Fetal Membranes

Question 1

Give an overview fetal membranes

Answer 1

• Amnion
The amnion is a membrane that closely covers the embryo when first formed.

• Umbilical vesicle/Yolk sac (development covered in Early Embryology)
The yolk sac is a membranous sac attached to an embryo, formed by cells of the
hypoblast adjacent to the embryonic disk.

• Allantois (development covered in CPR module)
The allantois is an extension of the posterior wall of the yolk sac. It degenerates to form
the median umbilical ligament in the adult.

• Chorion
The chorion is the outermost fetal membrane around the embryo which develops from an outer fold on the surface of the yolk sac.

Question 2

How is the amnion formed?

Answer 2

As implantation of the blastocyst progresses, a small space appears in the embryoblast, which forms the amniotic cavity

• At the same time, morphologic changes occur in the embryoblast that result in the formation of a flat, circular bilaminar plate of cells, the embryonic disc, consisting of two layers
• Epiblast
• Hypoblast

• Soon, amniogenic (amnion-forming) cells, amnioblasts, separate from the epiblast and form the amnion, which forms the roof of the amniotic cavity (the remaining epiblast form the floor of the cavity)

Question 3

What is the source of amniotic fluid?

Answer 3

Source:

• amnion, maternal blood, fetal urine, skin, secretions from respiratory tract

Question 4

What is the composition of amniotic fluid?

Answer 4

mostly water, small amounts of protein, glucose, vitamins, antigens, antibodies etc.

Question 5

What is the circulation of amniotic fluids?

Answer 5

Changes every 3 hours

Question 6

Describe the excretion of amniotic fluid

Answer 6

Fetus swallows the fluid

Question 7

What are the functions of amniotic fluid?

Answer 7

Functions:
• Shock absorber
• Maintains constant temperature
 • Movements
• Symmetrical growth
• Lung development
• Antibacterialactivity
• Hydrostatic wedge

Question 8

What is the amniotic sac?

Answer 8

• As the fetus grows, amniotic fluid volume increases
• Amniotic sac enlarges faster than the chorionic sac
• The amnion eventually fuses with chorion wall thus obliterating chorionic cavity and forming the single amniochorionic membrane
• Fetus now floats in amniotic fluid with cord attaching it to placenta

Question 9

What are the causes of oligohydramnios?

Answer 9

• preterm rupture of amniochorionic membrane
  
  • About 10% of pregnancies
• Urinary system. Anomalies
  
  • Bilateral renal agenesis
• Placental insufficiencies
  
  • Decreased blood circulation

Question 10

What are the functions of polyhydramnios?

Answer 10

• idiopathic (60%)
• maternal (20%)
  
  • multiple pregnancies
  • multiple diabetes
• fetal (20%)
  
  • digestive system anomalies
    
    • esophageal atresia
  • CNS anomalies
    
    • Meroencephaly
    • Anencephaly

Question 11

What is oligohydramnios?

Answer 11

• Low volume
  
  • below 400-500 ml in 3rd trimester
  • inadequate amount

Approximately 4% of pregnancies in US

Compilations

• Lung hypoplasia
• Facial or limb deformities
  
  • due to fetal compression by the uterine wall

Question 12

What is polyhydramnios?

Answer 12

• high volume
  
  • over 2000 ml in 3 rd trimester
  • Excessive amount

Approximately 1% of pregnancies in US

Complications

• preterm labor & delivery
• premature rapture of amniochorionic membrane

Question 13

What is the significance of the primary umbilical vesicle?

Answer 13

Primary umbilical vesicle (Exocoelomic cavity/yolk sac)

• First site of blood formation
• Gives rise to the primordial gut
• Primordial germ cells arise in the endodermal lining of the yolk sac

Question 14

What is allantois?

