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1 The Rockefeller University Press $30.00 J. Cell Biol. https://doi.org/10.1083/jcb.201610056 How do we define organoids and 3D cultures? That functional differentiation is dependent on 3D architecture has become accepted recently. Many papers over the last 50 years have shown that cells cultured in 2D are not representative of the in vivo situation. Structurally, 2D cultures do not provide the conditions for the organization and cellular relationships observed in vivo. Moreover, cell signaling networks are altered in 2D versus 3D, and this probably explains why drug screening outcomes many times do not reproduce the in vivo setting (Wang et al., 1998; Weaver et al., 2002). It is encouraging to see the recognition of the importance of 3D cultures to model signaling, differentiation, and drug development. Many of the studies use elegant images and sophisticated animations that are a delight to see and hear about and clearly show the similarity between organoids and the tissues and organs from which the cells were derived in vivo. We applaud the excitement and cheer the general enthusiasm that the work has deservedly generated. What is most exciting is that the combined effort is finally a critical mass and as a result has caught the attention of many new scientists who are emphasizing the importance of 3D culture by pointing out the relevance and the significance of this work to clinical research. However, the term “organoid” is being treated, or has come to imply, that this is a completely new field, separate from what several scientists from as early as the turn of the previous century have been engaged in for years, essentially in isolation, introducing the term 3D cultures, beginning the field of microenvironment, and pointing out the significance of tissue architecture. The first use of the words “three-dimensional culture models,” we believe, started with the assays developed by Barcellos-Hoff et al. (1989) and Petersen et al. (1992), although floating collagen gels were described in the 1970s and were certainly 3D (see Fig. 2). Before 2005, the word organoid was an extension of 3D cultures. Typically, it referred to small tissue fragments taken from organs, mostly epithelial tissues, separated from stroma by mechanical and enzymatic digestion and grown in different types of 3D gels to produce an organ-like structure. As an example, see Simian et al. (2001), in which rodent mammary fragments were grown in collagen gels to produce a branching structure resembling branching in the mammary gland of virgin mice, or Fata et al. (2007), in which rodent mammary fragments were grown in laminin-rich gels giving rise to alveogenesis. However, in the last decade, the meaning of “organoid” has lost precision and has come to cover a series of cell culture techniques that are not necessarily a single technique. Below are examples of definitions of organoids taken from some recent papers in appropriate journals for the field. We come across the following definitions: (1) “Various subfields use these terms either interchangeably or distinctly; for example, in the field of mammary gland biology, the term organoids refers to primary explants of epithelial ducts into 3D extracellular matrix (ECM) gels. Conversely, in studies of intestinal biology, organoids can refer to clonal derivatives of primary epithelial stem cells that are grown without mesenchyme or can refer to epithelial–mesenchymal co-cultures that are derived from embryonic stem cells or induced pluripotent stem cells” (Shamir and Ewald, 2014). (2) “Thus, we would like to define an organoid as containing several cell types that develop from stem cells or organ progenitors and self-organize through cell sorting and spatially restricted lineage commitment, similar to the process in vivo” (Lancaster and Knoblich, 2014). (3) “An organoid is now defined as a 3D structure grown from stem cells and consisting of organ-specific cell types that In the last ten years, there has been a dramatic surge in the number of publications where single or groups of cells are grown in substrata that have elements of basement membrane leading to the formation of tissue-like structures referred to as organoids. However, this field of research began many decades ago, when the pioneers of cell culture began to ask questions we still ask today: How does organogenesis occur? How do signals integrate to make such vastly different tissues and organs given that the sequence of the genome in our trillions of cells is identical? Here, we summarize how work over the past century generated the conceptual framework that has allowed us to make progress in the understanding of tissue-specific morphogenetic programs. The development of cell culture systems that provide accurate and physiologically relevant models are proving to be key in establishing appropriate platforms for the development of new therapeutic strategies. Organoids: A historical perspective of thinking in three dimensions