Ultracytochemical localizations of adenosine nucleotidase activities in the human term placenta, with special reference to 5'-nucleotidase activity

The ultracytochemical localizations of adenosine nucleotidase activities were investigated in the human term placenta. The enzymes studied were as follows: 5′-nucleotidase (5′-N), Ca++-activated adenosine triphosphatase (Ca++ ATPase), Mg++-activated adenosine triphosphatase (Mg++ ATPase) and nucleotide diphosphatase (NDPase). 5′-N activity was demonstrated using not only the lead nitrate method of Wachstein and Meisel but also the recently developed cerium and neodymium method as well.The reaction products for 5′-N activity were found on the external surface of the microvillous plasma membrane of the syncytiotrophoblast. Alpha-beta-methylene adenosine diphosphate (AOPCP), at a concentration of 2.0mM, effectively inhibited 5′-N activity. Ca++ATPase and Mg++ATPase activity were observed strongly on the microvillous membrane of the syncytiotrophoblast and weakly on the basal plasma membrane of the syncytiotrophoblast. NDPase, using ADP as a substrate, was localized on the microvillous membrane.These observations suggest that the syncytiotrophoblast is active in the nucleotide metabolism and that microvillous surface of the syncytium may play an important role in regulating the feto-placental-maternal microcirculation in the human term placenta.