Towards Three-Dimensional Dynamic Regulation and In Situ Characterization of Single Stem Cell Phenotype Using Microfluidics

Cell Culture in Microfluidic Droplets

Cell manipulation in droplets has emerged as one of the great successes of microfluidic technologies, with the development of single-cell screening. However, the droplet format has also served to go beyond single-cell studies, namely by considering the interactions between different cells or between cells and their physical or chemical environment. These studies pose specific challenges linked to the need for long-term culture of adherent cells or the diverse types of measurements associated with complex biological phenomena. Here we review the emergence of droplet microfluidic methods for culturing cells and studying their interactions. We begin by characterizing the quantitative aspects that determine the ability to encapsulate cells, transport molecules, and provide sufficient nutrients within the droplets. This is followed by an evaluation of the biological constraints such as the control of the biochemical environment and promoting the anchorage of adherent cells. This first part ends with a description of measurement methods that have been developed. The second part of the manuscript focuses on applications of these technologies for cancer studies, immunology, and stem cells while paying special attention to the biological relevance of the cellular assays and providing guidelines on improving this relevance.

Structural and Functional Mapping of Mesenchymal Bodies

The formation of spheroids with mesenchymal stem/stromal cells (MSCs), mesenchymal bodies (MBs), is usually performed using bioreactors or conventional well plates. While these methods promote the formation of a large number of spheroids, they provide limited control over their structure or over the regulation of their environment. It has therefore been hard to elucidate the mechanisms orchestrating the structural organization and the induction of the trophic functions of MBs until now. We have recently demonstrated an integrated droplet-based microfluidic platform for the high-density formation and culture of MBs, as well as for the quantitative characterization of the structural and functional organization of cells within them. The protocol starts with a suspension of a few hundred MSCs encapsulated within microfluidic droplets held in capillary traps. After droplet immobilization, MSCs start clustering and form densely packed spherical aggregates that display a tight size distribution. Quantitative imaging is used to provide a robust demonstration that human MSCs self-organize in a hierarchical manner, by taking advantage of the good fit between the microfluidic chip and conventional microscopy techniques. Moreover, the structural organization within the MBs is found to correlate with the induction of osteo-endocrine functions (i.e., COX-2 and VEGF-A expression). Therefore, the present platform provides a unique method to link the structural organization in MBs to their functional properties. Graphic abstract: Droplet microfluidic platform for integrated formation, culture, and characterization of mesenchymal bodies (MBs). The device is equipped with a droplet production area (flow focusing) and a culture chamber that enables the culture of 270 MBs in parallel. A layer-by-layer analysis revealed a hierarchical developmental organization within MBs.

Downstream bioprocessing of human pluripotent stem cell‐derived therapeutics

With the advancement in lineage‐specific differentiation from human pluripotent stem cells (hPSCs), downstream cell separation has now become a critical step to produce hPSC‐derived products. Since differentiation procedures usually result in a heterogeneous cell population, cell separation needs to be performed either to enrich the desired cell population or remove the undesired cell population. This article summarizes recent advances in separation processes for hPSC‐derived cells, including the standard separation technologies, such as magnetic‐activated cell sorting, as well as the novel separation strategies, such as those based on adhesion strength and metabolic flux. Specifically, the downstream bioprocessing flow and the identification of surface markers for various cell lineages are discussed. While challenges remain for large‐scale downstream bioprocessing of hPSC‐derived cells, the rational quality‐by‐design approach should be implemented to enhance the understanding of the relationship between process and the product and to ensure the safety of the produced cells.

Regeneration of irradiation-damaged esophagus by local delivery of mesenchymal stem-cell spheroids encapsulated in a hyaluronic-acid-based hydrogel

Radiation therapy (RT) is a typical treatment for head and neck cancers. Generally, prolonged irradiation of the esophagus causes esophageal fibrosis due to increased reactive oxygen species and proinflammatory cytokines. This study was designed to determine whether catechol-functionalized hyaluronic acid (HA-CA) hydrogel-encapsulated human mesenchymal stem-cell spheroids (MSC-SPs) could ameliorate damage to the esophagus in a mouse model of radiation-induced esophageal fibrosis. MSC-SPs were cultured in concave microwells 600 μm in diameter at a cell density of 1 × 10⁶ cells per mL. Most cells formed spheroids with a 100-300 μm size distribution in concave microwells. MSC-SPs were well maintained in the HA gel, and live-dead staining confirmed that most cells survived. The HA gel containing the MSC-SPs was then injected into the damaged esophageal layer. Inflammatory signs or adverse tissue reactions were not observed after esophageal injection of HA-gel-encapsulated MSC-SPs. Based on Masson's trichrome staining at 4 and 12 weeks postinjection, the inner esophageal layer (IEL) was significantly thinner in the MSC-SP + HA gel group compared to those in the other experimental groups. While the saline and HA gel treatments made the esophageal muscles loose and thick, the MSC-SP + HA gel group showed bundles of tightly packed esophageal muscles, as assayed by desmin immunostaining. qPCR analysis showed that epithelial genes tended to increase over time in the MSC-SP + HA gel group, and the expression of most fibrosis-related genes decreased. This study proposes the potential of using HA-CA-hydrogel-encapsulated MSC-SPs as a promising therapy against radiation-induced esophageal fibrosis.