Special Collection: Emerging Concepts in 3D Microtissues
Over the past decade, the concept of tissue engineering has been extended to include technologies that use multi-cellular aggregates, not only to repair or replace tissue, but as a stand-alone in vitro device (e.g. “organ-on-a-chip”) with well-defined biological outputs. The advantage of such systems is that they allow for culture of one or more cell types in 3D, which may promote tissue function that is more mimetic of in vivo state, while allowing high throughput sample testing and a large degree of control of external culture factors that may lead to more reproducible results than that found in the more complex in vivo environment. While the means employed to achieve these devices vary greatly, in this special collection, we focus attention on formation and use of scaffold-free cellular aggregates (3D microtissues).
Despite the fact that spheroid culture has been utilized in cancer research for decades, microtissues have only recently been explored for applications in tissue engineering and have found wide use. In this collection, we highlight papers with such diverse applications as forming microtissues to explore developmental processes, understand more about pathology progression, and screen drugs and other toxins. Specifically, Mondrinos et al. have prepared an organoid culture system to provide key insights on what soluble and extracellular matrix cues affect alveolar development, Turner et al. used an adipocyte spheroid model as an intriguing first approximation of inflammation that occurs in metabolic disease, and Li et al. have formed a clever high-throughput co-culture of primary hepatocytes and fibroblasts for screening liver toxicity. For orthopaedics, applications of microtissues have been succinctly summarized in the review by Klumpers et al, also included in this collection. In addition to these screening-type functions, other papers in this collection address the therapeutic potential of microtissues as direct tissue-engineered replacements of small tissue defects, or as building blocks for scale-up for whole-organ printing (see below).
Regardless of the intended primary application, two main challenges must be overcome before microtissue-related technologies can be widely adopted:
- Recapitulating enough of the biological response as is required for the intended application. The specific biological functions to be mimicked will depend on the application and tissue chosen, but a recurring question is what source of cells (human vs. animal) is needed to produce data that can be predictive of human responses.
- Maintaining cell viability and function. For smaller constructs, this may be affected by things like co-culture with other cells, while for larger, printed organ technologies, these concerns may center on diffusion limitations and construct perfusion. However, in either case, maintenance of important biological outputs over time is of utmost concern.
The articles highlighted in this special collection directly address several of these potential roadblocks. In regard to cell source, the methods paper by Beauchamp et al. demonstrates a hanging-drop aggregation method is sufficient to promote cardiomyocyte differentiation from human induced-pluripotent stem cells, thus potentially providing a patient-specific source of cells for both screening and therapies. In terms of microtissue function, Hoffecker et al. found that treatment with a ROCK inhibitor could affect cell sorting within the micromass to achieve close contact of immunoprotective mesenchymal stem cells and pancreatic islets to promote long-term survival of islet grafts. On a larger scale, Blakely et al. have developed a unique bioreactor-type system to specifically place microtissues in particular locations for eventual formation of tissue- or organ-size constructs with high cell viability.
Given the burgeoning interest in the field of microtissues (PubMed citations for this topic have increased three-fold in 2015-2010 compared to 2005-2010), there are many stimulating ideas about how to both better use and better create cellular aggregates. While the concerns about cell source and maintenance of key biological functions remain for the moment, the creativity, as well as potential commercializability, associated with this research area makes it an exciting extension of the tissue engineering field.