The junctional zone. (A) Necromys, early pregnancy (MAV/CEMAS 05). HE. The junctional zone (JZ) with simple structure facing towards the decidua. The border was not sharp with trophoblast derivatives (arrows) invading into the decidua. (B) Cerradomys, near term (MZUSP/APC 1157). The junctional zone had a folded structure arranged around the maternal blood channels (MBC). (C) Necromys, near term (MAV/CEMAS 02). TEM. Syncytial trophoblast (Syn TS) lined the maternal blood channels (MBC), associated with underlying cellular trophoblast (Cell TS). (D) Necromys, near term (MAV/CEMAS 02). TEM. In places the trophoblast separating the maternal blood spaces was represented only by a thin syncytial layer (arrow). (E) Necromys in early pregnancy (MAV/CEMAS 05). PCNA. Clustered groups of proliferating trophoblast cells (arrow) occurred in the junctional zone. (F) Necromys in mid gestation (MZUSP/APC 1246-1). PCNA. In more advanced stages, proliferating cells were widespread in the junctional zone. (G) Necromys in mid gestation (MZUSP/APC 1246-1). PCNA. Higher magnification. (H) Cerradomys, near term (MZUSP/APC 1157). HE. Among the spongiotrophoblast in the junctional zone, trophoblast giant cells (TGC) occurred. They were close to the maternal blood spaces and had large nuclei and prominent chromatin.