Gynecol. Invest. 7: 157-164 (1976)
Bulk Flows through Human Fetal Membranes D.R. A bramovich , K.R. P age and L. J andial Department of Obstetrics and Gynecology and Biophysical Chemistry Unit, Depart ment of Chemistry, University of Aberdeen, Aberdeen
Key Words. Amnio-Chorion ■Membranes • Amniotic fluid ■Bulk flows Abstract. Bulk water flows across term human amnio-chorion are studied in vitro. The hydrodynamic permeability is found to vary with both hydrostatic and osmotic pressure. The coefficient characterizing flows generated by hydrostatic pressure is substantially larger than that characterizing osmotic flows. The measurements are utilised to predict in vivo bulk flows across the amnio-chorion. These lead to the prediction that at most a flux of 34-83 ml/day may occur across amnio-chorion directed outwards from the amniotic cavity, the principal contribution to this arising from the effects of hydrostatic pressure.
In vivo studies have established that isotopically labelled water is dis tributed between the amniotic fluid, and maternal and fetal circulations at a far greater rate than would be expected from fetal swallowing and void ing alone [8]. These findings raise the possibility that significant net or bulk water flows can occur across the fetal membranes in utero. The evi dence is not direct however, as exchange diffusion may account for most of the observed movement of labelled water. The measurement of bulk water flows through tissues requires a degree of experimental control be yond that currently available in in vivo studies on humans. An alternative approach is to determine the hydrodynamic permeability coefficients of the tissue of interest using an in vitro method. A knowledge of these coefficients permits the estimation of a bulk flow in vivo provided the forces govern ing this flow are known. The most important forces in the present context are osmotic and hydrostatic pressures. Extensive studies of the osmotic properties of sepa
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Introduction
158
A bramovich/P age/J andiai.
rated amnion and chorion have been made by Seeds [14, 15]. B arton and B aker [1] appear to have made the only study on amnio-chorion of bulk flows produced by hydrostatic pressure, but these authors have not ap preciated the variation of the permeability with pressure. The object of the present paper is to establish the effects of osmotic and hydrostatic pressure on bulk flows through isolated amnio-chorion in vitro, and to discuss how these may relate to fluxes in the intact uterus.
Methods and Materials Specimens of term amnio-chorion were dissected immediately following delivery from a region 2-5 cm from the edge of the placenta. The specimens were kept moist at all times with gassed Gey’s solution [7] and mounted for study within 30 min of delivery. Osmotic studies were performed using solutions of T40 dextran (Pharmacca Ltd.) made up in Gey’s solution. All studies were carried out at a temperature of 38.010.2 °C. The effect of hydrostatic pressure on water flows through unstressed supported amnio-chorion was studied using the large aperture apparatus and method of P age et at. [11]. Water permeabilities were calculated using the expression.
(Lp)p = Jv/4P,
(1)
where Jv is the flow of water in ml/cm2 sec, /IP is the pressure differential in dyn/cm2 and (Lp)p is the hydrodynamic permeability coefficient for a given pressure differen tial. Osmotic studies on unstressed supported tissue were performed using an apparatus similar to that of F ranz and Van Bruggen [5J. A circular disc of tissue 2.5 cm in diameter was clamped at its periphery on a 3-mm broad perspex rim, and supported on one side by a porosity 2-glass sinter. Solutions of dextran T40 made up in Gey's solution were introduced into a 40-ml capacity compartment on the unsupported side of the tissue and mechanically stirred. The compartment on the supported side of the membrane had a 30-ml capacity and was filled with normal Gey’s solution. Fluxes were always measured by following the volume changes in the 30-ml compart ment. The properties of the tissue were characterized using the osmotic permeability coefficient (Lp)c defined by equation (2) (2)
where An represents the osmotic pressure. As shown by V ink [17] dextran T40 solutions are highly non-ideal in that they generate osmotic pressures much greater than the value obtained from the Van’t Hoff law of osmosis. The values of An used were therefore calculated from the product 0 RTc where 0 is the practical osmotic coefficient, R the gas constant, T the absolute temperature and c the dextran T40 concentration (in mol/ml calculated using a number average molecular weight of 29,500). The practical osmotic coefficient does not vary much with temperature so
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(Lp)c = h /A 7i,
Bulk Flows through Human Fetal Membranes
159
A P cm Hg
Fig.l. (Lp)p vs. membrane pressure differential for a typical experiment on sup ported tissue. The numbers by the points indicate the sequence of observation.
