Infographic: The Many Paths to Stem Cell-Based Embryo Models

For several years, researchers studied human embryonic stem cells (ESCs) to understand the unique features of these pluripotent cells, but on their own, they poorly resembled the complex structures that they derived from. While human embryos donated from in vitro fertilization clinics offered insights into the early development period, they are technically and ethically challenging to work with. In recent years, scientists developed a variety of stem cell-based models of embryos to overcome these challenges.

An illustration of five stem cell-based embryo models used to study embryonic development at different stages. Embryoid bodies, at the top of the page, are a cluster of embryonic stem cells (ESCs) that differentiate portions into ectoderm (orange), mesoderm (dark blue), and endoderm (pink). Micropattern colonies in the top right have ESCs that differentiate into organized 2D layers: Trophectoderm (green), endoderm (pink), mesoderm (dark blue), and ectoderm (orange). In the bottom right, blastoids are 3D models of the pre-implantation embryo, with trophectoderm (green), primitive endoderm (yellow) and epiblast (blue) cells. Post-implantation embryos (bottom left) see greater organization of the trophectoderm (green), epiblast (light blue), and primitive endoderm (yellow). Gastruloids, top left, mimic stages of gastrulation and early organogenesis. ESC clusters elongate in culture and begin to differentiate into the three germ layers, endoderm (pink), mesoderm (dark blue), and ectoderm (orange).

(1) Embryoid Bodies: In a first example of a human 3D embryo model, researchers grew ESCs on non-adhesive dishes in media without a differentiation inhibitor or growth factor to develop into embryoid bodies (EBs) that have all three embryonic germ layers: ectoderm, mesoderm, and endoderm. EBs aid in drug screening and developing some organoids.

(2) Micropattern Colonies: To study the spatial organization of human embryos, researchers treated ESCs with a growth factor and grew them in geometrically confined wells with extracellular matrix coating. These ESCs differentiated and assembled into radial layers of three embryonic germ tissues, or micropattern colonies.² Researchers use these spatially organized colonies to study gene expression and the role of cell density in embryo development.

(3) Blastoids: In 2021, four groups independently found that inhibiting growth and differentiation factors in naïve or primed pluripotent stem cells (PSCs) and culturing the cells on non-adherent hydrogel generated a blastocyst-like structure, a blastoid, with cells of the trophectoderm, the epiblast, and the primitive endoderm.^3-6 Blastoids resemble the pre-implantation period of embryo development.

(4) Post-Implantation Embryo Models: Scientists differentiated PSCs (naïve or primed stage) into lineages representing the primitive endoderm and trophectoderm and aggregated these with PSCs, modeling the epiblast, in defined ratios. These aggregates assembled into structures modeling the post-implantation embryo—an important milestone in developmental biology that could enable researchers to study this critical stage in the future.^7,8

(5) Gastruloids: To overcome limitations in studying embryonic development after 14 days, one group pre-treated ESCs with an activator of a proliferation pathway and then cultured the cells at a density of 300 cells in a low-adherence dish, prompting the ESCs to aggregate. Over 96 hours, they saw that these aggregates elongate, mimicking gastrulation, and cells begin to differentiate into those representing all three germ layers. These gastruloids allow scientists to study the earliest period of organ development that is prohibited in embryo research.⁹

Illustration of a timeline of important milestones in early embryogenesis currently replicated in stem-cell embryo models. Embryonic stem cells (ESCs, gray) differentiate and divide into multiple cells of either trophectoderm (green) or epiblast (light blue) lineages. As growth continues, some cells divide and mature into primitive endoderm tissue. After implantation, the tissues of the developing embryo further organize. At 14 days post fertilization, at the start of gastrulation, the cells of the epiblast begin to mature into the three germ layers, the endoderm (pink), mesoderm (dark blue), and ectoderm (orange).

After a sperm fertilizes an egg, the totipotent cell divides into a zygote. As division continues, some cells divide into the growing mass, distinguishing cells that form the embryo (epiblast) from those that form the placenta (trophectoderm). Further cell divisions lead to the formation of the primitive endoderm, which forms a blastocyst. This structure implants around one week after fertilization. As development continues, the epiblast invades inward toward the primitive endoderm. This begins the differentiation into the three germ layers that will construct all other bodily tissues: ectoderm, mesoderm, and endoderm.

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