Implantation of embryos in the uterus requires establishment of close contact between the blastocyst and the luminal epithelium. In many species, including humans, this is achieved by the absorption of uterine luminal fluid, resulting in uterine closure immobilising the blastocyst between the opposing uterine walls. To investigate the hypothesis that uterine glands play a significant role in luminal fluid absorption, we have studied the distribution of an extracellular marker (fluorescein-labelled dextran) within rat uterine tissue. Intact adult female Wistar rats weighing 200-250 g were pretreated for 3 days with either oestradiol (10 µg animal^-1 day^-1 by S.C. injection in arachis oil) or progesterone (5 mg animal^-1 day^-1 in arachis oil) (N = 4 rats per condition). A second series was undertaken to examine the effects of amiloride on progesterone-treated rats (N = 3). On the last day, the uteri immediately removed from humanely killed animals were gently inflated by syringe injection with Earle’s-Hepes (0.25 ml), isolated and opened along their anti-mesometrial border. Dissected full thickness mucosa with supporting myometrial layer intact was mounted as a 2-3 mm² sheet in a temperature-controlled perfusion chamber at 37 °C.

Laser scanning confocal microscopy showed that the endometrial glands change their fluid handling characteristics under different hormonal conditions. Under progesterone dominance, the glands showed an amiloride-sensitive dextran accumulation indicating sodium-dependent fluid absorption (n = 7; P < 0.001, ANOVAR vs. oestrogen-stimulated rats); this was absent in the oestrogen-dominated state (n = 4). The rate of fluid uptake in the progesterone-stimulated gland opening was estimated to be approximately 1 X 10^-4 cm s^-1, requiring a suction pressure of between 10 and 20 mmHg at the mucosal surface. This study provides the first direct evidence of fluid absorption by uterine glands. Such absorption may provide the mechanism for closure of the uterine lumen and immobilisation of the blastocyst necessary for implantation.