Advances in cancer modeling: fluidic systems for increasing representativeness of large 3D multicellular spheroids

No. 6 | Vol. 65 | 312 |2018 10.2144/btn-2018-0153 The representativeness of a cellular model is fundamental in preclinical cancer studies, being defined as the degree of likelihood between the in vitro model and the in vivo mimicked tumor situation. Although being the gold standard in cell biology for more than a half century, 2D cell cultures poorly represent the complex three-dimensionality of in vivo conditions. Size, heterogeneity and perfusion are three key aspects characterizing the behavior of the tumor and driving its progression. In this regard, it has been widely proven that 3D multicellular models enable a more reliable assessment of anti-cancer drugs and radiotherapy treatments, although a recent article emphasized that today there are no well-established approaches to obtain relevant biological data from these models [1]. Moreover, there are not even shared definitions of multicellular aggregates, spheroids, microtissues and organoids. Therefore, this work provides an opening rationale for the different terms today used for 3D cell cultures and discusses the benefits of using large-sized spheroids as 3D preclinical culture models. Finally, this work outlines the microfluidic systems that are now ready for 3D culture cultivation and manipulation in dynamic controlled conditions, this representing a further step towards more representative 3D in vitro cancer models. Based on the structural and consequently functional complexity, multicellular aggregates can be considered as the simplest 3D cell cultures, as they are characterized by a 3D cell–cell aggregation free from a defined structural obligation. The commonly used term spheroids should be referred to those multicellular aggregates producing their own extracellular matrix and having a nearly spherical shape [2]. When spheroids comprise more than one cell types that together accomplish a specific function, referring to them as microtissues is more appropriate. However, the use of the terms spheroids and microtissues is still often overlapping. It is worth noting that if the spheroids are composed of more than one cell type not accomplishing a specific function together, they can be better identified as co-culture spheroids. Finally, self-renewing multicellular aggregates that self-organize into ex vivo mini-organs are named organoids [3]. Whilst complex cell culture models enable representation of many in vivo parameters and interactions, spheroids are a simple yet effective model to represent in vivo tumor conditions while retaining handiness in their generation and use. Furthermore, it is known that when their diameter exceeds the threshold of approximately 500 μm, spheroids become more heterogeneous, since they develop a necrotic core surrounded by a viable rim of quiescent cells and an outer layer of proliferating cells. This structure mimics the one of vascular tumor nodules and microregions of solid tumors, where cells next to capillaries are actively proliferating and distant inner cells stay quiescent or die due to mass transport limitations. Accordingly, stratified large-sized spheroids are a more representative model of in vivo intra-tumor heterogeneity, an aspect widely reported to impact on many cancer-specific traits such as clonogenicity, invasive potential and response to drugs [2]. A further improvement in the representativeness of a cellular model can be achieved by taking into account the tumor microenvironment, which has been extensively proven to play a key role in several steps of tumor progression, from initial neoplastic transformation to an eventual metastatic invasion. Traditional culture methods are static and thus often limiting for reshaping Advances in cancer modeling: fluidic systems for increasing representativeness of large 3D multicellular spheroids