LONDON – After the human brain organoid, here comes the gastruloid – a 3D organized model of key elements of the human embryo at around 18 to 21 days old.
The model will enable researchers to work around international legal restrictions that prevent human embryos being cultured in the laboratory after 14 days, to study the transformation of the blastula from a simple, spherical ball of cells into a gastrula containing the three germ layers that give rise to all the body’s major systems.
The gastruloids are formed from human embryonic stems cells in which WNT, a signaling pathway that is essential for body axis specification in early embryos, is activated. After activation, 400 cells per plate are seeded into well-plates, where they form compact spherical aggregates.
The aggregates then progressively break symmetry, forming elongated structures with three germ layers arranged spatiotemporally. Transcriptional analyses of those structures tracked expression of 1,023 genes, finding representatives of all three germ layers, and showing gastruloids reflect key elements of the plan of a human body.
Gastruloids model “part of the blueprint of a human” making it possible “to witness the developmental processes that until now have been hidden from view and from study,” said Alfonso Martinez-Arias of the Department of Genetics at Cambridge University, who led the research, published in Nature on June 11, 2020.
Gastrulation has been referred to as “the most important stage of our life,” but up to now, it has not been possible to study it in humans, said Joyce Harper, head of the Reproductive Science and Society group at University College London, commenting on the research. “This work will allow many key studies to be done so we can learn about early human development and when it goes wrong,” she said.
Magdalena Zernicka-Goetz, professor of mammalian development and stem cell biology at Cambridge University (who was not involved in the research), agreed, saying, “This breakthrough gives us direct access to the study of processes that go awry in a huge range of human diseases when the embryo is just 3 weeks old.”
The advance of culturing human gastruloids builds on earlier research by Martinez-Arias in recreating the early stages of mouse embryogenesis. He has shown that early patterns of gene expression in developing mouse gastruloids show very large similarities, over space and time, with those seen in mouse embryos.
For example, HOX genes are expressed at defined points along the head to tail axis of gastruloids in the same order as seen in mouse embryos.
Comparing mouse and human gastruloid gene expression profiles in this latest paper, Martinez-Arias observed a high degree of conservation between the two. But there also were differences, suggesting species-specific regulation might be occurring in the two models.
Jeremy Green, professor of developmental biology at King’s College London, said that moving the gastruloid research from mouse to human embryology is significant. “It highlights the amazing power of self-organization of cells and tissues, given the right conditions,” he said. “We are beginning to get a pretty good handle on how to set the self-organization process in motion.”
However, in common with other researchers who have cultivated 3D organoids, ranging from kidney to liver to brain models, Martinez-Arias describes a trial and error process of establishing the correct inputs needed to prompt different human embryonic stem cell lines to form gastruloids.
The work is subject to a patent application filed by Cambridge University’s technology transfer office, Cambridge Enterprise.
Green questioned the current utility of the human gastruloids, saying they represent a tool for understanding basic biology but “are quite a long way away” from being useful for things like drug screening, which requires scalability and reproducibility.
No doubt such protocols will emerge. For now, gastruloids can be seen as a step forward in the in vitro modeling of gastrulation and understanding of the program of gene expression driving it. Transcriptome analysis of human gastruloids at 72 hours old displays a clear signature of events that give rise to muscle, bone and cartilage.
Comparing the gastruloids to the Carnegie embryo collection in the National Museum of Health and Medicine in Washington, Martinez-Aria suggests that at 72 hours old, they might serve as models for some of the features of CS8 or early CS9 human development, 18 to 21 days post fertilization.
With the current protocol, the majority of human gastruloids retract after 72 hours and culturing them beyond that point will require further study.
Notably, the gastruloids do not develop any of the neural cell lineages that would go on to form the brain, nor any of the extra-embryonic cells needed for implantation in the uterus.
The researchers say that raises important questions about the self-organization of the human body plan, but also removes ethical considerations associated with prolonged culturing of human embryos.