Oral-nasal membranes are not the major route for fetal absorption of amniotic fluid arginine vasopressin

Cellular localization of vascular endothelial growth factor in ovine placenta and fetal membranes

To further understand the role of vascular endothelial growth factor (VEGF) in mediating angiogenesis and vascular permeability during development in the sheep placenta and fetal membranes, we examined the localization of VEGF mRNA and protein in placental, chorionic and amniotic tissues by in situ hybridization and immunohistochemistry in ovine fetuses at 62, 102 and 141 days gestation (term=150 days). In the placenta, VEGF mRNA expression and VEGF protein immunostaining were strong in cytotrophoblasts surrounding the villi. In addition, VEGF protein was localized in smooth muscle cells around fetal and maternal blood vessels and in the maternal epithelium. There was no apparent difference in placental VEGF mRNA or protein levels associated with advancing gestation. In the fetal membranes, VEGF mRNA was detected in the amniotic epithelium and the chorionic cytotrophoblastic cell layer. The intensity of the hybridization signals in both amnion and chorion appeared low at 62 days, moderate at 102 days and high at 141 days gestation. VEGF protein was detected in amniotic epithelium and chorionic cytotrophoblasts at all gestational ages studied. The increase in VEGF gene expression in fetal membranes as term approaches suggests that during fetal development VEGF may promote the vascularity and permeability of the microvessels which perfuse the fetal membranes, as well as permeability of the amniotic membrane itself. Thus VEGF may participate in the regulation of amniotic fluid volume.

Effect of esophageal ligation on amniotic fluid volume and urinary flow rate in fetal sheep

OBJECTIVE

Although the fetus normally swallows large volumes of amniotic fluid each day, it is unclear whether amniotic fluid volume increases after fetal esophageal obstruction or whether fetal urine production changes. Our objective was to determine the effects of fetal esophageal ligation on amniotic fluid volume and urinary flow rate over time.

STUDY DESIGN

Seven late-gestation fetal sheep underwent esophageal ligation, and 7 served as time control animals. The urachus was ligated to eliminate urine flow to the allantoic cavity. On days 1, 3, 5, 7, and 9 after surgery, we measured the composition of amniotic fluid, fetal urine, and fetal and maternal blood, as well as amniotic fluid volume and fetal urinary flow rate. A 3-factor analysis of variance was used for statistical analysis.

RESULTS

Amniotic fluid volume did not change with time in the control group, averaging 876 +/- 142 mL (mean +/- SEM), and it decreased in the esophageal ligation group (P =.020), averaging 309 +/- 75 mL on day 9. Fetal urinary flow rate was lower (P =.0063) in the esophageal ligation group (431 +/- 27 mL/d) than in the control group (631 +/- 54 mL/d). There were no differences in fetal or maternal blood compositions between the two groups. Amniotic fluid sodium and chloride increased in the ligated animals.

CONCLUSION

Polyhydramnios did not occur after esophageal ligation, even though the fetuses excreted approximately 4000 mL of urine over the 9-day study period. This suggests that intramembranous absorption is substantially increased. With only small changes in amniotic solute concentrations, intramembranous solute absorption must occur simultaneously with water, suggesting a near-zero reflection coefficient for solutes. We speculate that fetal urine, lung secretions, or both contain a factor that increases intramembranous permeability.

Amniotic fluid composition changes during urine drainage and tracheoesophageal occlusion in fetal sheep

OBJECTIVE

Recently an intramembranous pathway was reported in the ovine fetus as a route for the rapid exchange of water, ions, and molecules between the amniotic fluid and the fetal blood that perfuses the fetal surface of the placenta and the fetal membranes. Our study was designed to test the hypothesis that the amniotic fluid composition would gradually equilibrate with fetal plasma when the major flows to and from the amniotic compartment were eliminated.

STUDY DESIGN

Eleven near-term fetal sheep underwent ligation of the urachus to eliminate the allantoic fluid. An inflatable cuff was placed around the esophagus and trachea, and catheters were placed in the fetal urinary bladder, fetal circulation, and maternal circulation. At > or = 5 days after surgery the animals were subjected to either a control experiment or a continuous urine drainage plus tracheoesophageal occlusion for 8 hours.

RESULTS

During the urine drainage plus occlusion study, amniotic fluid osmolality (p < 0.0001), Na+ (p < 0.0001), K+ (p < 0.01) Cl- (p < 0.001), and lactate (p < 0.001) increased compared with the control experiment. These corresponded to 50% reductions in the gradients for osmolality and Na+ between fetal plasma and amniotic fluid; the K+ gradient increased, and the Cl- gradient reversed. The percentage increases in amniotic Na+, K+, Cl-, and lactate were all 10% at 8 hours.

CONCLUSION

These observations suggest that water is absorbed from the amniotic fluid through the intramembranous pathway into the fetal circulation at a rate of 1.25% of the total amniotic volume per hour or approximately 240 ml/day.

Vascularization of the ovine amnion and chorion: a morphometric characterization of the surface area of the intramembranous pathway

OBJECTIVE

The purpose of this study was to characterize the vascularization of the ovine amnion, allantois, and chorion.

STUDY DESIGN

A white silicone vascular casting material was infused into both umbilical arteries of nine fetal sheep ranging in age from 58 to 142 days' gestation. A morphometric analysis of photomicrographs of the membranes was then performed with computerized image analysis techniques.

RESULTS

After removal of the uterus, the fetus was surrounded by a layer of white silicone-filled microvessels in the chorion. The amniotic membrane after separation from the chorion was covered by a fine mesh of microvessels, whereas the allantois was avascular. The amniotic membrane readily separated into an outer vascularized layer and an inner, avascular layer containing the amnion. Approximately 50% of the surface of the chorionic membrane was covered by microvessels; this appeared independent of gestational age. At midgestation 30% of the surface of the amnion was covered by microvessels, and this decreased to 17% at 142 days. Relative to fetal weight, the amniotic and chorionic vascular surface areas decreased from 6 to 0.3 and from 15 to 1.5 cm2/gm fetal weight, respectively, over the last half of gestation.

CONCLUSIONS

There is an extensive network of microscopic fetal blood vessels within the ovine chorion and covering the outer surface of the amnion. These vessels appear to be ideally situated to facilitate a direct exchange of water and solutes between amniotic or allantoic fluids and fetal blood through the intramembranous pathway.