Answer 14

• Arises as a small diverticulum(outpouching) from the caudal wall of the primitive umbilical vesicle and extends into the body stalk (3rd week)
• Another early site of blood formation (3rd to 5th week)
• Its blood vessels form the umbilical arteries and veins
• Obliterates and forms the urachus (median umbilical ligament postnatally)

Question 15

What is the significance of the chorion?

Answer 15

Extraembryonic (primary) mesoderm
fills the space between the trophoblastic wall & and the amniotic sac and the primary umbilical vesicle. This surrounds the amnion and primary umbilical vesicle.

Question 16

Describe formation of the chorionic cavity

Answer 16

• Clefts appear in the extraembryonic mesoderm called extraembryonic coelomic spaces
• Clefts fuse to form a cavity called the extraembryonic coelom the precursor to the chorionic cavity
• Extraembryonic coelom splits the extraembryonic mesoderm into two layers:
• extraembryonic splanchnic mesoderm  surrounds the umbilical vesicle
• extraembryonic somatic mesodermlines trophoblast and covers amnion

Question 17

What are the components of the chorionic sac and wall?

Answer 17

Chorion sac :
• Chorion wall
• Chorionic cavity
Chorion wall is made of:
• Syncytiotrophoblast
• Cytotrophoblast
• Somatic layer of extraembryonic mesoderm

Question 17

What are the components of the chorionic sac and wall?

Answer 17

Chorion sac :
• Chorion wall
• Chorionic cavity
Chorion wall is made of:
• Syncytiotrophoblast
• Cytotrophoblast
• Somatic layer of extraembryonic mesoderm

Question 18

Describe the development of chorionic villi

Answer 18

• Chorion wall presents finger-like projections called villi
• There is local proliferation of cytotrophoblast cells into the syncytiotrophoblast—> primary chorionic villi
• Penetration of extraembryonic mesoderm into the core of the primary chorionic villisecondary chorionic villi
• Blood vessels develop in the extraembryonic mesoderm core of the 2nd villi tertiary chorionic villi

Question 19

What is the Hydratiform Mole?

Answer 19

• Sometimes the embryo dies and the chorionic villi do not complete their development; that is, they do not become vascularized to form tertiary villi.
• These degenerating villi may form cystic swellings, called hydatidiform moles. These moles exhibit variable degrees of trophoblastic proliferation and produce excessive amounts of human chorionic gonadotropin.
• In about 5% of such cases, these moles develop into malignant trophoblastic lesions, called choriocarcinomas.

Question 20

What is the chorion wall?

Answer 20

• Chorion sac enlarges towards the uterine cavity and the villi on that part of the chorionic wall degenerate. This part of the chorion wall is called the smooth chorion
• Villi on the chorion sac towards the uterine wall grow and become bushy. This part of the chorion wall is called villous chorion
• Intervillous spaces are derived from the lacunae spaces that developed in the syncytiotrophoblast in 2nd week of development (review from Early Embryology DLA)

Question 21

What are the layers of the decidua (endometrium of pregnant uterus)?

Answer 21

• Decidua basalis is the endometrium deep to and in contact with villous chorion
• Decidua capsularis is the endometrium related to the smooth chorion & separates it from the uterine cavity. It overlies the conceptus.

• Decidua parietalis refers to the rest of endometrium not
directly related to the chorion sa

Question 22

Give a general description of the placenta

Answer 22

Placenta
General description:
• Discoid in shape
• 20 cm in diameter
• 3 cm thick
• 1⁄2 kg in weight

Formation:
• Maternal component – formed from the decidua basalis
• Fetal component – formed from the villous chorion. Attached to the maternal part of placenta by a cytotrophoblastic shell
• Cotyledons – placental septa (wedge shaped areas of decidua) project toward the chorionic plate (“roof” of fetal placenta) and divide the fetal parts of the placenta into irregular convex areas

Question 23

What are the functions of the placenta?

