An organoid model to study the human placenta

In vitro generated trophoblast organoids closely resemble the human villous placenta.

Throughout a pregnancy, the developing fetus is supported by the placenta, an extraembryonic organ that maintains the connection to the maternal uterine wall and performs a complex set of functions from nutrient supply, gas exchange to hormone production and immune responses. Despite its essential role in human development and evidence that implicates placental dysfunction as a major cause of pregnancy disorders, we still know very little about the placenta. To date, research in this area has been hampered by a lack of accurate and reproducible models to study it.

Now, a Nature article from Ashley Moffett, Graham Burton and colleagues at the University of Cambridge, UK, shows that a long-term, genetically stable organoid model of the human trophoblast can be established from human placenta samples [1].

After the embryo implants into the uterus, the trophectoderm cells of the blastocyst expand and create the trophoblast, which develops into the placenta. In the first trimester of human pregnancy, the region in which maternal tissue interfaces with fetal tissue, the placental villus, consists of different trophoblast cell types with varied functions. Among them are villous cytotrophoblast (VCT) cells, which are highly proliferative at this stage, sit at the top of the villus and give rise to syncytiotrophoblast (SCT) and extravillous trophoblast (EVT) cells. SCT cells make up the outer epithelium of the fetal placenta and produce hormones, whereas EVT cells anchor the placenta to the maternal decidua, the uterine lining.

Building the organoid system, the researchers first isolated proliferating trophoblast cells from first trimester human placentas, seeded them into Matrigel drops and cultured them in a specifically developed trophoblast organoid medium. The organoids were genetically stable after many passages and selected cultures were still growing after a year, thus exceeding the timespan of a human pregnancy. The authors confirmed fetal and trophoblast identity of their organoids. Transcriptomic and genome-wide methylation analysis further showed high correlation between trophoblast organoids and natural first trimester placental villi.

As the cellular environment of the trophoblast is highly complex and functionally diverse, close recapitulation of the in vivo architecture is essential. The authors could show that primary morphological features were reproduced, the VCT cells were proliferating, SCT markers were present and, importantly, placental hormone secretion was detectable. They also verified the presence of four commonly accepted trophoblast-specific criteria: a set of protein markers, HLA class I profile, methylation of the ELF5 promoter, and expression of microRNAs from the chromosome 19 microRNA cluster. Finally, by tweaking the protocol, differentiation of EVT cells was also achieved and cells were capable of Matrigel invasion, mimicking their in vivo capacity to grow out of the placenta and migrate into the maternal decidua.

Interestingly, the researchers also found that by careful selection of media components they could derive both decidual glandular (maternal) and trophoblast (fetal) organoid cultures from the same placental sample of a pregnant donor. This finding further verifies the importance of robust validation of cell/organoid identity and origins in these experiments. The presence of maternal epithelial cells from the uterus in addition to fetal placenta tissue is common in donated samples and should be taken into account.

In summary, although there of course remain technical, anatomical and functional limitations, the methodology described in this study generates organoids that closely resemble the villous placenta in vivo and are an important step forward in placental modelling. They provide an important tool to study human placental development, the interactions between the trophoblast and the maternal environment as well as physiological and pathological changes that occur during pregnancy.

References

  1. Turco, M. Y. et al. Trophoblast organoids as a model for maternal–fetal interactions during human placentation. Nature https://doi.org/10.1038/s41586-018-0753-3 (2018).
  2. Image credit: Wikimedia Commons.


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