Embryoid Bodies Fail to Produce Structures with Any Similarity to the Embryo

Developmental biology Organoids from organoids to gastruloids Last year was hailed as ‘the year of the organoid’ as news of mini- brains, livers and intestines grown in the laboratory hit the headlines. Here, Dr David Turner explains the latest in vitro systems for modelling human development and disease, including embryonic stem cells, organoids and gastruloids ome of the most important these questions, traditionally using genetic shown to play an important role in experimentally) that is cheaper than Embryoid bodies fail patterning the early embryo, and it is this questions in biology relate experiments in embryos from animals development, are very difficult to assess; in vivo work, is ethically responsible, and well-grounded understanding that enables to how we are formed and such as the fruit fly, frog, chicken, zebrafish there is a great deal of expense involved in can be utilised to ask specific questions to produce structures us to guide ESCs towards specific tissue develop. For example, how is it and mouse. In terms of mammalian maintaining mouse lines (and generating regarding developmental processes and types and cell fates by applying signals or that a single fertilised egg is able, development, the mouse has proven new ones with specific traits), and it is differentiation, which can eventually be with any similarity modifying cells’ gene expression in culture. Sin a relatively short space of time, to exceptionally useful as a model system. difficult and technically challenging to targeted to understanding human For example, neural tissues can be to the embryo produce so many different cell types, Although many developmental processes experimentally manipulate the mouse development and disease. generated by applying retinoic acid or tissues and organs, and position them so and patterning events are conserved embryo at the early stages. inhibiting bone morphogenetic protein precisely within the body? Why does throughout diverse animal species, how There are also ethical considerations STEM CELLS, DISEASE (BMP) signalling, and beating human development occasionally go sure are we that what we see in the mouse to take into account when using mice AND DEVELOPMENT blastocysts by Evans and Kaufmann in cardiomyocytes can be enriched through wrong, producing birth defects and (and other model systems) recapitulates and their embryos (although a number of Embryonic stem cells (ESCs) offer an the 1980s). application of BMP, Activin A and vascular pathological conditions? human development? Mice and other initiatives are underway to dramatically alternative and, arguably, parallel route The pluripotent, self-renewing trait endothelial growth factor. The answers to these questions are vital model systems have a number of reduce or replace the numbers of mice to dissecting the development of embryos, of ESCs is essential during development It is often the case that by using a tissue in uncovering the mechanisms behind limitations. For example: it is not always and other animals involved in experiments delineating the processes and mechanisms and useful experimentally, as it prevents culture-based approach, new insights can disease states, eventually instructing us in possible to discount the effects of and refine their use, such as the utilised physiologically. ESCs are a premature exhaustion of the cells as they be gained regarding the signals involved in ways to treat them through regenerative redundancy between genes when assessing National Centre for the Replacement, self-renewing, pluripotent population are cycled into different cell fates in vivo, cell fate specification. Examples of this medicine, or manage their severity through the effect of mutant phenotypes (i.e. Refinement and Reduction of Animals in of cells derived from the early mouse and enables us to culture them indefinitely include our recent work on the role of Wnt/ better medicines and pharmaceuticals. biological systems are buffered, and Research, NC3Rs). embryo which can give rise to all the in the laboratory. β-Catenin and fibroblast growth factor We can use a number of experimental multiple genes may have similar functions); What is needed is a fully tractable tissues and organs of the embryo proper Careful experimentation has determined signalling in generating a population of tools and techniques to tease apart some of mechanical forces, which have been system (one that’s easy to control (they were first isolated from mouse many of the genes and signals involved in cells that serves as a pool for generating 14 / The Biologist / Vol 64 No 5 Vol 64 No 5 / The Biologist / 15 Developmental biology Organoids Embryoid bodies, Fig. 2. An array of fluorescent light a) Colorectal organoid different organ micrograph types have been