Modeling human peri-implantation placental development and function†

Abstract

It is very difficult to gain a better understanding of the events in human pregnancy that occur during and just after implantation because such pregnancies are not yet clinically detectable. Animal models of human placentation are inadequate. In vitro models that utilize immortalized cell lines and cells derived from trophoblast cancers have multiple limitations. Primary cell and tissue cultures often have limited lifespans and cannot be obtained from the peri-implantation period. We present here two contemporary models of human peri-implantation placental development: extended blastocyst culture and stem-cell derived trophoblast culture. We discuss current research efforts that employ these models and how such models might be used in the future to study the "black box" stage of human pregnancy.

Keywords: extended blastocyst culture; human; implantation; placenta; pluripotent stem cells; trophoblast stem cells (TSC).

Figure 1

Carnegie images depicting stage 4, 5a, and 5c human embryos. These histological sections from the original Carnegie collection represent approximately days 6–12 postfertilization. At stage 4 the polar trophectoderm attaches and adheres to the decidualized endometrium. By stage 5, the CTB has given rise to a multinucleated primitive syncytium that has begun to invade and form lacunae. Carnegie stage images are from the Virtual Human Embryo Project at Louisiana State University with permission to reprint (http://virtualhumanembryo.lsuhsc.edu).

Figure 2

Extended human blastocyst culture. Human embryos grown in extended embryo culture at day 10 demonstrating the syncytiotrophoblast (STB) marker CGB (red, left panel), and day 12 demonstrating the MTB marker HLA-G (red, right panel). Scale bar, 150 μm.

Figure 3

Enriched pathways in cells from the peri-implantation stage placenta. A) Gene ontology (GO) enrichment analysis reveals a transition of pathways enriched in CTB cells collected at different time points during development. B) CTB cells go through a transitory process of differentiation, remaining in the cell cycle while also displaying features of STB and MTB cells before leaving the cell cycle to become terminally differentiated to the STB and MTB lineages. This figure was adapted from ref. with permission.

Figure 4

Modeling the human placenta in vitro using stem cells. Various cell type conversions between primary cells, pluripotent stem cells, and trophoblast stem cells. Reference numbers for original studies are shown next to the arrows. Human ESC and iPSC are derived from the blastocyst ICM and various somatic cells, respectively. By exposing undifferentiated ESC and iPSC to BMP4 and signaling inhibitors, the cells differentiate into a mixture of trophoblast subtypes including extravillous trophoblast and multinucleated syncytiotrophoblast [66]. Human trophoblast stem cells (TSC) can be derived from trophoblast cells of both blastocysts and first trimester placental villi. Distinct types of pluripotent stem cells (PSC), primed-, naïve-, expanded-, and BMP4 primed-PSC can be converted into TSC. Induced TSC are generated by exposing reprogrammed cells to TSC culture medium using otherwise standard iPSC generation methods. TSC generate extravillous trophoblast and syncytiotrophoblast with distinct differentiation conditions.