Chapter 2 How Do the Placenta and Fetal Membranes Form?

9 Chapter 2 How do the placenta and fetal membranes form? Chorionic cavity Amniotic cavity Decidua basalis What are the fetal membranes 9 (basal plate) Development of the uteroplacental circulation 10 Further development of chorionic villi 10 Chorion frondosum (chorionic plate) Formation of the placenta and placental circulation 11 Placental membrane and placental functions 12 Structure of the full-term placenta 12 The umbilical cord 13 Twins and their fetal membranes 13 During fertilization and initial formation of the blastocyst the early embryo receives its nutrition by diffusion through the zona pellucida from the accumulated fl uid found in the blastocoele. However, after the blastocyst has hatched from the zona pellucida, allowing attachment to the uterine epithelium on the fi fth or sixth day after fertilization, the embryo grows faster, thus the need for a more effi cient method of nutrition becomes essential. From 12 days until full term, the developing embryo and fetus obtain their nutrition from maternal blood. This is achieved by the formation of a placenta and the development of the Chorion laevae uteroplacental circulation. In response to the circulating progesterone and to the Decidua capsularis blastocyst, the stromal cells of the endometrium become large and accumulate glycogen. At 12 days the Decidua parietalis syncytiotrophoblast of the blastocyst begins to erode the endometrium and the maternal vessels at the implantation site become congested and dilated. These cellular changes, together with an increase in endometrial vascularization, are known as the decidual reaction. Within a few days the decidual reaction spreads throughout the endometrium, which now is known as the decidua. As the implanted Fig. 2.1 Development of the deciduae at 12 weeks. embryo bulges into the uterine lumen the decidua becomes identifi able as three discrete areas at the implantation site (Fig. 2.1). The decidua underlying the embryo is called the decidua basalis, which forms the maternal face of the is a double-layered membrane formed by the trophoblast placenta. The decidua capsularis lines the superfi cial part and the extra-embryonic mesoderm, which eventually will of the embryo bulging into the uterine lumen and the give rise to the fetal part of the placenta. From 12 days until remainder of the decidua is called the decidua parietalis. the end of embryonic period the developing embryo is suspended in the chorionic cavity. The expansion of the amniotic sac obliterates the chorionic cavity and the only What are the fetal membranes? connection between the embryo and the chorion is via a thick plate of mesoderm called the connecting stalk. The term fetal membrane is applied to those structures The yolk sac and its diverticulum, the allantois, are the derived from the blastocyst which do not contribute to the major means of nutritional exchange mechanisms in other embryo. The amnion, the chorion, the yolk sac and mammals. However, in humans the yolk reserves are poor, the allantois make up the fetal membranes (Fig. 2.2). The and part of the yolk sac is incorporated into the embryo to amnion lines the amniotic sac and protects the embryo form the gut tube. The allantois, which serves as a reservoir from physical injury. The amniotic sac enlarges rapidly due for fetal urine in other mammals, becomes attached to the to an increase in the volume of amniotic fl uid. The chorion urinary bladder..

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