Placenta of discordant twins: lack of change in histochemically detectable enzyme activities

We localised three important enzymes histochemically in placental trophoblasts from women who gave birth to dichorionic discordant twins, in which the co-twin was affected by foetal growth restriction (FGR). The enzymes studied were adenosine diphosphate-degrading enzyme (ADP-degrading enzyme, plasma membrane enzyme), cytochrome c oxidase (mitochondrial enzyme), and glucose-6-phosphatase (endoplasmic reticular enzyme). We compared these enzyme activities and their distribution patterns among placentas of the smaller (FGR) co-twin, larger co-twin, pre-eclamptic singleton with FGR, and normal singletons with birth weight of appropriate for their gestational ages. In FGR co-twin placentas, the intensity and localisation pattern of these three enzymes did not differ from those seen in the larger co-twin and normal singleton placentas. Decreased ADP-degrading activity and cytochrome c oxidase negative mitochondria, which were characteristic features of pre-eclamptic trophoblasts, were not observed in FGR co-twin placentas. These observations indicated that, in the FGR co-twin, enzyme-histochemically detectable trophoblastic cell dysfunction may be absent, or if present, less prominent, compared with pre-eclamptic FGR. We previously reported that placental trophoblasts from singleton idiopathic FGR also showed no reduction in these enzyme activities. In mechanism and pathophysiology, FGR in dichorionic discordant twins may be quite different from pre-eclamptic FGR, but somewhat resembles idiopathic FGR, though all three disorders lead to placental insufficiency, resulting in limited foetal growth.