generated using organoid culture techniques. the body axis,1 or how cells resolve binary Examples include fate decisions depending on the signalling colorectal and environment they are exposed to and the gastric organoids time in which they see these signals.2 (a and b), mini- brains (c) and ESCs are, when compared with in vivo b) Gastric kidney organoids studies in the mouse, exceptionally easy to organoid (d) as well as manipulate (genetically and chemically), forebrains, optic reduce the necessity for animal cups and liver experimentation and are orders of organoids (not magnitude less expensive than keeping mice c) Mini-brain pictured). in the laboratory. However, there are a few Images courtesy of disadvantages that have the potential to a) Cellesce compromise our complete reliance on ESCs b) Kyle McCracken with respect to in vivo work. c) Jürgen Knoblich at One of these involves the topographical the Institute of differences between a 2D layer of cells Molecular grown on a plate and the 3D nature of Biotechnology, a whole embryo – the 3D spatial Vienna d) Atsuhiro Taguchi organisation in the embryo between cells and Ryuichi and tissues is not replicated on a tissue Nishinakamura via culture plate. This limitation can to some Taguchi et al. Cell extent be solved by culturing cells as 3D d) Kidney organoid Stem Cell 2014. aggregates known as embryoid bodies (Fig. 1). To generate these bodies, cells are typically plated in little droplets on the inside lid of a tissue culture dish and (Fig. 1), they spontaneously differentiate and development, from human induced between the observed outcomes, and the form spherical structures on the same inverted so they form aggregates at the form self-organised, stratified tissues. pluripotency stem cells (iPSCs; cells frequency of these outcomes is low.8 scale as the early mouse embryo at the bottom of a droplet through gravity (Fig. However, within the last decade, there has from adult organisms that have been If organoids are to be used as model blastocyst stage. This is in contrast to 1). Cells grown in this way produce many as generating precursor populations for been something of a revolution in what can essentially reprogrammed to their systems for disease states or understanding most other organoid systems or embryoid cell types associated with the three further differentiation protocols. be accomplished in this field, with the rise embryonic state) and exposing them to development, as well as being used bodies where larger numbers of cells are embryonic germ layers over time, and can of organoids (Fig. 2). the Zika virus, they were able to determine clinically for drug screening or for typically used. even form spontaneously beating regions THE ORGANOID REVOLUTION Generally speaking, cellular material the neural cells the virus preferentially regenerative medicine, it is essential that Over time, and under appropriate as cardiac precursors are generated. Early studies using 3D structures, in parallel (such as mouse or human ESCs, tissue targeted and how microcephaly may be they are reliable, reproducible and signalling conditions, these spherical Unfortunately, embryoid bodies are highly with work on embryoid bodies, used fragments, primary cells and so on) that is established6 (Fig. 2). quantifiable. Advances need to be made so aggregates or, rather, embryonic disorganised3,4 and fail to produce artificial scaffolds and matrices to provide grown in 3D, and over time, can form As well as modelling the progression that organoid systems can be precisely organoids, begin to show something structures with any similarity to the embryo. support for growing tissues in what is structures very similar in the patterning of disease states, organoids may have controlled and manipulated with remarkable, undergoing many of the Their usefulness is therefore limited to known as mechanically supported air-liquid and, sometimes, function to their in vivo uses in regenerative medicine. Using minimal experimental variation. morphological and patterning events of broad questions on the signals required for interface culture. When skin or oesophageal counterpart5 following the same cellular material from patients through the early embryo. Furthermore, they differentiation of various cell types, as well primary keratinocytes are grown in this way developmental progression as in the generating iPSCs, a patient’s own cells can GASTRULOIDS – EMBRYONIC undertake a process similar to gastrulation embryo. Brain (mini-brains), optic cup and be guided towards the required cellular ORGANOIDS in the embryo, generating cell types that gut organoids have been produced, to lineage or fate, and the organoids that are With this in mind, our laboratory in correspond to the three germ layers.9,11,13 Monolayer Culture

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