STAR VOLUME ESTIMATES OF THE INTERVILLOUS CLEFTS IN THE HUMAN PLACENTA - HOW CHANGES IN UMBILICAL ARTERIAL-PRESSURE MIGHT INFLUENCE THE MATERNAL PLACENTAL CIRCULATION

The study estimated the mean volume of the clefts between adjacent vil li in the normal term human placenta, and the effect on this of increa sing the fetal perfusion pressure, using a new stereological tool call ed the 'star volume estimator'. This enables the measurement of irregu lar and complex structures, including both particles and cavities, in a mathematically defined and unbiased manner. To achieve this, a total of ten term placentae delivered by caesarian section were obtained. F our fetal arteries supplying opposite quadrants of the placental disc were perfusion-fixed under Standard pressures of 40, 60, 80 and 100 mm Hg respectively. Stereological estimates relating to the star volume o f the clefts between the villi, and to the volume density of the inter villous space were obtained. There was a significant rise in the star volume of the intervillous clefts from 26.8 x 10(4) m(3) at 40 mm Hg t o 75.1 x 10(4) m(3) at 100 mmHg (F=75, df=1,38, P<0.05). The volume de nsity of the intervillous space remained unchanged, thus obviating the possibility of fluid leakage into the intervillous space accounting f or this change. It is concluded that the fetal vasculature provides hy draulic support to the villous tree, and that changes in the umbilical perfusion pressure can therefore alter the disposition of the villi w ithin the intervillous space. As fetal blood pressure rises, for examp le during acute hypoxic episodes, the villi will move apart. The enlar gement of the clefts between adjacent villi will have a secondary effe ct upon the maternal circulation, promoting more even perfusion of the intervillous space at higher overall flow rates. This mechanism might provide a physical means by which the maternal and fetal circulations can locally interact in the villous haemochorial placenta so as to ac hieve optimal exchange conditions.