Galectin-9 has been recently considered as a novel marker for the mid- and late-secretory phases of human endometrium and decidua. The aim of this study was to investigate the subcellular distribution of galectin-9 in the endometrial epithelium, especially during the frame of the implantation window. Endometrial biopsies in the proliferative, early, and mid-secretory phases from women with regular menstrual cycle were studied using several approaches, including scanning electron microscopy, immunostaining for light and transmission electron microscopies (TEM), immunoblotting, and statistical analysis of the area-related numerical densities of galectin-9-bound nanogold. Images of immunostaining for light microscopy demonstrated a strong expression of galectin-9 at the luminal and glandular endometrial epithelium in the mid-secretory phase compared to the proliferative and early secretory phases. Data of immunoblotting revealed a molecular weight of 36 kDa band with high intensity in the mid-secretory samples. Photomicrographs of immunogold staining for TEM illustrated the localization of galectin-9 in the uterodomes. Statistical and morphometric analysis showed a significantly higher area-related numerical density of galectin-9-bound nano-golds in the uterodomes compared to that of the uterodome-free areas of the luminal epithelium (p<0.001). This is the first demonstration of the molecular localization of galectin-9 in the bulbous ultrastructure of the human endometrial epithelium, called uterodomes. High expression of galectin-9 at uterodomes during the frame of implantation window suggests that galectin-9 can be considered as a marker of endometrial receptivity and should play an important role during the initial events of human embryo implantation.
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