By the end of the first week of development, the trophoblast attaches to the epithelium of the endometrium (inner part of the uterus wall). The trophoblast cells will then proliferate on the contact surface with the endometrium, supposedly under the influence of uterine growth factors. This initiates the cytotrophoblast. Some of the cytotrophoblast cells fuse to form the syncytiotrophoblast. The syncytiotrophoblast grows into the endometrium (through the fusion of more cytotrophoblast cells) and prompts the stromal tissue of the endometrium to differentiate into decidual cells. Halfway through the second week, the embryo is fully implanted in the decidua (= endometrium after implantation), and the cytotrophoblast is now surrounded by the syncytiotrophoblast. The placenta consists of both maternal and fetal components. As maternal part of the placenta the wall of the uterus invaded by the trophoblast cells is defined. The fetal part of the placenta is formed from the chorion. The chorion consists of an inner layer of cytotrophoblast cells, and the syncytiotrophoblast, which will eventually form the separating barrier between mother and child. Both trophoblast layers only contribute to the formation of the extra-embryonic structures and not to the development of the embryo. Around day 9, cavities, called lacunae, are formed in the syncytiotrophoblast. At this stage the placenta is not yet functional.
The maternal capillaries from the decidua will later connect to these lacunae creating the intervillous spaces between the villi of the developing placenta.
Please note: At no time is there a direct contact or mixing of maternal and fetal blood!
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