Junctions on the maternal blood surface of the human placental syncytium.

Terminant chorionic villi of the human placenta at 8, 9, 12, 14 weeks' gestation and at full term have been studied in freeze-fracture replicas and in thin sections. In replicas, maculae occludentes of different size have been detected in all placental samples on the cleaved plasma membrane of the free (maternal) surface of the syncytiotrophoblast. In thin sections they are located in narrow invaginations of the syncytium luminal plasma membrane. These invaginations are in turn similar to flattened vesicles (also associated with junctions) located in the underlying syncytioplasm. Possible origin and function of the junctions on the syncytium free surface are discussed.