Bioprinting has been widely applied in tissue engineering and regenerative medicine due to its powerful ability to control large-scale depositions of cells and biocompatible materials. Although robust bioprinting modalities such as multimaterial, in situ, freeform and smart material bioprinting have recently been developed, these methods compromise the suitability of the cellular environment to improve printability and resolution. This tradeoff has precluded the high cellular density and permissiveness necessary to recreate the complexity of native tissue architecture and function.
In vivo, tissue formation relies heavily on a tightly regulated morphogenetic program that allows groups of cells to locally interact and self-organize. Iterative interactions between these locally developing tissue units guide successive cycles of cellular differentiation and patterning that establish biological complexity over a large scale. Because of their unique self-organization potential, stem cell-derived organoids are promising tissue mimetics that are unmatched by engineering methods in terms of reproducing local features of tissue architecture and cell-type composition. However, because organoids cannot be grown beyond the millimetre scale, they lack architectural features of native organs that would allow the emergence of higher-level functional characteristics. An important step towards in vitro tissue and organ development for regenerative medicine involves controlling the self-organization potential of mammalian cells at the macroscopic scale, but this remains challenging with existing technologies. A better control over tissue size and architecture could ultimately provide artificial organs to be used for drug screening or eventual organ replacements, lessening the burden on animal testing and removing the long wait times for transplants.
Here we introduce a three-dimensional (3D) bioprinting concept for guiding tissue morphogenesis across more physiologically relevant scales and directly within highly permissive extracellular matrices (ECMs) that facilitate effective multicellular self-organization. Our approach, termed bioprinting-assisted tissue emergence (BATE), uses stem cells and organoids as spontaneously self-organizing building blocks that can be spatially arranged to form interconnected and evolving cellular constructs . Hence, each cell or cellular aggregate that would normally develop into a relatively randomly shaped small organoid can be coerced to fuse and reorganize, following the geometry and constraints imposed by 3D printing. Using this versatile strategy, large-scale cellular constructs can be printed with key cell types—for example, parenchyma and its corresponding stroma, or different epithelial cells from the gastrointestinal tract—in an effort to reproduce the tissue–tissue interactions seen in native organ development or in homeostasis1.
Brassard, J.A., Nikolaev, M., Hübscher, T. et al. Recapitulating macro-scale tissue self-organization through organoid bioprinting. Nat. Mater.20, 22–29 (2021). https://doi.org/10.1038/s41563-020-00803-5