Intramembranous Absorption of Water from the Ovine Allantoic Cavity

Abstract

The ovine allantoic cavity may receive up to 500 ml/day of fetal urine in late gestation even though there is no anatomic outlet from the allantoic space. The present study was designed to test the hypothesis that the fluid which moves across the allantoic membrane is absorbed through the intramembranous pathway directly into the fetal circulation. Thirteen chronically catheterized fetal sheep were utilized in three groups: 1) 6 control experiments, 2) 9 experiments with injection of 500 ml of warmed distilled water into each horn of the allantoic cavity, and 3) 7 experiments with fetuses killed prior to the intraallantoic water injection. Allantoic osmolality was unchanged during control experiments and decreased following water injection from 280 mOsm/kg to 65 mOsm/kg (P < 0.0001). The injected water was rapidly absorbed into the fetal circulation resulting in a decrease in fetal plasma osmolality (7.0 ± 1.1 [SE] mOsm/kg, P < 0.0001). Amniotic fluid osmolality decreased (6.7 ± 1.3 mOsm/kg, P < 0.001) but this occurred later (P < 0.05) than the decrease in fetal plasma osmolality. There was no change in maternal plasma osmolality when comparing the 3 groups. These results suggest that direct absorption through the intramembranous pathway into the fetal circulation may be the primary means by which fluid is removed from the ovine allantoic cavity. Thus, the intramembranous pathway appears to play an important role in the regulation of allantoic fluid volume and composition.

Key Words:

• Allantoic fluid
• Amniotic fluid
• Intramembranous pathway
• Amnion
• Chorion
• Fetus