Association of immunoreactive eosinophil major basic protein with placental septa and cysts.

A protein that is immunochemically indistinguishable from the major basic protein of the eosinophil granule is present at elevated concentrations in the plasma of pregnant women. Major basic protein has been localized to placental trophoblasts known as X cells. Because X cells are located in placental septa and septal cysts, we tested whether the numbers of these structures are correlated with plasma levels of major basic protein. Data analysis revealed strong positive correlations between maternal plasma major basic protein concentrations and the number of placental septa, septal cysts, and subchorial cysts and the presence of subchorial fibrin deposits. No significant correlation was found between plasma major basic protein levels and other variables examined, such as placental weight. Thus major basic protein is a specific marker for septa and cysts, presumably because it is an X-cell product. The study of major basic protein may aid in delineating the function and ontogeny of X cells, as well as their role in fibrin deposition and in septa and cyst formation.