Regional ultrastructural differences in placental villi in cotyledons of a mature human placenta.

Earlier histological, histochemical, biochemical and autoradiographic studies revealed a marked difference in the morphology and biology of placental villi dependence upon their localization within the materno-fetal circulation units (placentones). The quantitative differences demonstrated characterize the villi in the centres of the circulation units as morphologically and functionally less differentiated than the villi in the periphery. In the present study it should be scrutinized whether these differences can also be proven on the ultramicroscopic level. Small and large villi from both regions (centre and periphery) were studied. Two well-defined syncytial areas, namely syncytium directly overlying a fetal vessel and syncytium over a Langhans cell were studied separately. The following structures were chosen for study and counted by the "grid" method. Microvilli, rough endoplasmic reticulum with parallel channels, vesicular rough endoplasmic reticulum, and the number mitochondria. The ultrastructural studies support the earlier histological, enzyme-histochemical, biochemical and autoradiographic findings, which suggested that the villi in the periphery of the placentones have a higher degree of differentiation (maturation) than those in the centre. There are no qualitative differences between the two regions of the placentone but only quantitative differences which reflect the maturity of the villi.