Scientists have discovered that they can generate induced blastoids, or iBlastoids, that emulate natural human blastoids. Because iBlastoids are built of reprogrammed skin cells, they represent research material that may come to be more readily available than natural human blastoids, which are typically donated to researchers following in vitro fertilization.
iBlastoids, however, may be subjected to the sorts of ethical and legal constraints that have complicated the use of natural human blastoids. These constraints include the “14-day rule,” which prohibits in vitro experimentation on embryos beyond 14 days. In general, iBlastoids might be viewed as a step toward the engineering of human embryos.
Although iBlastoids may stir up ethical and legal disputes, they are meant to advance research of indisputable value—research that could reveal the causes of early miscarriage and infertility, research that could solve longstanding mysteries of early human development.
The potential research value of iBlastoids has been explored by the scientists who first generated them. These scientists, led by José M. Polo, PhD, a professor of anatomy and developmental biology at Monash University, were studying reprogrammed fibroblasts when they unexpectedly discovered that these cells, when aggregated, could form cellular structures presenting blastocyst-like cavitation.
“To determine the true nature of these structures, we performed a battery of molecular and functional assays,” the scientists reported. “These assays demonstrated that the structures were blastocyst-like structures that could model several aspects of blastocyst biology.”
Details of the scientists’ work appeared March 17 in Nature, in an article titled, “Modeling human blastocysts by reprogramming fibroblasts into iBlastoids.”
“[iBlastoids] model the overall architecture of blastocysts, presenting an inner cell mass-like structure, with epiblast- and primitive endoderm-like cells, a blastocoel-like cavity, and a trophectoderm-like outer layer of cells,” the article’s authors wrote. “Single-cell transcriptomics further confirmed the presence of epiblast-, primitive endoderm-, and trophectoderm-like cells. Moreover, iBlastoids can give rise to pluripotent and trophoblast stem cells and are capable of modeling, in vitro, several aspects of the early stage of implantation.”
In other words, iBlastoids model the overall genetics and architecture of human blastocysts, including an inner cell mass-like structure made up of epiblast-like cells, surrounded by an outer layer of trophectoderm-like cells and a cavity resembling the blastocoel.
Members of the Polo Lab succeeded in generating the iBlastoids using nuclear reprogramming, a technique that allowed them to change the cellular identity of human skin cells that—when placed in a 3D “jelly” scaffold known as an extracellular matrix—organized into blastocyst-like structures, or iBlastoids.
“iBlastoids will allow scientists to study the very early steps in human development, and some of the causes of infertility, congenital diseases, and the impact of toxins and viruses on early embryos—without the use of human blastocysts,” said Polo, the senior author of the Nature paper. “Importantly, [the research could be performed] at an unprecedented scale, accelerating our understanding and the development of new therapies.”
In human embryos, the epiblast goes on to develop into the embryo proper, while the trophectoderm becomes the placenta. However, iBlastoids are not completely identical to a blastocyst. “Early blastocysts are enclosed within the zone pellucida, a membrane derived from the egg that interacts with sperm during the fertilization process and later disappears,” Polo noted. “As iBlastoids are derived from adult fibroblasts, they do not possess a zona pellucida.”
The research is published as the International Society for Stem Cell Research is about to release guidelines for research on modeling human embryos in vitro following 2017 and 2018 reports on the generation of mouse “blastoids” in vitro, as well as advances in the generation of human stem cells that replicate aspects of early embryonic development.
It is not known whether the new guidelines will reference the study published in Nature, which is the first to produce an integrated stem cell model that closely mimics key fate and spatiotemporal decisions made by the early human embryo. However, in a paper published in Stem Cell Reports last February (2020), the Society stated, “[If] such models could be developed for the early human embryo, they would have great potential benefits for understanding early human development, for biomedical science, and for reducing the use of animals and human embryos in research. However, guidelines for the ethical conduct of this line of work are at present not well defined.”
Although there is no legislative precedent with respect to working with human integrated stem cell models of blastocysts, such as iBlastoids, all experiments had Monash University Human Ethics approval in compliance with Australian law and international guidelines referencing the “primitive streak rule,” which states that human blastocysts cannot be cultured beyond the development of the primitive streak, a transient structure that appears at day 14 in embryonic development.
Under these legislative recommendations, although iBlastoids are different from blastocysts, the Polo Lab did not culture their iBlastoids beyond day 11 in vitro. Also, the iBlastoids were monitored closely for the appearance of primitive streak–associated genes.
Infertility and miscarriage can be caused by early-stage human embryos failing to implant or failing to progress at the time of implantation. This takes place in the first two weeks after conception, when women do not even know they are pregnant. These “silent” miscarriages are likely to represent a significant proportion of the total number of miscarriages that occur and, according to Polo, the generation of iBlastoids provides a model system that will enable insights into this early stage of pregnancy.