Early human development is an incredibly complex and well-orchestrated process. Researchers have been trying to understand the mechanisms involved in transitioning from a ball of undifferentiated cells with particular specialty into organized functional tissues. Understanding these mechanisms is fundamental to advancing human embryology and the study of human health. 

To this purpose, researchers have relied on animal embryos, primarily from mice, or discarded human embryos from in vitro fertilization (IVF) treatments. Yet, animal embryos do not fully represent human biology, and human embryos are scarce and using them brings on ethical concerns. To bypass these problems, stem cell biologists have developed human embryo models in vitro. 

The human blastocyst is the structure formed from the fertilized egg at around day 6 of development. The blastocyst divides into three structures: the trophoblast, the epiblast, and the hypoblast. All the cells that eventually form every tissue in the human body arise from the epiblast. (The trophoblast and the hypoblast will form the tissue outside an embryo that interacts with maternal tissue.) 

Researchers from the Living Systems Institute at the University of Exeter, UK, were able to mimic these first stages of human development by growing stem cells. The cells self-organized and specialized similarly to the three-structure blastocyst. These lab-grown models are called blastoids because of their resemblance to the human blastocyst in structure and gene expression profile. 

This was not the first time researchers have formed blastoids; two other teams previously established the first-ever human in vitro blastoids. However, their models were difficult to grow; since they reported only around 20 percent of blastoid formation. In contrast, researchers in this study developed a more reliable and efficient protocol, reaching about 80 percent of blastoid formation. 

More optimizations will be needed to fully recapitulate the behavior of human blastocysts. Nevertheless, reliably producing in vitro blastoids on demand provides an alternative for researchers to study early human embryogenesis. These models will allow to improve IVF treatments and to better understand infertility and developmental disorders.