Intermediate trophoblast: a distinctive form of trophoblast with specific morphological, biochemical and functional features.

Morphological and immunohistochemical analysis of placental tissue from 12 days to term using antibodies directed against the unique carboxyl terminal peptide of human chorionic gonadotrophin beta subunit, human placental lactogen (hPL) and pregnancy-specific beta I-glycoprotein reveals that an intermediate form of trophoblast with distinctive features exists. This cell has a diverse morphological expression and is located overlying chorionic villi, in the trophoblastic columns, basal plate and the trophoblastic shell. Although all three placental proteins are localized in this cell the predominant hormone is hPL, which can serve as an immunocytochemical marker. One of the primary functions of this cell is in implantation and in the establishment of the uteroplacental circulation since it extensively invades the spiral arteries at the placental site. It is proposed that this distinctive form of trophoblast be termed 'intermediate trophoblast'.