On the factors which contribute to thinning of the villous membrane in human placentae at high altitude. I. Thinning and regional variation in thickness of trophoblast.

Sections of human term placentae delivered at low and high altitude to indigenous and non-indigenous populations in Bolivia were analysed by stereological methods. Estimates were obtained of the arithmetic and harmonic mean thicknesses of villous membrane and of trophoblast. These values were then used to derive indices of thickness variation. No significant ethnic, sex or interaction effects were found. At high altitude, the villous membrane was significantly more variable in thickness owing to a lower harmonic mean but the same arithmetic mean thickness. Alterations in the arithmetic and harmonic mean thicknesses of the trophoblast contributed to the thinning of the villous membrane. The trophoblast was thinner and more irregular in thickness at high altitude. These findings are discussed in the context of placental diffusing capacity and the possible mechanisms for formation of vasculosyncytial membranes.