Fetal microcirculation of abnormal human placenta. II. Scanning electron microscopy of placental vascular casts from fetus with severe erythroblastosis fetalis.

The latex injection-corrosion cast technique coupled with scanning electron microscopy was applied to the study of severe Rh sensitization of the placenta. The main observed ultrastructural changes in fetal vasculature in this type of placenta were very similar to those of the small for gestational age placenta; large-sized vessels exhibited less branching than those of the normal placenta, and the capillary network showed numerous vascular buds and "H"-shaped anastomoses. In addition, in Rh-sensitized placentas, the capillary network was remarkably sparse, and the cast also revealed smooth-surfaced veins with distinct U shapes undulating through the placental stroma. These U-shaped veins may be the result of umbilical flow impedance with subsequent umbilical venous hypertension, which stretches the veins into undulating form.