Chapter 2 How do the placenta and fetal membranes form?
In response to the circulating progesterone and to the blastocyst, the stromal cells of the endometrium become large and accumulate glycogen. At 12 days the 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 embryo bulges into the uterine lumen the decidua becomes identifiable 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 placenta. The decidua capsularis lines the superficial part of the embryo bulging into the uterine lumen and the remainder of the decidua is called the decidua parietalis.
What are the fetal membranes?
The term fetal membrane is applied to those structures derived from the blastocyst which do not contribute to the embryo. The amnion, the chorion, the yolk sac and the allantois make up the fetal membranes (Fig. 2.2). The amnion lines the amniotic sac and protects the embryo from physical injury. The amniotic sac enlarges rapidly due to an increase in the volume of amniotic fluid. The chorion is a double-layered membrane formed by the trophoblast and the extra-embryonic mesoderm, which eventually will give rise to the fetal part of the placenta. From 12 days until 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 connection between the embryo and the chorion is via a thick plate of mesoderm called the connecting stalk.
Clinical boxAmniotic cavity disorders
Amniotic fluid serves as a cushion for the embryo and fetus, and prevents attachments between the skin of the embryo and surrounding tissues. In a condition called oligohydramnios, the volume of amniotic fluid is too low, resulting in compression of the fetus by the uterine wall. If the amniotic fluid is in excess, then a polyhydramnios results (see Chapter 5). Tears or ruptures in the amnion result in amniotic bands forming which can surround limbs and may result in various fetal anomalies.
Development of the uteroplacental circulation
The umbilical arteries carry the deoxygenated blood from the embryo to the chorion and umbilical veins return the oxygenated blood to the embryo. The uterine vessels supply the maternal blood to the trophoblastic lacunae. When the cytotrophoblast cells grow into the lacunae as the primary chorionic villi, the primitive uteroplacental circulation is established. The initial formation of primary chorionic villi is explained in Chapter 1.
Further development of chorionic villi
From day 15, the primary chorionic villi begin to branch, and the extra-embryonic (chorionic) mesoderm invades the core of the primary villi, thus converting them into secondary chorionic villi (Fig. 2.3). The secondary villi line the entire surface of the chorion, and soon blood vessels develop within the mesenchyme of these villi that connect to the umbilical vessels in the embryo. Villi containing blood vessels are called tertiary villi (Fig. 2.4).
