Intralobular variations in barrier thickness in the mature human placenta

Following perfusion fixation, the harmonic mean thickness of the villous membrane was measured for terminal villi sampled at various sites within a single lobule. Comparisons were made between central and peripheral regions of the lobule at the levels of the basal plate, the mid-zone and just under the chorionic plate. Ten placentae were examined, and in each case the centre of a lobule was identified by the presence of a villus-free cavity. At each level the harmonic mean thickness was significantly reduced in the peripheral regions compared to the central regions. By contrast, no change was found in the arithmetic mean thickness, the volume fraction of the fetal capillary lumina, or the mean capillary diameter. This variation in harmonic mean barrier thickness may be related to the pattern of maternal blood flow through the lobule and any PO2 gradient that results therefrom. It is clear that changes in the thickness of vasculosyncytial membranes can take place independently of any alterations in the calibre of the underlying fetal capillary network.