A new word has been added to the lexicon of human stem cell research – blastoids, aka “blastocyst-like structures.”
Unlike the familiar three-layered embryo that emerges during the third week of prenatal development, the earlier blastocyst resembles a fluid-filled soccer ball, with a smear of cells on the interior face destined to develop into the embryo. It has only three cell types, but those distinctions are the first inklings of differentiation.
Blastoids have the potential to provide insights into the critical crosstalk vital for development to continue, and may illuminate how toxins, viruses, and mutations can derail the journey, leading perhaps to a better understanding of infertility and early pregnancy loss. And although the researchers who introduce blastoids in two papers published today in Nature point out that the structures are not the precise equivalent of blastocysts and are not quite synthetic human embryos, ethical objections are almost certain to arise.
The two groups used different cell sources, but their blastoids are similar in shape, size, cell number, and representation of the three cell lineages present in blastocysts (inner cell mass or epiblast, trophectoderm, and hypoblast).
“I wouldn’t call it a synthetic embryo. These are clusters of cells generated from skin or NIH-registered human embryonic stem cell lines that are good models for the blastocyst during the short period of pre-implantation,” said Jun Wu, of the Hamon Center for Regenerative Science and Medicine at the University of Texas Southwestern Medical Center and corresponding author of one Nature report. They used human pluripotent stem cells (PSCs) derived from WIBR3 human embryonic stem cells (hESCs) from the NIH registry to fashion their “human blastoids,” and also used induced pluripotent stem (iPS) cells.
Jose Polo, from Monash University and colleagues report in the second Nature paper that they created “iblastoids” from reprogrammed adult human skin fibroblasts. The “i” refers to “induced,” but unlike the usual fate of iPS cells, the researchers didn’t let the cells fully differentiate. Polo and Wu explained the process during a Nature press briefing.
Insights into early human development
“We started with skin cells, and before they become iPS cells, we use those intermediates to make an iblastoid. To understand more about how these intermediate cells work, we put them into a 3D interaction so they could talk to each other. We realized after 6 days that they were forming structures that looked like bowls, so we analyzed and characterized them to understand what they were, exactly. And we concluded that these structures were blastocyst-like, and because we make them out of reprogramming cells, we called them induced blastoids,” said Polo.
Either route to creating and nurturing the blastoids is painstaking. “The efficiency of generating blastoids from naïve hES or iPS cells varies between lines, genetic backgrounds, and batches, but in general it’s about 10-20%. Once we generate blastoids and culture them, the efficiency sharply drops compared to growing human embryos,” from IVF, explained Wu.
Both approaches enabled the researchers to visualize, and describe in molecular terms using single-cell RNA sequencing to capture gene expression, the cell-to-cell crosstalk of very early human development.
“The start of human development has been a black box for a century. We can test hypotheses without using human embryos. Because we can produce blastoids in large quantities, we can perform perturbations, screening them with chemicals and toxins that may affect human development during this period and look at genes and pathways,” said Wu.
For example, the group used the blastoids to show that specific forms of protein kinase C are critical in forming the cavity, which past work on mouse blastoids hadn’t revealed.
Polo mentioned potential uses of blastoids in dissecting the genetic controls that underlie cell identities. “So far, over the last several decades, we’ve worked with cells in isolation. Now we have the possibility to study these cells when they are talking with other cells, which is what happens in the natural environment. This will allow us to advance our knowledge on how cell identities and decisions are made and will lead to understanding how we can solve infertility and work out how toxins and viruses affect these early stages.”
Steering clear of the 14-day mark
A key part of the word “blastoid” is “oid,” which denotes a resemblance – think humanoid – but not identity.
Amander Clark, from UCLA and part of the iblastoid team, explained why blastoids aren’t the equivalent of blastocysts. “There’s no implantation and no uterine cells. These blastocyst-like structures attach to tissue culture plastic and are not transferred to a uterus or uterus-like structure to test implantation, and there’s no placenta because that requires cell types from the embryo and the mother. “ Nor does she consider blastoids to be the equivalent of blastocysts obtained from IVF “leftovers” donated for research.
Added Wu, “It’s important to note that a blastoid is not an embryo. It is a collection of cells that undergoes initial stages of embryogenesis. It is a good model and can bypass some problems of studying” early development.
Both teams were careful to let their creations proceed only to the equivalent of day 10 of human development, before the 14-day limit that the International Society for Stem Cell Research (ISSCR) recommends. That’s when the primitive streak forms, the first hint of a depression, the notochord, that will give rise to the nervous system. But Clark points out that 14 days is well before a true nervous system forms.
Polo explained that the goal was to culture the initial cells for the minimal time at which a battery of tests would reveal changes, both visible and gene expression ups and downs indicating primitive streak formation. “When we saw that the structures could attach and spread and form some cells of a placenta, we stopped. It wasn’t just a ball of cells, but functional. We felt that was enough to say, ok, these are blastocyst-like structures. Now we need to go back and see how the process of development is regulated.”
The Wu team called it quits before day 14 too. “We saw trophoblast (placenta) starting to mature, but we stopped the experiment after 4 days of additional culture, which was equivalent to about day 10 of human embryos. In addition, we also found from single-cell RNA sequencing some cells in blastoids that are not in human blastocysts,” which may have come from culture conditions, Wu said. The identity of those other cells remains unclear.
Bioethics implications of blastoids
Despite caveats from the researchers at the press briefing that the blastoids are NOT blastocysts, the headline of an accompanying News and Views, “First complete model of the human embryo,” is bound to raise objections, or at least questions. For what, exactly, is a “complete” model?
In the essay, Yi Zheng and Jianping Fu, from the University of Michigan, note that bioethical questions will emerge as this line of research goes forward, yielding structures that are even closer to actual pre-implantation embryos. “Thus, the continuous development of human embryo models, including human blastoids, calls for public conversations on the scientific significance of such research, as well as on the societal and ethical issues it raises,” they write.
Several bioethicists convened by the Science Media Centre Roundup agree with the researchers on the exciting possibilities of further research using blastoids, but caution that more work is needed to align each step with the in vivo situation, and determine the degree to which each step is recapitulated. Concluded Dr Peter Rugg-Gunn, Group Leader at the Babraham Institute, “if the structures can shed light on how the cell types in a blastocyst communicate with each other and also help identify the key factors that are required for lineage formation and development, then this will be a very informative cell model.”
Polo sums up the potential value of this new entrant into stem cell research.
“Blastoids will allow us to study the very early steps of human development without having to use blastocysts donated from IVF or animal models. They will open a big window into the initial weeks of human development.”