An ultrahistochemical study of the placental content of respiratory enzymes in normal and prolonged pregnancies.

The placental content of malate dehydrogenase and lactate dehydrogenase has been studied at the ultrastructural level. In the trophoblast both enzymes have a predominantly mitochondrial localization although occasional membrane-bound extra-mitochondrial activity is also seen. As compared with the first trimester placenta, there is a highly significant decrease in the activity of both enzymes within the trophoblast at term; this may represent an adaptative change to reduce placental oxygen consumption and increase the amount of oxygen available to the fetus. In prolonged pregnancies, there is a continued decline in trophoblastic malate dehydrogenase activity but the lactate dehydrogenase activity tends to increase, these changes suggesting that there is a switch from oxidative pohosphorylation to anaerobic glycolysis. It is not clear whether this change to a less efficient mode of energy generation is due to an intrinsic ageing change within the trophoblast or is secondary to placental ischaemia.