Localization of Mucin 1 in endometrial luminal epithelium and its expression in women with reproductive failure during implantation window

Pinopode and Mucin 1 (MUC1) have both been proposed as morphological and molecular markers of endometrial receptivity for implantation. However, their spatial relationship in luminal epithelium and its association with reproductive failure are still unclear. This was a prospective cohort study conducted at a university assisted reproductive unit including 9 receptive control women, 18 infertile women, and 22 women with recurrent implantation failure (RIF) and 22 women with recurrent miscarriage (RM). Endometrial tissues were obtained at 7 days after luteinizing hormone surge during implantation window. Luminal epithelium was examined by scanning electron microscopy (SEM). MUC1 localization in luminal epithelium was detected by scanning immunoelectron microscopy (SIM) and compared by modified double immunofluorescence (mIF) staining without antigen retrieval. SEM did not show any significant difference in percentage of secretory cells, any stage of pinopodes, and ciliated cells between control, infertility, RIF and RM groups. SIM identified MUC1 was mainly localized on surface of ciliated cells and at the bottom of cilia, not secretory cells and pinopodes, and its specific localization was validated by mIF staining. MUC1 expression in ciliated cells in control women was significantly higher than those women with reproductive failure, but there was no significant difference between RIF and RM. In conclusion, MUC1 is mainly expressed in ciliated cells, not secretory cells and pinopodes, of the endometrial luminal epithelium during implantation window. The specific expression of MUC1 in the ciliated cells in receptive control women is higher than that of women with reproductive failure during implantation window.