its value determined at 25 °C may be used without significant error at 38 °C. Values of An at 25 °C were calculated from the virial coefficients given for dextran T40 by V i n k , and divided by the corresponding Van’t Hoff law pressures. The results yielded values of
Bulk Flows through Human Fetal Membranes D.R. A bramovich , K.R. P age and L. J andial Department of Obstetrics and Gynecology and Biophysical Chemistry Unit, Depart ment of Chemistry, University of Aberdeen, Aberdeen
Key Words. Amnio-Chorion ■Membranes • Amniotic fluid ■Bulk flows Abstract. Bulk water flows across term human amnio-chorion are studied in vitro. The hydrodynamic permeability is found to vary with both hydrostatic and osmotic pressure. The coefficient characterizing flows generated by hydrostatic pressure is substantially larger than that characterizing osmotic flows. The measurements are utilised to predict in vivo bulk flows across the amnio-chorion. These lead to the prediction that at most a flux of 34-83 ml/day may occur across amnio-chorion directed outwards from the amniotic cavity, the principal contribution to this arising from the effects of hydrostatic pressure.
In vivo studies have established that isotopically labelled water is dis tributed between the amniotic fluid, and maternal and fetal circulations at a far greater rate than would be expected from fetal swallowing and void ing alone [8]. These findings raise the possibility that significant net or bulk water flows can occur across the fetal membranes in utero. The evi dence is not direct however, as exchange diffusion may account for most of the observed movement of labelled water. The measurement of bulk water flows through tissues requires a degree of experimental control be yond that currently available in in vivo studies on humans. An alternative approach is to determine the hydrodynamic permeability coefficients of the tissue of interest using an in vitro method. A knowledge of these coefficients permits the estimation of a bulk flow in vivo provided the forces govern ing this flow are known. The most important forces in the present context are osmotic and hydrostatic pressures. Extensive studies of the osmotic properties of sepa
Downloaded by: University of Chicago Library 205.208.116.24 - 4/13/2018 8:21:16 AM
Introduction
158
A bramovich/P age/J andiai.
rated amnion and chorion have been made by Seeds [14, 15]. B arton and B aker [1] appear to have made the only study on amnio-chorion of bulk flows produced by hydrostatic pressure, but these authors have not ap preciated the variation of the permeability with pressure. The object of the present paper is to establish the effects of osmotic and hydrostatic pressure on bulk flows through isolated amnio-chorion in vitro, and to discuss how these may relate to fluxes in the intact uterus.
Methods and Materials Specimens of term amnio-chorion were dissected immediately following delivery from a region 2-5 cm from the edge of the placenta. The specimens were kept moist at all times with gassed Gey’s solution [7] and mounted for study within 30 min of delivery. Osmotic studies were performed using solutions of T40 dextran (Pharmacca Ltd.) made up in Gey’s solution. All studies were carried out at a temperature of 38.010.2 °C. The effect of hydrostatic pressure on water flows through unstressed supported amnio-chorion was studied using the large aperture apparatus and method of P age et at. [11]. Water permeabilities were calculated using the expression.
(Lp)p = Jv/4P,
(1)
where Jv is the flow of water in ml/cm2 sec, /IP is the pressure differential in dyn/cm2 and (Lp)p is the hydrodynamic permeability coefficient for a given pressure differen tial. Osmotic studies on unstressed supported tissue were performed using an apparatus similar to that of F ranz and Van Bruggen [5J. A circular disc of tissue 2.5 cm in diameter was clamped at its periphery on a 3-mm broad perspex rim, and supported on one side by a porosity 2-glass sinter. Solutions of dextran T40 made up in Gey's solution were introduced into a 40-ml capacity compartment on the unsupported side of the tissue and mechanically stirred. The compartment on the supported side of the membrane had a 30-ml capacity and was filled with normal Gey’s solution. Fluxes were always measured by following the volume changes in the 30-ml compart ment. The properties of the tissue were characterized using the osmotic permeability coefficient (Lp)c defined by equation (2) (2)
where An represents the osmotic pressure. As shown by V ink [17] dextran T40 solutions are highly non-ideal in that they generate osmotic pressures much greater than the value obtained from the Van’t Hoff law of osmosis. The values of An used were therefore calculated from the product 0 RTc where 0 is the practical osmotic coefficient, R the gas constant, T the absolute temperature and c the dextran T40 concentration (in mol/ml calculated using a number average molecular weight of 29,500). The practical osmotic coefficient does not vary much with temperature so
Downloaded by: University of Chicago Library 205.208.116.24 - 4/13/2018 8:21:16 AM
(Lp)c = h /A 7i,
Bulk Flows through Human Fetal Membranes
159
A P cm Hg
Fig.l. (Lp)p vs. membrane pressure differential for a typical experiment on sup ported tissue. The numbers by the points indicate the sequence of observation.
its value determined at 25 °C may be used without significant error at 38 °C. Values of An at 25 °C were calculated from the virial coefficients given for dextran T40 by V i n k , and divided by the corresponding Van’t Hoff law pressures. The results yielded values of
