Scientists at the Weizmann Institute of Science have developed a novel method to create human embryo models from stem cells. These models closely mimic the development and structure of real human embryos, and may offer new insights into the mysteries of early human life.
Stem Cells: The Building Blocks of Life
Stem cells are cells that have the potential to differentiate into many different types of cells, such as skin, muscle, nerve, or blood cells. They are essential for the development and maintenance of the human body. However, not all stem cells are equal. Some are more versatile than others, depending on their stage and source.
One of the most versatile types of stem cells are called pluripotent stem cells, which can become almost any cell type in the body. These stem cells can be derived from adult cells, such as skin cells, that have been reprogrammed to return to a more primitive state. They can also be obtained from human embryonic stem cell lines that have been cultured in the lab for years.
However, even pluripotent stem cells have some limitations. They cannot become certain cell types that are essential for the formation of the embryo, such as those that form the placenta and the chorionic sac. These cell types originate from an earlier stage of the embryo, called the naïve state, which occurs around day 7 of its existence, when it implants itself in the womb.
A New Method to Generate Naïve Stem Cells
To overcome this limitation, a research team headed by Prof. Jacob Hanna at the Weizmann Institute of Science developed a new method to generate naïve stem cells from pluripotent stem cells. They used a technique called reprogramming, which involves exposing the cells to certain factors that induce them to revert to an earlier state.
The researchers divided the pluripotent stem cells into three groups. The first group was left as it was, to develop into the embryo. The second group was reprogrammed to become naïve stem cells, which were then coaxed to become trophoblasts, the cells that form the placenta. The third group was also reprogrammed to become naïve stem cells, which were then induced to become extraembryonic endoderm, the cells that form the chorionic sac.
The researchers then combined these three groups of cells in a special culture medium that mimicked the conditions of the womb. They observed that the cells spontaneously organized themselves into structures that resembled human embryos at around day 11-12 of their development. These structures had all the hallmarks of a post-implantation embryo, such as a bilaminar disc, a primitive streak, and a pro-amniotic cavity.
A Window into the Black Box of Human Development
The human embryo models created by Hanna’s team are unprecedented in their accuracy and complexity. They are the first to contain all the cell types and structures that are essential for the embryo’s development, and to show dynamic progression to the next developmental stage.
These models may provide an invaluable opportunity to study the early stages of human development, which are largely unknown due to ethical and technical challenges. During these stages, the clump of cells that implants itself in the womb becomes, within three to four weeks, a well-structured embryo that already contains all the body organs.
“The drama is in the first month, the remaining eight months of pregnancy are mainly lots of growth,” Hanna says. “But that first month is still largely a black box. Our stem cell–derived human embryo model offers an ethical and accessible way of peering into this box. It closely mimics the development of a real human embryo, particularly the emergence of its exquisitely fine architecture.”
The models may also have implications for improving fertility treatments, preventing birth defects, and understanding diseases that originate from defects in early embryonic development.
The research was published in Nature on September 7th, 2023.