Immunohistochemical demonstration of pregnancy-associated plasma protein A (PAPP-A) in the syncytiotrophoblast of the normal placenta at different gestational ages.

The immunoperoxidase technique was used to study the localization of pregnancy-associated plasma protein A (PAPP-A) in formaldehyde-fixed paraffin-embedded tissue from normal human placentae at the gestational age of 8, 15 and 40 weeks. Sections of formaldehyde-fixed tissue treated with a proteolytic enzyme and incubated in antiserum against PAPP-A either raised in goats or rabbits showed that PAPP-A was distributed in the cytoplasm of the syncytiotrophoblast. The protein was not found in the cytotrophoblast. Sections without pretreatment with trypsin and incubation in goat anti-PAPP-A showed no staining reaction, whereas incubation in rabbit anti-PAPP-A revealed a staining of the syncytiotrophoblast surface. The results indicate that PAPP-A is probably synthesized in the syncytiotrophoblast.