Background and aims. Preeclampsia is a dangerous pregnancy condition without a cure, with a root cause in placental dysfunction. Although emerging evidence suggests the origins may arise earlier, during the implantation stage. Preliminary data shows that estrogen receptors and FKBPL are dysregulated in cases of preeclampsia, however, the interaction between these key regulators and their role in successful implantation have not yet been established. Endometrial organoids offer a new experimental model of critical processes occurring in early pregnancy.

Methods. Uterine horns from non-pregnant wildtype and Fkbpl+/- transgenic mice (n=3) were isolated and digested, with proestrus cycle confirmed by ensuring vaginal opening was open and red. Cells were plated and incubated to allow differential attachment (stromal cells attach, while epithelial cells remain floating). The epithelial cells were embedded in Matrigel for up to 12 days. Organoid formation and growth were captured by live cell imaging. Organoids were passaged to ensure expandability and exposed to Estradiol-17b (1nM) to show physiological hormone responsiveness. Organoids were harvested, embedded and sectioned for analysis of protein expression (FKBPL, ERa and ERb), alongside primary uterine tissue.

Results. Cells encapsulated within the Matrigel self-formed organoids within 2-3 days and demonstrated luminal epithelial structure. The presence of epithelial cells was confirmed by immunofluorescence labelling for markers E-cadherin and CD31-. While there was no significant impact on organoid number or size with Fkbpl knockdown, mean organoid number was significantly increased with red isolated uteri across both genotypes, suggesting a different stage of estrous cycle.

Conclusion. This study provides a new 3D in vitro platform recapitulating implantation in pregnancy and enables investigation into how FKBPL dysregulation and estrogen signalling may contribute to the genesis of preeclampsia.