Stereological studies on the oxygen diffusing capacity of human placentae from low and high altitudes
These studies have quantified human placental development throughout 10-41 weeks of `normal' gestation, and compared the structure of term organs delivered at low (400m) and high (3600m) altitudes in Bolivia. Quantitative analyses of histological preparations were performed using stereological techniques. Salient structural parameters were then combined with physicochemical constants (gleaned from the literature) to estimate the oxygen conductances of the maternal and fetal erythrocytes and plasmas, the trophoblast and the stroma. These were combined to yield Dp, the total morphometric diffusing capacity for oxygen (in ml O2/min/torr). During the third trimester, villous trees expand predominantly by the elaboration of terminal villi. This increases the surface areas and decreases the thicknesses of the trophoblast and the stroma (the two compartments which offer the greatest resistance to oxygen diffusion). The sources of expansion of the villous trees may lie in the transformation of immature intermediate villi into stem and mature intermediate villi. Hypoxically stimulated capillary elongation within mature intermediate villi may be involved in initiating the production of new terminal villi. It is interesting that the total length and surface area of terminal villi were maintained in highland organs while those of larger diameter villi were significantly lower than in lowland controls. Total villous surface area was significantly reduced in highland organs while capillary surface was comparable to lowland estimates. However, reductions in harmonic mean trophoblast and stromal thicknesses helped to conserve the conductance of the trophoblast and to increase that of the stroma above lowland values. Dp increased significantly but specific Dp remained constant throughout gestation. Although the main stressor at high altitude is hypobaric hypoxia, no significant altitudinal differences in Dp were found. The intrauterine growth retardation consistently observed and high altitudes therefore occurs despite placental adaptations which maintain Dp at lowland values.