RESUMEN

This report presents microscopic observation on morphological and immuno- cytochemical characteristics of the trophoblast cells of the human decidua at the implantation site in the first trimester pregnancy .The infiltration of a unique type of large cells different from cytotrophoblast (CT) and syncytotrophoblast (ST) was found in the decidua at the implantation site. These unique cells are termed inter- mediate trophoblast (IT). They are typically mononucleate, but may be binucleate or multinucleate. The mononucleate cells vary in shape from round, polyhedral spindle shaped. Their cytoplasm is typically abundant and eosinophilic or amphophilic. The nuclei may vary in size and shape. These cells usually distribute around the spiral arterioles, differse blood vessel wall, penetrate into the lumen, and even replace the blood vessel wall totally. Immunocytochemically, both these cells and ST are stained positive for HCG and HPL. However, the HCG-stained Cell population of IT.is much lower than that of ST,while the of HPL stained cell population of IT is significantly higher than that of ST.On the other hand,neither HCG nor HPL are positive in CT. The results of SP-1 ?-HCG usually go with those of HCG.in CT, ST and IT.The PAPP-A gives non specific staining result. It is believed that IT,with its distinct morphological and immunocytochemical feats,is regarded as a transitional form in the shift from CT to ST.