Organoid Models of Human Endometrial Development and Disease

The endometrium is a complex multicellular tissue lining the uterus that plays a crucial role in human reproduction. This multi-layered organ is composed of a functionalis layer, adjacent to the uterine lumen and a basalis layer, attached to the myometrium. These layers comprise several cell types including luminal and glandular epithelial cells, endometrial stromal cells, immune cells and vascular cells forming the spiral arterioles (Kobayashi and Behringer, 2003; Roy and Matzuk, 2011; Hart, 2016). This unique tissue undergoes remarkable dynamic remodeling orchestrated by menstrual repair, proliferation and differentiation processes, which are carefully controlled by female sex hormones during the menstrual cycle. The remodeling requires a delicate interplay of cellular and molecular events that enable the endometrium to be receptive for embryo implantation. During the proliferative (or follicular) phase, both the endometrial glands and stroma of the functionalis layer proliferate in response to rising estrogen levels coming from the ovarian follicular. Consequently, the thickness of the endometrium increases. Then, the functionalis layers maturate during the secretory (or luteal) phase with the concomitant presence of secretory and ciliated cells in response to progesterone. In the absence of conception, this tissue sheds before regenerating for a subsequent menstrual cycle (Kobayashi and Behringer, 2003; Roy and Matzuk, 2011). Defects in endometrium remodeling and function can lead to the development of various types of disorders that affect considerable numbers of women. These includes infertility, pregnancy disorders, endometriosis and endometrial cancers (Hart, 2016). For instance, endometriosis a condition in which endometrial tissues proliferate outside the uterus leading to pelvic pain, excessive bleeding and infertility affects between 10 and 15% of all women of reproductive age (Giudice and Kao, 2004; Vercellini et al., 2013). Endometrial cancer, the most common malignancy of the female genital tract, affects ~3% of women, with the highest rates registered in North America and Europe (Lortet-Tieulent et al., 2017; Urick and Bell, 2019). Even if progress has been made in the knowledge of the endometrium structure and function, little is known regarding the molecular and the cellular mechanisms involved in this dynamic remodeling in both physiological and pathological conditions. The main obstacle of such studies is the lack of accurate models. Many insights have been provided thanks to studies using mouse models. However, these models do not accurately recapitulate the specificities of human endometrial development and function. As a matter of fact, endometrium decidualization in mice and humans are quite different. Decidualization of the endometrium occurs in rodents exclusively when there is an incoming embryo or in response to mechanical injury, whereas in humans the endometrium undergoes decidualization in a cyclic manner, regardless of the presence of an embryo (Gellersen et al., 2007; Peterse et al., 2018). Therefore, findings obtained with these animals often cannot be directly translated to humans. Immortalized or carcinoma-derived cell lines, such as Ishikawa cells (derived from epithelial endometrial cells) or St-T1b cell (derived from stromal endometrial cells) are also available for in vitro investigations. While these cells are easily cultured for long periods (Table 1), there are general limitations of using such cell lines including their genetic background, the potential changes acquired following transformation and during their establishment in culture (Mannelli et al., 2015). Alternatively, primary endometrial cells can be isolated, but these are difficult to maintain and expand in long term culture. Most of these cells lose their phenotype and hormone responsiveness in culture (Mannelli et al., 2015). Moreover, two dimensional (2D) cell cultures do not faithfully mimic in vivo three dimensional (3D) structure and function. These limitations prompted researchers to find novel strategies to model human endometrium. To that end, 3D-culture methods, namely organoids hold promise as models to better mimic in vivo human endometrium. Table 1 Comparative advantages and drawbacks of the current in vitro endometrial models.