Placental villous structure and cells present in the uteroplacental unit
Fetal blood vessels are separated from maternal blood by the feto–maternal interface composed by a layer of CTBs and another of STBs. Vessels from both sides are composed by vascular smooth muscle cells (VSMCs) and endothelial cells (ECs). The vessels containing arterial blood are presented in red, and the vessels containing venous blood are in blue. The fetal side (space in yellow) has the chorionic plate and the chorionic membranes; Hofbauer cells are macrophages of fetal origin. The extravillous trophoblasts (EVTBs) are invasive trophoblasts responsible for remodeling maternal spiral arteries, allowing increased blood flow in the villous. The decidua on the maternal side of the placenta have a large quantity of immune cells, such as macrophages, natural killer (NK) cells, lymphocytes T (T cells) and dendritic cells (DCs). The decidua are the modified portion of the endometrium developed after the implantation as a result of intense remodeling and immune cell recruitment. On the right, a scheme shows the dynamic expression of ACE2 protein (in green) in each placental cell type during early pregnancy, late pregnancy and chorioamnionitis.