs) was stenosis. We investigated the role of macrophages in the formation of TEVG stenosis in a murine model. We analyzed the natural history of TEVG macrophage infiltration at critical time points and evaluated the role of cell seeding on neovessel formation. To assess the function of infiltrating macrophages, we implanted TEVGs into mice that had been macrophage depleted using clodronate liposomes. To confirm this, we used a CD11b-diphtheria toxin-receptor (DTR) transgenic mouse model. Monocytes infiltrated the scaffold within the first few days and initially transformed into M1 macrophages. As the scaffold degraded, the macrophage infiltrate disappeared. Cell seeding decreased the incidence of stenosis (32% seeded, 64% unseeded, P=0.024) and the degree of macrophage infiltration at 2 wk. Unseeded TEVGs demonstrated conversion from M1 to M2 phenotype, whereas seeded grafts did not. Clodronate and DTR inhibited macrophage infiltration and decreased stenosis but blocked formation of vascular neotissue, evidenced by the absence of endothelial and smooth muscle cells and collagen. These findings suggest that macrophage infiltration is critical for neovessel formation and provides a strategy for predicting, detecting, and inhibiting stenosis in TEVGs.

Recent advances have allowed for three-dimensional (3D) printing technologies to be applied to biocompatible materials, cells and supporting components, creating a field of 3D bioprinting that holds great promise for artificial organ printing and regenerative medicine. At the same time, stem cells, such as human induced pluripotent stem cells, have driven a paradigm shift in tissue regeneration and the modeling of human disease, and represent an unlimited cell source for tissue regeneration and the study of human disease. The ability to reprogram patient-specific cells holds the promise of an enhanced understanding of disease mechanisms and phenotypic variability. 3D bioprinting has been successfully performed using multiple stem cell types of different lineages and potency. The type of 3D bioprinting employed ranged from microextrusion bioprinting, inkjet bioprinting, laser-assisted bioprinting, to newer technologies such as scaffold-free spheroid-based bioprinting. This review discusses the current advances, applications, limitations and future of 3D bioprinting using stem cells, by organ systems.