Modeling human trophoblast, the placental epithelium at the maternal fetal interface

Abstract

Appropriate human trophoblast lineage specification and differentiation is crucial for the establishment of normal placentation and maintenance of pregnancy. However, due to the lack of proper modeling systems, the molecular mechanisms of these processes are still largely unknown. Much of the early studies in this area have been based on animal models and tumor-derived trophoblast cell lines, both of which are suboptimal for modeling this unique human organ. Recent advances in regenerative and stem cell biology methods have led to development of novel in vitro model systems for studying human trophoblast. These include derivation of human embryonic and induced pluripotent stem cells and establishment of methods for the differentiation of these cells into trophoblast, as well as the more recent derivation of human trophoblast stem cells. In addition, advances in culture conditions, from traditional two-dimensional monolayer culture to 3D culturing systems, have led to development of trophoblast organoid and placenta-on-a-chip model, enabling us to study human trophoblast function in context of more physiologically accurate environment. In this review, we will discuss these various model systems, with a focus on human trophoblast, and their ability to help elucidate the key mechanisms underlying placental development and function. This review focuses on model systems of human trophoblast differentiation, including advantages and limitations of stem cell-based culture, trophoblast organoid, and organ-on-a-chip methods and their applications in understanding placental development and disease.

Figure 1.. A)

An illustration of a blastocyst-stage human embryo. Both human embryonic stem cells (ESC) and trophoblast stem cells (TSC) have now been established, derived from the inner cell mass (ICM) and trophectoderm (TE) of human embryos. B) An illustration of early gestation (first trimester) human placenta. While primary cytotrophoblast (CTB) derived from this tissue have been the gold standard in studies of human trophoblast differentiation for many years, recent studies have established protocols for derivation of trophoblast stem cells (TSC) and trophoblast organoids, which can self-replicate and be further differentiated into both syncytiotrophoblast and extravillous trophoblast. Utility of both embryo- and first trimester placenta-derived cells is limited by ethical issues as well as their unknown disease potential. C) An illustration of human placenta at delivery. Current reprogramming technologies enable us to generate induced pluripotent stem cells (iPSCs) starting with somatic cell types, including placental cells at delivery. This has been done mostly using mesenchymal stem cells (MSCs) derived from the umbilical cord, but can also potentially be done using cytotrophoblast (CTB) as starting material. iPSCs (as well as ESC) can be differentiated into trophoblast using various protocols. Application of TSC/organoid culture media to iPSC/ESC-derived trophoblast has the potential to generate TSC-like cells. At the same time, TSC-specific transcription/reprogramming factors could potentially be used to generate “induced trophoblast stem cells” (iTSC), directly from MSCs or CTBs. This model system is potentially powerful, because it will be applicable not only to normal pregnancies but also to pregnancies associated with placenta-based dysfunction. Note that the blue arrow indicates a method which has already been established and applied, the green arrow represents technology that is available but has yet to be applied, and the red arrow points to protocol(s) that have yet to be developed.