Spheroids as powerul 3D-cell culture intermediates

Spheroids: What are they and what are their uses?

In an organ, a cell needs to be considered together with its whole environment. Each cell is surrounded by a myriad of other cells, either of the same types or not. This entire environment allows cell interactions and contacts in 3 dimensions and also creates physical constraints. The classical culture cell models are for now mainly in 2D, meaning that this environment is far from the 3D environment of a cell located in an organ (other cells, extracellular matrix, interstitium…). There is a need for a 3D model with the advantage of 2D cell line cultures (easy to do, reproducible, cheap) and approaching the 3-dimensional organization of an organ. The solution is called spheroids. This very interesting solution came out in the 1970s. Spheroids are 3D spherical organoids displaying an increase in cell density and biological functionality in comparison to classical monolayer cell culture. Spheroids are often considered as an intermediate to mimic the level of organization of a tissue/organ.

Fig 1: Spheroid characteristics. Spheroids show increased biological function and increased cell density in comparison to cell monolayer.

Spheroids and tumor studies

Solid tumors are growing in a 3-dimensional architecture and possess a heterogeneous vasculature. This results in a variety of exposure to O2, nutrients, chemical agents… More generally, there is a gradient of diffusion which considerably limits the tumor response to treatments. The generation of tumor spheroids is used as an intermediate model between 2D cell culture and tumor explants culture. Spheroids may contain only one single type of tumor cell or be more complex by containing multiple cell types. Cancer researchers may be brought to measure the impact of hypoxia, oxidative stress, cancer cell metabolism, cell migration and invasion, the tumor microenvironment signaling crosstalk, and may be involved in the discovery of new anti-cancer drugs. Microfluidic chips are, for now, probably the best devices to perform such cancer research.

Fig 2: Key features comparison of cell monolayer, spheroids and in vivo tumor. — Fig 2: Key features comparison of cell monolayer, spheroids, and in vivo tumor.

Spheroids and microfluidics for new Organ-on-a-chip devices

Organs-on-a-chip and more specifically, “tumors-on-a-chip” are revolutionary microengineered biomimetic models. It consists of manufacturing a functional unit of a tumor in a microfluidic device. The device allows the culture of either monocellular spheroids or the co-culture of tumor cells with other cell types in a relevant architecture that replicates the tumor microenvironment. The microfluidic device integrates a chamber allowing the attachment of the spheroids, the perfusion of the culture medium with or without a dissolved chemical.

In addition, Cherry Biotech implemented its cutting-edge microfluidic temperature controller to this spheroid-on-a-chip development. This is the “SpOC” project where the Cherry R&D team aims at mimicking the 3D-cellular microenvironment of an in vivo model using spheroids. The chip is composed of an inert plastic polymer, channels, and may integrate a porous membrane for advanced medium perfusion. The tumor spheroid grows in the chamber and the metastatic cells are carried away by the medium flow and might be detected afterward. Custom microfabrication of the chip and real-time data recording are some of the advantages of this technique. Real-time optical microscopy is a useful tool to follow the spheroid geometry and measure the impact of a chemical on it. Moreover, the “spheroïd-on-a-chip” model provides new avenues for genomic and drug screenings, in addition to the detection of circulating tumor cells (CTC).