Embryonic innovation: demystifying human placenta formation using stem cells

The discovery of a gene essential to early embryonic development sheds light on the preliminary stages of human placenta formation.

A team led by scientists from the University of California San Diego (CA, USA) has used human pluripotent stem cells to model the early formation of the placenta, potentially furthering our understanding of pregnancy loss, reproductive disorders and placental disease.

The trophectoderm is the outermost layer of cells in the blastocyst, the developmental stage that occurs 5–6 days after fertilization. It is the first tissue to differentiate during mammalian development and eventually forms the trophoblast compartment of the placenta.

Fundamental to processes including embryo cavitation, implantation and pregnancy initiation, abnormalities in the establishment and differentiation of the trophectoderm during the first phases of placenta formation have been linked to pregnancy loss and placental-associated disorders. However, the mechanisms that underpin this formative step remain poorly defined.

While animal models have been used to identify key signaling pathways involved in trophectoderm specification, differences during early development limit the translation of these findings to our own species, necessitating the need for human-specific models. Thus far, these have been hindered by ethical considerations and technical complications.

Cradle cultures: growing stem cell-derived developmental cell models in vitro

How are three stem cell-derived developmental cell models furthering our understanding of post-implantation human embryo development? And why have recent advancements in these human embryo-like models spurred ethical discussion and the need to refine our definition of ‘embryo’?

In the new study, the researchers used human pluripotent stem cells to create a model of trophectoderm formation that overcame these obstacles. They established a two-step protocol to first convert pluripotent stem cells into trophectoderm-like cells by treating them with the signaling protein BMP4, before steering them toward differentiating into mature trophoblasts or human trophoblast stem cells.

They further optimized the initial step by adding IWP2, an inhibitor of the WNT signaling pathway, to prevent mesoderm differentiation and obtain a more homogeneous population of trophectoderm-like cells.

Their model of trophectoderm induction was then used to investigate chromatin remodeling and transcriptional dynamics that shape the formation of the early placenta.

Using temporal activation data, collected via qPCR and immunofluorescence, the team discovered that a gene called VGLL1 was expressed downstream of key trophectoderm transcription factors during early placenta formation and was seemingly essential for the differentiation of trophoblasts by up-regulating markers of mature trophectoderm.

To confirm this, they knocked down VGLL1 by establishing human embryonic stem cell lines that express shRNA specific to the gene. Cells lacking VGLL1 failed to differentiate into bona fide trophoblast stem cells, suggesting that, although the gene isn’t necessary to initiate placenta formation, it is required to complete it.

Moreover, the researchers used ChIP-seq to demonstrate that VGLL1 directly regulates the chromatin modifier gene KDM6B, which may act as a potential mediator of VGLL1-dependent gene expression.

“We identify VGLL1 as a key regulator linking multiple signaling networks to gene expression and epigenetic control,” the researchers concluded. “Our findings reveal a species-specific mechanism of placental initiation with broad implications for understanding reproductive disorders, pregnancy loss, and advancing stem cell-based models to study and potentially treat human placental disease.”