In vitro models of the human endometrium: evolution and application for women's health

Abstract

The endometrium is the inner lining of the uterus that undergoes complex regeneration and differentiation during the human menstrual cycle. The process of endometrial shedding, regeneration, and differentiation is driven by ovarian steroid hormones and prepares the endometrium and intrauterine environment for embryo implantation and pregnancy establishment. Endometrial glands and their secretions are essential for pregnancy establishment, and cross talk between the glandular epithelium and stromal cells appears vital for decidualization and placental development. Despite being crucial, the biology of the human endometrium during pregnancy establishment and most of pregnancy is incomplete, given the ethical and practical limitations of obtaining and studying endometrium from pregnant women. As such, in vitro models of the human endometrium are required to fill significant gaps in understanding endometrial biology. This review is focused on the evolution and development of in vitro three-dimensional models of the human endometrium and provides insight into the challenges and promises of those models to improve women's reproductive health.

Keywords: cell culture; endometrium; human; organoids.

Figure 1

The 3D cell culture models of the endometrium and pregnancy. Earlier models suspended endometrial stromal cells in collagen, Matrigel or fibrin-agarose gels, and overlaid epithelial cells, forming a lumenal-like epithelial monolayer. Endometrial epithelial organoids have been developed that can be passaged and cryopreserved, and they also exhibit hormonal responses similar to the in vivo environment. Scaffold-free models allow cells to self-organize, where compact endometrial stromal cells are surrounded by a layer of epithelial cells. Future 3D models will incorporate matrices, like those of the polyethylene glycol (PEG) hydrogel and other alternative matrices that can be manipulated to reflect the in vivo endometrial ECM and incorporate multiple endometrial cell types.

Figure 2

Human endometrial epithelial organoids (EEOs) characteristics. (A) Organoids form in Matrigel under WNT-activating conditions and express FOXA2, a marker of the GE. (Scale bars: left: 500 μm, right: 75 μm.). (B) Hormone-responsive genes in EEOs are determined by bulk RNA-seq analysis following treatment with E2 or E2 and MPA treatment [61]. (C) Single-cell RNA-seq analysis discovered that human EEOs are composed of many different cell types that respond to hormone treatment [61].