Final answer:
The placenta is formed from fetal tissue from the trophoblast and maternal tissue from the endometrium. The chorionic villi of the trophoblast invade the endometrium to facilitate nutrient exchange, while the maternal portion develops from the endometrial lining's deepest layer.
Step-by-step explanation:
The placenta forms from the combination of fetal tissue and tissue from the structure of the endometrium. The maternal portion of the placenta, called decidua basalis, develops from the deep layer of the endometrial lining of the uterus. Meanwhile, the fetal portion arises from the trophoblast, more specifically from a layer of cells that expand to become the chorion and its extensions, the chorionic villi. These villi penetrate into the endometrium and facilitate the exchange of nutrients, oxygen, and waste between the mother and the developing fetus through a network of blood vessels.
The syncytiotrophoblast and cytotrophoblast, constituents of the trophoblast, along with the extraembryonic mesoderm, proliferate and form the chorionic membrane enveloping the conceptus. This chorionic membrane is crucial, as it develops into the chorionic villi which deeply embed into the endometrial tissue, forming the interface for maternal-fetal exchange. The fetal blood vessels, including the three umbilical vessels connecting the growing embryo to the placenta, develop within these villi.
It's through this elaborate organ, the placenta, where the mother's blood and the fetus’s blood can exchange substances without mixing, ensuring the fetus is nourished and protected throughout the pregnancy. Notably, blood from the mother flows into the space surrounding the chorionic villi, while the fetus's blood circulates within the vessels contained within the villi themselves.