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So, you may remember that the placenta is the composite structure of embryonic and maternal tissues that supplies nutrients to the developing embryo. The placenta is a unique organ, found only in mammals, that allows the mother to provide a very large amount of nutrients to her offspring for an extended period of time before they are even born.
The placenta serves as an interface between the mother and the developing fetus and has three main jobs:
The placenta is composed of both maternal tissue and tissue derived from the embryo. The chorion is the embryonic-derived portion of the placenta. It is composed of trophoblasts, which you may remember are the cells that made up the outer cell layer of the blastocyst. You may also remember that during implantation the trophoblasts multiply in number and extend into the uterine wall. They eventually form finger-like structures called chorionic villi, which are finger-like structures of the placenta composed of embryo-derived trophoblasts. The chorionic villi are surrounded by maternal blood which comes into direct contact with the embryonic trophoblast cells. The intervillous space is the part of the placenta that surrounds the chorionic villi and contains maternal blood.
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Maternal blood flows from the mother's circulatory system, through the intervillous space and then re-enters the mother's blood vessels. Fetal blood flows from the fetus into two main arteries in the umbilical cord, through the capillary network of the chorionic villi and is returned to the fetus by the umbilical vein. The maternal blood that enters the placenta is nutrient and oxygen rich.
A growing fetus obviously requires lots of nutrients and oxygen that it cannot provide for itself, so the fetal blood that flows through the capillaries of the chorionic villi is nutrient and oxygen poor. Because of this disparity in the nutrient and oxygen concentration between the fetal blood in the villi and the maternal blood in the intervillous space, nutrients and oxygen diffuse from the maternal blood, into the villi and into the fetal blood.
Conversely, the fetal blood contains high concentrations of carbon dioxide and other waste products, while the maternal blood in the intervillous space contains low concentrations of these waste products. As a result, carbon dioxide and other waste products diffuse out of the fetal blood, across the trophoblasts of the chorionic villi and into the maternal blood that surrounds the villi. The mother's lungs, kidneys and liver will easily remove these waste products from her blood. Notice that in the placenta, the maternal and fetal blood does not mix. The two blood supplies remain separated by the fetal trophoblasts and endothelial cells. However, nutrients and waste products, including oxygen and carbon dioxide, are free to diffuse from one blood supply to the other down concentration gradients.
Embryonic trophoblasts are not incorporated into the tissues of the growing fetus, but they are still very important and versatile cells.
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After the chorionic villi are established, many of these trophoblasts fuse together to form very large syncytiotrophoblasts, which are large, multinucleated trophoblasts that are formed by the fusion of several smaller cells. These syncytiotrophoblasts are a main component of the chorionic villi and are the cells that are in direct contact with the maternal blood. Syncytiotrophoblasts are also the major source of most placental hormones, including human chorionic gonadotropin, which is also known as hCG.
Yet other trophoblasts form columns of cells which serve to anchor the placenta to the uterus. These junctional trophoblasts secrete extracellular matrix proteins that help them anchor the placenta to the uterus.
Let's review. The placenta is the composite structure of embryonic and maternal tissues that supply nutrients to the developing embryo. The placenta serves three main functions:
The placenta is composed of both maternal tissue and tissue derived from the embryo. The chorion is the embryonic-derived portion of the placenta. It is composed of fetal blood vessels and trophoblasts which are organized into finger-like structures called chorionic villi. The chorionic villi are surrounded by the intervillous space, which is the part of the placenta surrounding the chorionic villi that contains maternal blood. Maternal blood flows from the mother's circulatory system, through the intervillous space and then re-enters the mother's blood vessels. Nutrients and oxygen diffuse from the maternal blood, into the villi and into the fetal blood. At the same time, carbon dioxide and other waste products diffuse out of the fetal blood, across the trophoblasts of the chorionic villi and into the maternal blood surrounding the villi.
Embryonic trophoblasts are very versatile cells. Some trophoblasts fuse together to form very large syncytiotrophoblasts, which are large, multinucleated trophoblasts that are formed by the fusion of several smaller cells. These syncytiotrophoblasts are a main component of the chorionic villi and are the cells that are in direct contact with the maternal blood. They are also the major source of most placental hormones.
Yet other trophoblasts form columns of cells which serve to anchor the placenta to the uterus. These junctional trophoblasts secrete extracellular matrix proteins that help them anchor the placenta to the uterus.
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