Trophoblast Organoids: Capturing the Complexity of Early Placental Development In Vitro.

First trimester placental development comprises some of the most critical yet understudied events that impact fetal development. Improper placentation leads to a host of health issues that not only impact the fetal period but also influence offspring throughout their lives. Thus, a paradigm to study early placental development is necessary, and this has spurred on the pursuit of new in vitro model systems that recapitulate specific aspects of placentation. One of the most complex and translationally valid models to arise are organoids, three-dimensional structures comprising multiple differentiated cell types that originate from a common progenitor population. Trophoblasts are the progenitor cells of the placenta, serving as the proliferative base for placental development. Recent advances have enabled the derivation of organoids from primary tissue, yet access to first trimester human samples is ethically constrained; derivation from established trophoblast stem cell lines is an alternative source. Organoids have already proven useful in generating insights into molecular events that underlie trophoblast differentiation, with the identification of new cell subtypes that are primed to differentiate down different paths. In this review, (1) we recap early pregnancy development events, (2) provide an overview of the cellular complexity of the placenta, (3) discuss the generation of organoids from tissue versus cellular sources, (4) highlight the value of translational animal models, and (5) focus on the complexities of the molecular regulation of trophoblast organoid development, differentiation, and function.