Guiding tissue-scale self-organization
A bioprinting approach that utilizes organoid-forming stem cells as a living ink within hydrogels guides tissue-scale self-organization to generate more realistic gastrointestinal and vascular tissue constructs.
Organoids are miniature stem-cell-derived tissues in a dish that form in vivo-like fine-grained tissue structures and cellular heterogeneity through self-organization1. These features have provided new insight into the mechanisms of development and disease progression. However, organoids fail to recapitulate many coarse-grained structural features found in vivo that span length scales from hundreds of micrometres to centimeters. This naturally leads to the question of what developments are necessary to expand the applicability of organoid models to these larger-scale features of tissues. Towards answering this question, Matthias Lutolf and colleagues report a simple and accessible bioprinting method that uses organoid-forming cellular inks2. Their method establishes a basis for combining coarse-grained structural control provided by bioprinters with the fine-grained structural control that emerges from organoid self-organization.
The promise of organoids as tools for basic research, regenerative medicine and disease modelling derives from the elaborate tissue structures that arise spontaneously during their self-organization. However, the consequence of allowing tissues to self-organize without the constraints provided by the surrounding embryo is that tissues often form in unexpected or uncontrolled ways. Culturing organoids in three-dimensional extracellular matrix (ECM) hydrogels, such as the reconstituted basement membrane, Matrigel, provides many important mechanical and biological cues for organoid self-organization. However, the resulting organoids can adopt a wide variety of sizes, shapes and cell-type compositions arising from stochastic processes together with micro-environmental and cellular heterogeneity. Furthermore, many of our organs exhibit complex structure at length scales that span hundreds of micrometres to centimetres. These features of self-organizing tissues arise from interactions among cells nearby in space (as occurs in current organoid cultures) together with cues from surrounding embryonic tissues that are missing from organoid cultures. Reintroducing such cues by implanting organoids into live animals has been used to promote the development of larger-scale tissue structures3. However, these processes remain challenging to recapitulate in an organoid culture. Therefore, there is great interest in adapting many of the top-down tools of tissue engineering — micropatterning, bioprinting and photolithography, among others — to arrange additional tissue types in time and space to better guide organoid self-organization. Ultimately, combining the strengths of top-down fabrication methods with the bottom-up self-organizing capacity of living cells will be critical to achieving the potential of building more complex and functional tissues and organs.
In this proof-of-concept, Lutolf and colleagues demonstrate the generation of centimetre-scale tissue features constructed from dissociated organoid progenitors, mesenchymal cells and endothelial cells after extrusion from a simple bioprinter built from a syringe pump and microscope (Fig. 1a). The microscope provides precise movement and positioning using the stage controls and real-time visual feedback, thus allowing the user to fine-tune the extrusion parameters by eye. A dense suspension of cells in media is printed directly into liquid ECM gels in the minutes before it solidifies. The authors could control cell density, nozzle size and speed of extrusion to regulate the morphology of the tissue. The printed tissues, initially simple lines and dots suspended in the ECM gel, condense into continuous tissues as they grow, then develop microstructures as they begin to self-organize into organoids (Fig. 1b). Intestinal organoids and endothelial cells both formed lumens, while intestinal organoids additionally developed crypt structures along the outer surface of tubes that resemble the crypts distributed throughout the small intestine. More complex tissues with multiple cell types were also generated by using this method sequentially with different cellular inks. In one particularly remarkable demonstration, distinct stem cells isolated from the intestine and stomach were combined to form a hybrid stomach and intestine organoid, mimicking the gastrointestinal junction with organ-specific features such as a smooth gastric zone and crypt-covered intestinal zone. Using a strategy akin to that published in a recent study4, the authors also deposited distinct cell types sequentially into the same matrix by extruding droplets of stromal cells at specific locations. The presence of stromal cells led to an increase in the lumen diameter, permitting the perfusion of the intestinal organoid.1
Gartner, Z.J., Hu, J.L. Guiding tissue-scale self-organization. Nat. Mater.20, 2–3 (2021). https://doi.org/10.1038/s41563-020-00885-1