This module describes the structure and the differentiation of the tissues that the fetal membranes and placenta form, from the moment of implantation of the blastocyst into the uterine wall up to the end of the intrauterine development.
The development (10.1) of the extra-embryonic membranes begin at the moment of the differentiation of the blastocyst cells into an embryoblast and a trophoblast. The embryoblast forms the later embryo and the trophoblast the components of the embryonic appendage organs (stage 2)
The forming of the placenta (10.2) is induced by the syncythiothrophoblast of the blastocyst, which triggers the decidual reaction of the uterine wall (stage 5a). This change of the endometrium depends on the stimulation by the hormones released by the ovary and placenta. The differentiation of the placenta begins with the formation of lacunae in the syncythiothrophoblast (stage 5b) that are filled with maternal blood, which stems from the spiral arteries. The feto-placental circulation begins in the 3rd week, when the fetal vessels connect the placenta with the tissues of the embryonic body. Over the course of the pregnancy, the placenta adapts itself to the needs of the growing embryo (development of the placental villi).
In humans, the placenta is hemo-chorial, discoid, pseudo-cotyledonic, decidual and chorio-allantoid.
The placental circulation system (10.3) consists of a fetal and a maternal one, separated from one another by the placental barrier. The barrier controls the metabolic exchange processes between the embryo and mother (10.4). In addition, in the course of a normal pregnancy, the placenta fulfills other important physiologic tasks (e.g., endocrine).
In multiple pregnancies, the development of the fetal membrane is subject to certain changes (one distinguishes between dizygotic and monozygotic twins) (10.5.).
The umbilical cord (10.6) develops from the body stalk. With the development of the amniotic cavity the umbilical cord becomes enveloped by the amniotic epithelium and towards the end of the pregnancy, contains only two umbilical arteries and one umbilical vein. These structures are surrounded by connective tissue that stems from the extra-embryonic mesoblast. The embryonic umbilical cord, which swims in the amniotic cavity, lengthens with the progressive development of the embryo. The cavity is filled with amniotic fluid (10.8) which looks after various mechanical and metabolic functions between the mother and fetus. Bands in the amniotic fluid can lead to fetal abnormalities.
Of the various pathologies, which can influence placental development, we cite (10.9): complications of fetal (fetal erythroblastosis, chorion carcinoma, hydatid mole) or maternal (toxemia of pregnancy, eclampsia, diabetes).
In addition, there exist anomalies that are connected with an abnormal implantation location (ectopic pregnancy) as well as with the development of the placenta (placenta praevia). The insertion location of the umbilical cord at the placenta can also vary (marginal or eccentric insertion). All these anomalies lead either to an abnormal development of the fetus, complications at delivery, or to a miscarriage.