Surface exchange area of the porcine placenta: morphometry of anisotropic interdigitating microvilli.

Abstract

The pig has a diffuse epitheliochorial placenta. The foeto-maternal surface area (Sv(F), Sv(M)) is enlarged at three levels: level I: macroscopic folds of uterine mucosa covered by foetal membranes; level II: light microscopic folds of uterine epithelium covered by trophoblastic epithelium; level III: ultrastructural formation of both maternal and foetal microvilli in an interdigitating pattern. This study presents a morphometric method, using intersection counting, to estimate the surface area of these interdigitating, highly oriented microvilli at level III, i.e. the 'true' surface exchange area. The method is applied to different stages in pregnancy. The epithelial surfaces are enlarged by a factor 8-11 due to the formation of microvilli. This factor varies during pregnancy, being lower early in gestation and before delivery, and for the first time it is shown to be significantly different for mother and foetus. At level III the foetal surface of placenta is larger than the maternal indicating that the two opposing surfaces are not congruent at the ultrastructural level and that membrane-linked placental transport may be different in mother and foetus. A three-compartment model is suggested for placental exchange with maternal, intermicrovillous and foetal compartments.