An immunohistological comparison of the secretory capacity of villous and extravillous trophoblast in the human placenta.

Using an immunoperoxidase technique the localization of hCG, hPL, SP1, PAPP-A and PP5 within villous and extravillous trophoblast has been studied. The villous syncytiotrophoblast appears to be the sole significant source of hCG, SP1, PAPP-A and PP5 but hPL is also present in the infiltrating extravillous trophoblast. Within the interstitial extravillous trophoblast the proportion of cells staining positively for hPL increases progressively as the cells extend deeper into the uterine tissues but all the vascular extravillous trophoblast within the lumina of the spiral arteries stains positively for hPL at all levels within the decidua and inner myometrium. It is not clear why the synthetic capacity of the infiltrating trophoblast is limited to the production of hPL. We suggest that this might be indicative of selectivity of a specific subpopulation of cytotrophoblastic cells. It is unlikely that the physiological effects of infiltrating trophoblast are mediated by hPL, this substance being probably only a marker of a cell population with a particular invasive capacity.