Researchers have made considerable progress in the construction of small, functional tissue sections called organoids over the past decade, enabled by a combination of better understanding the mechanisms involved in regeneration and embryonic development of tissues, advances in 3-D bioprinting, guidance of cell behavior via appropriate provision of signal molecules, and the generation of environments that mimic an existing tissue environment. Every tissue requires its own specific recipe of signals and environment in order to form a functional organoid, but researchers have demonstrated the manufacture of organoids for liver, kidney, lung, and thymus, among others.
Organoids are tiny, usually a millimeter or two in size. They are a stepping stone to the generation of patient matched replacement organs on demand, given a cell sample as a starting point. Ever since the first organoids were generated, however, the blocking challenge to scaling up engineered tissue in size has been the inability to generate organoids that incorporate functional blood vessel networks. In cross-sections of natural tissues, several hundred capillaries pass through every square millimeter. Absent this microvasculature, blood (and thus the necessary oxygen and nutrients for cell survival) cannot perfuse through more than a few millimeters into tissues.
Producing vasculature in engineered tissues has proven to be challenging. That it is so challenging is why considerable effort has gone towards establishing decellularization of donor organs and xenotransplantation of genetically engineered pig organs as a basis for expanding the pool of organs available for transplantation. Of late some progress has been made on methods of 3-D printing organoids that contain an initial vasculature that is dense enough to sustain the tissue, albeit short of the natural capillary density. Today's open access paper describes an example of the opposite approach, which is to find a suitable combination of cells, signals, and environment that causes a microvasculature to form within the organoid as it grows.
Organoids derived from human induced pluripotent stem cells (hiPSCs) are state of the art cell culture models to study mechanisms of development and disease. The establishment of different tissue models such as intestinal, liver, cerebral, kidney, and lung organoids was published within the last years. These organoids recapitulate the development of epithelial structures in a fascinating manner. However, they remain incomplete as vasculature, stromal components, and tissue resident immune cells are mostly lacking. All these cell types derive from mesenchymal tissue and it is well known that epithelial-mesenchymal interactions play a fundamental role during tissue development.
Recent publications addressed this issue, especially with regard to organoid vascularization. Researchers demonstrated that human blood vessels self-organize and can be grown in vitro. In order to vascularize cerebral organoids, others added endothelial cells to the system. But blood vessels are more complex than an endothelial tube. Larger vessels consist of multiple layers that contain cell types such as endothelial and smooth muscle cells, while even small capillaries rely on the support of pericytes and a basal lamina. Other groups generated vascularized neural organoids consisting of blood vessels and microglia. However, in these cases, the heterologous vessels as well as microglia are host derived and invade the neural organoid after transplantation.
We propose that the directed incorporation of mesodermal progenitor cells (MPCs) into organoids will overcome the aforementioned limitations. In order to demonstrate the feasibility of the method, we generated complex human tumor as well as neural organoids. We show that the formed blood vessels display a hierarchic organization and mural cells are assembled into the vessel wall. Moreover, we demonstrate a typical blood vessel ultrastructure including endothelial cell-cell junctions, a basement membrane as well as luminal caveolae and microvesicles. We observe a high plasticity in the endothelial network, which expands, while the organoids grow and is responsive to anti-angiogenic compounds and pro-angiogenic conditions such as hypoxia. We show that vessels within tumor organoids connect to host vessels following transplantation.