There were controversial data concerning the localizations of alkaline phosphatase (ALP) and acid phosphatase (ACP) in the human term placenta. These enzymes were reinvestigated histochemically using the lead method. The recently developed cerium method was also employed.
Intense ALP reaction was observed on the plasma membrane of the microvilli at the luminal surface of the syncytiotrophoblasts. Moderate activity of this enzyme was also observed on the basal plasma membrane of the syncytiotrophoblasts and on the plasma membrane of the cytotrophoblasts. The ALP activity was markedly suppressed in a medium containing the enzyme inhibitors, i.e. 1.0mM bromotetramisole or 30mM L-phenylalanine. However, the preheating treatment (65°C, 30min) did not affect the activity of ALP. The ACP reaction products were recognized on the lysosomes and on the Golgi apparatus in the syncytiotrophoblasts as well as in the cytotrophoblasts. Both the lead method and the cerium method yielded consistent results.
As ALP and ACP are fundamental enzymes in enzyme histochemistry, these results might help to investigate the localizations of other specific enzymes, such as Na⁺/K⁺ATPase, Ca⁺⁺ATPase or 5′-nucleotidase, in the human term placenta.