Uterine natural killer (uNK) cells. (A/B) Oligoryzomys in mid gestation (MZUSP 31167). PAS-positive cells accumulated near the maternal blood vessel (MA) in the decidua (D). Some were binucleated (arrows). (C) Oligoryzomys in early pregnancy (MZUSP 32735). Positive response to vimentin indicated that the cells were of mesenchymal origin, representing maternal uNK cells, not trophoblast. (D) Necromys in early pregnancy (MAV/CEMAS 04). Vimentin. In early gestation, the uNK cells were frequent also near to the outer margin of the placental disk (CA), which consisted of spongiotrophoblast and was vimentin-negative. (E) Necromys in early pregnancy (MAV/CEMAS 04). Immunostaining for cytokeratin. The negative response indicated that the invasion of trophoblast cells was limited to areas near the placenta. (F) Euryoryzomys in early pregnancy (MAV/SJB 01). TEM. The uNK cells in the decidua possessed large spherical nuclei (N) with irregular shape and distinct nucleoli (NC).