Embryology Course - Session 4

8/6/2019 Embryology Course - Session 4

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Left-out Materialsy 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 universityy T he 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 aloney T he only materials discussed in this session regarding this chapter are:

y Changes in the Trophoblasty T he Placenta (only the essential topics)y T he Umbilical Cordy Twinning (only an outline)



Changes in the Trophoblasty 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



Changes in the Trophoblasty Initially the barrier between maternal and fetal blood is composed of four layers: syncytium ,

cytotrophoblast , mesoderm (connective tissue), and capillary endothelium

y B y the beginning of the forth month, cytotrophoblast cells and some connective tissue begin todisappear , progressing from the smaller to the larger villi, and thus the barrier is reduced to thesyncytium and the capillary endothelium only



Chorion and Deciduay 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 smoothy 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 ; withfurther development the capsularis stretches and degenerates thus the chorion laeve comes intocontact with the oppose uterine wall, the decidua parietalis ; the amnion also enlarges and fuses withthe chorionic cavity to form the amniochorionic membrane which ruptures during labor



Chorion and Decidua



F usion of Amniotic and Chorionic Cavitiesy The amniotic cavity enlarges further and fuses with the chorionic cavity to form the amniochorionic

membrane which ruptures during labor



T he Placentay B y 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 placentainto cotyledons ; the lacunae remain in contact



Placental Circulationy H igh 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



Ex change through Placenta



Amnion and Umbilical Cordy 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

y F urther enlargement of the amnion causes it to surround all the structures together and give them anamniotic lining , the primitive umbilical cord , and later completely obliterates the chorionic cavity

y Following the physiological herniation and obliteration of the yolk sac and its vessels, the onlycontents of the umbilical cord are the umbilical vessels covered by Wharton s jelly



Twinningy Twins are either dizygotic (fraternal) or monozygotic (identical)y Dizygotic twins result from simultaneous ovulation of two oocytes and their fertilization by two

different spermatozoa ; they are not similar in their features



Dizygotic Twinsy Dizygotic twins result from simultaneous ovulation of two oocytes and their

fertilization by two different spermatozoa ; they are not similar in their features



Twinningy Monozygotic twins result from the splitting of a single zygote at various stages: two-cell stage ,

blastocyst stage , or the bilaminar germ disc stage



Monozygotic Twinsy Monozygotic twins result from the splitting of a single zygote at various stages such as the blastocyst

stage ; mi x ing of placental vasculature can cause erythrocyte mosaicism



B irth Defects and Prenatal Diagnosisy Regarding chapter 8, a good study scheme can be proposed as the following:

y T he whole chapter should be read and understood with keeping the headlines and general topics in mindy

T he subjects Types of Abnormalities and Principles of Teratology should be well-memorizedy S pecial 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 thequestions of the chapter, at the back of the book

y A strategy should be built for trying to memorize the table Teratogens Associated with H umanMalformations and linking its items to the longer e x planations given in the te x t that follows it

Embryology Course - Session 4

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