Term ovine placental vasculature: comparison of sea level and high altitude conditions by corrosion cast and histomorphometry.

The placental vascular architecture differs significantly at high altitude from that at sea level in the human and guinea-pig. Four sheep between 137 and 140 days of gestation, kept near sea level throughout gestation, were used as a normoxic control group for comparison of the placental vasculature with 10 other ewes, kept at high altitude (3820 m above sea level; Barcroft Laboratory, White Mountain Research Station, CA, USA). Placentomes from both groups were prepared for histology and scanning electron microscopy of vascular corrosion casts. Singular perfusion of fetal placentae, as well as combined maternal/fetal injection was performed. The influence of long-term hypoxaemia was determined by qualitative and semi-quantitative evaluation of corrosion casts and histological sections. The fetal vessel casts show a distinct difference in the arrangement of vessels of all sizes in response to long-term hypoxaemia. In the control group, stem arteries and veins are straight and parallel. In contrast, this is much less evident in the hypoxaemic group because arterioles and venules branch off the stem vessels more frequently and in an irregular manner. This leads to a capillary bed that is much more dense due to increased branching and capillary coiling. These observations are confirmed by histomorphometry. In the fetal vessels of high altitude sheep placentomes, we observed a decreased number of vascular cross sections (21.6 +/- 4.7 SEM versus 27.7 +/- 4.0 SEM; P = 0.02). However, the average luminal size per cross section (77.9 +/- 10.5 microns2 SEM versus 59.4 +/- 7.4 microns2 SEM; P = 0.004) was increased at high altitude and the percentage of lumina of the total area (5.7 +/- 0.5 SEM versus 5.3 +/- 0.3 SEM; P = 0.09) indicated a trend towards an increase. In maternal vessels of high altitude placentomes, the number of vessel cross sections (6.5 +/- 0.7 SEM versus 6.0 +/- 0.5 SEM; P = 0.2) remained unchanged, whereas the average luminal size (1108 +/- 122 microns2 SEM versus 844 +/- 77 microns2 SEM; P < 0.001) and the percentage of lumina out of the total area (20.9 +/- 1.8 SEM versus 17.5 +/- 1.7 SEM; P < 0.001) were increased. The interhaemal distance appeared to be slightly but not significantly increased at high altitude. These findings indicate that at high altitude the sheep placenta develops an increased materno-fetal absorptive surface to help guarantee substance exchange.