Coordinated changes in uterine epithelial cells (UECs) and trophoblastic cells are critical in the early stages of blastocyst implantation. Maternal-foetal communication includes biochemical signalling within the uterine lumen contributing to a receptive environment. One mechanism of this communication is via UEC exocytosis. Proteins involved in exocytosis (including SNAP23, Syntaxin-2, VAMP and Munc 18) are present in UECs during uterine receptivity, however the regulation of exocytosis is currently unknown.
The hormone secretin (Sec) is known to be released from decidual cells during early pregnancy and together with its receptor (SecR) has been shown to regulate epithelial cell exocytosis in other tissues. Sec and SecR has not been studied in UECs during uterine receptivity.
Immunofluorescence microscopy and western blotting identified a statistically significant increase in apical SecR staining in uterine epithelial cells at the time of uterine receptivity in rats. Secretin was found in UECs and in trophoblastic cells of mouse blastocysts.
An increase in vesicles within the apical cytoplasm of UECs is one of the many changes seen at the time of uterine receptivity. This in conjunction with our other work demonstrating key exocytosis regulating proteins in UECs and uterine luminal fluid at the time of uterine receptivity suggests that exocytosis is occurring across the apical membrane. The presence of SecR in apical UECs and secretin in both UECs and trophoblast cells at the time of uterine receptivity could indicate that exocytosis is partially regulated by secretin and its receptor. Hence secretin-regulated exocytosis may play a role in regulation of uterine luminal fluid composition and contribute to uterine receptivity and successful implantation.