Role of endometrial stromal and epithelial primary cells in matrix‑free spheroid assembly: Insight into the early initiation of endometriosis

The etiopathology of endometriosis is poorly understood, and retrograde menstruation continues to be the most accepted theory to explain its development. Endometrial stromal cells (eSCs) and endometrial epithelial cells (eECs) have been extensively studied to elucidate their role in the establishment of disease. The present study evaluated the role of eSCs and eECs in the formation of endometrial spheroids in a matrix-free model resembling regurgitated endometrial cells in the peritoneal environment. Primary eSCs and eECs were isolated from the eutopic endometrium of women with stage IV endometriosis (n=4) and cocultured at different cell type ratios (eSCs:eECs: group HS, 9:1; group SE, 1:1; and group HE, 1:9). The morphology of the spheroids and the participation of each cell type were evaluated by microscopy. The eSCs were the main part of the spheroids at all ratios assessed and were in the inner part of the spheres. The increase in the proportion of eSCs was directly proportional to the increase in spheroid size. On day 3, the HS group had a radius of 37 (SD [+ or -]9.90), the SE group had a radius of 24 [micro]m (SD [+ or -]4.17) and the HE group had a radius of 20 [micro]m (SD [+ or -]5.40). The spheroids were mainly composed of an eSCs core surrounded by an eECs lining. The mesenchymal stem cell marker, CD146, was expressed even by the cells on the surface of the spheroids, which should be epithelial. On the whole, the present study demonstrates that, both eECs and eSCs actively contribute to the formation of endometrial spheroids. Interactions between eSCs and eECs are required for the formation of stable spheroids, underscoring the putative role of eECs and eSCs crosstalk in cell survival in peritoneal fluid.