Fig. 2
From: Recent insights on indirect mechanisms in developmental toxicity of nanomaterials
Scheme of the human placental barrier in early and late pregnancy. In the first trimester, the placental barrier consists of the syncytiotrophoblast (ST), cytotrophoblasts (CT), basal lamina (BL) and the endothelial cells (E) of the fetal capillaries (FC). Other cell types in the villous mesoderm include fibroblasts (F) and Hofbauer cells (HC). Various immune cells are also present in the maternal decidual tissue, including dendritic cells (DC), macrophages (MP), uterine natural killer cells (uNK), T cells (TC) and B cells (BC). Extravillous trophoblasts (EVT) of the anchoring villi invade the maternal spiral arteries (SA) and form a plug that prevents entry of maternal blood into the intervillous space, and uterine glands (UG) provide histiotrophic nutrition. After the first trimester, the EVT plug is released and placental villi are now surrounded by maternal blood. Towards the end of pregnancy, the placental barrier decreases in size by thinning of the ST layer and spreading of the CT layer, and the FCs move towards the periphery of the floating villi