Development and characterization of functional sheep endometrial luminal epithelial organoids

The luminal epithelium (LE) serves as the primary interface for embryo-maternal communication during implantation. In ruminants such as sheep, the LE orchestrates critical reproductive events, including noninvasive adhesion, conceptus elongation, and pregnancy recognition. However, existing models face significant limitations: conventional two-dimensional (2D) cultures fail to recapitulate endometrial complexity, while current organoid systems are predominantly derived from glandular epithelium. Herein, we report the establishment of a robust three-dimensional (3D) culture system for generating ovine endometrial luminal epithelial organoids using an optimized expansion medium (ExM) supplemented with CHIR99021, Y-27632, SB202190, and the EphrinA1 ligand. The resulting organoids exhibited characteristic luminal epithelium features, including polarized architecture, mucin secretion, and high expression of lineage markers (keratin 18 (KRT-18), trophoblast cell surface antigen 2 (TROP2), and epithelial cell adhesion molecule (EpCAM)). Moreover, we developed an apical-out polarity model that faithfully recapitulates the in vivo luminal orientation. Transcriptomic analysis confirmed the close resemblance between organoids and native luminal epithelium, as well as maintained hormonal responsiveness. Functional validation demonstrated the organoids' capacity to promote blastocyst expansion and trophoblast proliferation in co-culture systems, as well as substrate-specific adhesion that was enhanced by β-estradiol (E2) + medroxyprogesterone acetate (MPA) treatment. Notably, we identified erythropoietin-producing hepatocellular receptor A (EphA) signaling as a novel regulator of stemness properties during organoid development. The successful generation of functional luminal epithelial organoids thus provides a physiologically relevant model that mirrors native endometrial structure, hormone responses, and embryo interactions. As such, it offers a valuable platform for investigating the mechanisms of endometrial receptivity and for screening potential therapeutics aimed at improving reproductive efficiency in ruminants.