Fine structure of human chorionic membrane. Ultrastructural and histochemical examinations.

Normal human term chorionic membrane was examined by electron microscopy and histochemistry. In comparison with previous studies of the human chorion, our findings showed the following new morphological aspects: (1) The cellular junctions between adjacent trophoblastic cells were made up not only of desmosomes but also of gap junctions and occasional tight junctions. (2) The epithelial basement membrane was stratified and discontinuous. (3) Besides the fibroblasts and macrophages (Hofbauer cells), the connective tissue of the human chorion also contained myofibroblasts. Histochemically, the surface coat of the plasma membrane of the chorionic epithelial cells, the microfibrils, and the proteoglycans associated with the collagen fibrils showed a positive reaction to ruthenium red. This was not so for the basement membrane of the chorionic epithelium. Since the tight junctions are seldom present in the chorionic membrane, they are apparently not significant for the paracellular protein transfer. Similar to the amnion epithelium, it is assumed that the intercellular spaces and the discontinous basement membrane of the chorionic epithelium represent a paracellular route for non-placental protein transfer. As the myofibroblasts in human amnion, we suggest that the myofibroblasts in the chorionic membrane may contribute to the protection of the fetal membranes from overdistension.