Stereological studies on the true thickness of the villous membrane in human term placentae: a study of placentae from high-altitude pregnancies.

Stereological principles were used to calculate functionally significant dimensions of the human villous membrane, its arithmetic mean thickness (Ta) and its harmonic mean thickness (Th). The former is proportional to tissue mass and oxygen consumption, the latter to diffusional resistance. For a group of 15 term placentae from uncomplicated pregnancies at high altitude, the average values were Ta = 4.44 micron, Th = 3.56 micron and Ta/Th = 1.26. The latter figure provides a useful quantitative expression for the efficiency of the membrane in gas and metabolite diffusion. It implies that vasculosyncytial membranes and syncytial knots decrease resistance to diffusion by 26 per cent, compared with that of a membrane with uniform thickness throughout. The methods are simple to apply and provide better estimates of true thickness than do measurements confined to thin sections.