The document summarizes key topics from a chapter that will be discussed in a class session, including changes in the trophoblast, the placenta, umbilical cord, and twinning. For trophoblast changes, it notes that cytotrophoblast cells invade maternal arteries and convert them to increase blood flow, and that the barrier between maternal and fetal blood is initially four layers but is reduced to two layers by the fourth month. For the placenta, it describes its two components - fetal chorion frondosum and maternal decidua basalis - and notes that decidual septa penetrate the placenta and divide it into cotyledons. It also briefly outlines placental circulation and exchange. For twinning

2. Left-out Materials
 Due to the nature of the chapter, which is easy to understand and has no role in
understanding the other chapters, most of the material of this chapter will not be
discussed in our session but they are still included at university
 The reason is that here we are mainly focusing the subjects that link to each other and
those that may be difficult to understand by reading the book alone
 The only materials discussed in this session regarding this chapter are:
 Changes in the Trophoblast
 The Placenta (only the essential topics)
 The Umbilical Cord
 Twinning (only an outline)

3. Changes in the Trophoblast
 Cytotrophoblast cells from the ends of the stem villi invade maternal spiral arteries and undergo an
epithelial-to-endothelial transition to convert the small-diameter high-resistance arteries to large-
diameter low-resistance vessels to increase blood flow

4. Changes in the Trophoblast
 Initially the barrier between maternal and fetal blood is composed of four layers: syncytium,
cytotrophoblast, mesoderm (connective tissue), and capillary endothelium
 By the beginning of the forth month, cytotrophoblast cells and some connective tissue begin to
disappear, progressing from the smaller to the larger villi, and thus the barrier is reduced to the
syncytium and the capillary endothelium only

5. Chorion and Decidua
 With further development, chorionic villi on the embryonic pole grow further and this part of the
chorion is called chorion frondosum, while those on the abembryonic pole degenerate and this part is
called chorion laeve and it is smooth
 The decidua is the functional part of the uterine wall shed during parturition; over the chorion
frondosum it is called decidua basalis, while over the chorion laeve it is called decidua capsularis; with
further development the capsularis stretches and degenerates thus the chorion laeve comes into
contact with the oppose uterine wall, the decidua parietalis; the amnion also enlarges and fuses with
the chorionic cavity to form the amniochorionic membrane which ruptures during labor

6. Chorion and Decidua

7. Fusion of Amniotic and Chorionic Cavities
 The amniotic cavity enlarges further and fuses with the chorionic cavity to form the amniochorionic
membrane which ruptures during labor

8. The Placenta
 By the forth month, the placenta has two components: fetal component (chorion frondosum) and
maternal component (decidua basalis); portions of the decidua basalis (called decidual septa)
penetrate down into the lacunae (without reaching the chorionic plate) and these divide the placenta
into cotyledons; the lacunae remain in contact

9. Placental Circulation
 High blood pressure in endometrial arteries pours blood into the lacunae (or intervillous spaces) and
bathes the villi; negative pressure pushes the blood from the lacunae into the endometrial veins

10. Exchange through Placenta

11. Amnion and Umbilical Cord
 The primitive umbilical ring is the oval line of attachment of the amnion to the embryo’s ectoderm;
around the fifth week, it transmits: the connecting stalk, the yolk stalk, and the communication of
extra- and intraembryonic cavities; the yolk sac is still in the chorionic cavity
 Further enlargement of the amnion causes it to surround all the structures together and give them an
amniotic lining, the primitive umbilical cord, and later completely obliterates the chorionic cavity
 Following the physiological herniation and obliteration of the yolk sac and its vessels, the only
contents of the umbilical cord are the umbilical vessels covered by Wharton’s jelly

12. Twinning
 Twins are either dizygotic (fraternal) or monozygotic (identical)
 Dizygotic twins result from simultaneous ovulation of two oocytes and their fertilization by two
different spermatozoa; they are not similar in their features

13. Dizygotic Twins
 Dizygotic twins result from simultaneous ovulation of two oocytes and their
fertilization by two different spermatozoa; they are not similar in their features

14. Twinning
 Monozygotic twins result from the splitting of a single zygote at various stages: two-cell stage,
blastocyst stage, or the bilaminar germ disc stage

15. Monozygotic Twins
 Monozygotic twins result from the splitting of a single zygote at various stages such as the blastocyst
stage; mixing of placental vasculature can cause erythrocyte mosaicism

16. Birth Defects and Prenatal Diagnosis
 Regarding chapter 8, a good study scheme can be proposed as the following:
 The whole chapter should be read and understood with keeping the headlines and general topics in mind
 The subjects “Types of Abnormalities” and “Principles of Teratology” should be well-memorized
 Special attention should be given to the topic of the indications for prenatal screening, about which
information can be found at the end of the topic “Chorionic Villus Sampling” and in the answer to one of the
questions of the chapter, at the back of the book
 A strategy should be built for trying to memorize the table “Teratogens Associated with Human
Malformations” and linking its items to the longer explanations given in the text that follows it