Research into renal organoids, also called mini kidneys, has increasingly shown their value. Most organoids are cultured in vitro; however, organoids grown under this condition lack the vasculature to provide cells with oxygen and nutrients and facilitate communication between cell types. The absence of vessels prevents researchers from reproducing essential processes such as blood filtration, reabsorption, and urine production.1 2 Blood filtration is done through specialized glomerular and tubular tissue compartments, integrated into a complex vascular network. Although renal organoids have such compartments, the development of their vasculature is limited in regular in vitro cultures. 3
Figure 1: A kidney organoid. 4
There are several approaches to obtain a vascularized organoid, they can be divided into in vitro and in vivo vascularization: in vitro techniques include needle hydrogel molding, sacrificial molding, and bioprinting. In vivo vascularization, approaches can be achieved by transplanting organoid models built in vitro into a host. 5 The development of a vascularized organoid could serve as a platform to model kidney disease and improve kidney drug toxicity testing. Furthermore, using the mini kidneys as building blocks for replacement therapies would also be possible. 1 2 3
Homan et al. (2019) analyzed the relationship between environmental stimuli and the increase in vascularization and maturation, promoted in renal tissue derived from human pluripotent stem cells in vitro. The researchers developed an in vitro method for culturing renal organoids underflow on millifluidic chips. The dynamic culture conditions cause the endogenous pool of endothelial progenitor cell organoids to expand and generate vascular networks with perfusable lumens surrounded by mural cells. The results revealed that flow-cultured vascularized renal organoids had more mature podocytes and tubular compartments with increased cell polarity and adult gene expression compared to static controls, confirming the need to induce substantial vascularization in renal organoids to implement kidney disease and regeneration studies. 3
Figure 2: A glomerulus in a kidney organoid cultured under flow showing invasion by a single capillary, an important step in nephron development. In dynamic culture conditions, the shear stress caused by the liquid flow influences the cell morphology and behavior 1
As a replacement for traditional methods, Lawlor et al. (2020) applied extrusion-based 3D bioprinting to provide a fast, high-yield generation of renal organoids that have high viability and highly reproducible cells. This approach demonstrated that three-dimensional bioprinting allows manipulation of organoid size, substantially increasing nephron yield proportionally to the starting cell number. This facilitates the fabrication of uniformly patterned sheets of renal tissue with functional proximal tubular segments, a constituent structure of the nephron. 6
A single-cell suspension was used to create a moist cell paste free of any carrier hydrogel, then loaded into a syringe for automatic deposition using a 3D bioprinter. The resulting bioprinted organoids showed spontaneous nephron formation over the subsequent 20 days of culture. Bioprinting applied to the manufacturing of renal organoids offers improvements in yield, quality control, scale, and structure, facilitating in vitro and in vivo applications of renal tissue. 6
REFERENCES
1. Boettner, B. Engineered miniature kidneys come of age. Harvard Gazette https://news.harvard.edu/gazette/story/2019/02/engineered-miniature-kidneys-come-of-age/ (2019).
2. Nanyang Technological University. Mini kidneys grown from stem cells give new insights into kidney disease and therapies. Science Daily (2019).
3. Homan, K. A. et al. Flow-enhanced vascularization and maturation of kidney organoids in vitro. Nat. Methods 16, 255–262 (2019).
4. Utrecht University. Researchers develop mini kidneys from urine cells. https://www.drugdiscoverytrends.com/researchers-develop-mini-kidneys-from-urine-cells/ (2019).
5. Zhao, X. et al. Review on the Vascularization of Organoids and Organoids-on-a-Chip. Front Bioeng Biotechnol 9, 637048 (2021).
6. Lawlor, K. T. et al. Cellular extrusion bioprinting improves kidney organoid reproducibility and conformation. Nat. Mater. 20, 260–271 (2020).