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. 2021 Sep 7;22(18):9675. doi: 10.3390/ijms22189675

Figure 1.

Figure 1

Schematic illustrating placental development—from blastocyst implantation to spiral artery remodeling and villous formation. Different events, such as proliferation of cytotrophoblast (CTBs), differentiation of extravillous trophoblasts (EVTs), formation of the EVT plug in the spiral artery, formation of Hofbauer cells (HBCs), and remodeling of the spiral arteries are shown. The dramatic changes of O2 levels in the villous region during the 1st trimester (2–3%) and 2nd trimester (8–10%) compared with the relatively constant O2 levels in decidua (5–6%) are highlighted. uNK, uterine natural killer; dMϕ, decidua macrophages; O2, oxygen. Reprinted with permission from Ref. [5]. Copyright 2008 Chang C-W, et al.