Equine endometrial organoids preserve tissue structure and cycle-stage transcriptional identity†

The endometrium is a hormonally responsive tissue that undergoes cyclic remodeling. Although endometrial organoids have been established in several species, detailed characterization remains limited. Here, we assessed the structural and molecular fidelity of equine endometrial organoids across reproductive cycle stages and during extended culture. Organoids were generated from biopsies collected during estrus and diestrus and analyzed using histology, immunohistochemistry, electron microscopy, and bulk- and single-cell RNA sequencing. Organoids formed polarized cystic structures composed of columnar cells with microvilli, tight junctions, and secretory vesicles. Compared to native tissue, organoids showed higher expression of genes involved in proliferation and metabolism, and lower expression of genes related to differentiation, angiogenesis, and immune responses. Single-cell analysis identified diverse epithelial and stromal populations in both tissue and organoids. While most cell types were preserved, organoids were enriched in progenitor-like cells but underrepresented in ciliated, proliferative glandular, endothelial, smooth muscle, and antigen-presenting cells. Cycle-specific differences were observed in morphology, hormone receptor expression, and gene expression profiles. Estrus-derived organoids showed increased proliferation and metabolic activity. Although organoids retained transcriptional signatures reflective of the hormonal cycle stage of the source tissue, these signatures faded with prolonged culture, despite overall transcriptomic stability. In summary, equine endometrial organoids replicate key features of the native tissue, retain reproductive cycle characteristics, and maintain transcriptomic stability over time. Endometrial organoids provide a robust platform to study the equine endometrium, though native tissue differences should be considered in the experimental design and data interpretation.