Answer 23

• Protection
• Nutrition
• Respiration
• Excretion
• Hormone production

Question 24

Describe the placenta atfter birth

Answer 24

Maternal surface: • rough shows fetal cotyledons demarcated by grooves (formerly occupied by placental septal) Fetal surface: • Smooth, covered with the transparent amniotic membrane • Umbilical cord is attached to this surface • Umbilical vessels - Branches of the vessels ramify on this surface deep to the amnion

Question 25

Describe placental circulation

Answer 25

The intervillous spaces are filled with maternal blood from the spiral arteries (branches of uterine arteries) • Tertiary villi float in the blood in the intervillous space * Gaseous exchange takes place between the maternal blood & blood in fetal capillaries * No intermingling of fetal and maternal blood

Question 26

Describe the placental membrane

Answer 26

* The placental membrane consist of extra fetal tissue separating the maternal and fetal blood. * Initially consists of 4 layers: * Syncytiotrophoblast,cytotrophoblast, connective tissue of the villi, endothelium of fetal capillaries. * After 20 weeks, cytotrophoblast disappear leaving the membrane consisting of 3 layers * In some areas, there is thinning of the membrane where the syncytiotrophoblast comes in direct contact with the fetal endothelium to form a vasculosyncytial placental membrane * The membrane becomes thinner as pregnancy progresses

Question 27

Explain the hemolytic disease of the unknown

Answer 27

Some fetal blood may pass to the maternal blood through microscopic breaks in the placental membrane. If the fetus is Rh-positive and the mother is Rh-negative, the fetal cells may stimulate the formation of anti-Rh antibody by the mother’s immune system. This can cause hemolysis of fetal Rh-positive blood cells and anemia in the fetus. Fetus may die unless delivered early or given transfusions of packed Rh- negative blood cells. Relatively uncommon

Question 28

What is the umbilical cord?

Answer 28

* Short cords can be ruptured * Long cord can twist around the neck * Long cords can form knots * Aberrant number of umbilical vessels

Question 29

Describe the placental abnormalities

Answer 29

* Placenta accreta * Abnormal adherence of chorionic villi to myometrium of the uterine wall. * Placenta increta – chorionic villi invade deeply into the myometrium Placenta percreta • Chorionic villi penetrates the entire uterine wall to and/or through the perimetrium. • 3rd trimester bleeding. * Placenta previa * Implantation close to or overlying the internal os * Placenta bridges the cervical opening. * Severe bleeding in later part of pregnancy

Question 30

What are dizygotic twins?

Answer 30

Most common type. • Origin from 2 zygotes. • Fertilisation of 2 oocytes by 2 sperms. • 2 amnions and 2 chorions. • Placentas may be fused • May be of the same sex or different sexes

Question 31

What are monozygotic twins?

Answer 31

``` Origin from a single zygote • Genetically identical and same sex • Division of embryoblast into two embryonic primordia (occurs at the blastocyst stage) • 2 amniotic sacs • 1 chorionic sac • 1 placenta ```

Question 32

What is twin transfusion syndrome?

Answer 32

``` Twin transfusion syndrome • Preferential shunting of arterial blood in one twin to venous blood in the other. • Donor twin (small and anemic) • Recipient twin (large and polycythemic) ```

Question 33

What are conjoint twins?

Answer 33

Incomplete division of the embryonic disc or fusion of adjacent embryonic discs

Question 34

Describe the maternal component of the placenta

Answer 34

1. Consists of the decidua basalis, which is derived from the endometrium of the uterus located between the blastocyst and the myometrium. 2. The decidua basalis and decidua parietalis (which includes all portions of the endometrium other than the site of implantation) are shed as part of the afterbirth. 3. The decidua capsularis, the portion of endometrium that covers the blastocyst and separates it from the uterine cavity, becomes attenuated and degenerates at week 22 of development because of a reduced blood supply. 4. The maternal surface of the placenta is characterized by 8–10 compartments called cotyledons (imparting a cobblestone appearance), which are separated by decidual (placental) septa. 5. The maternal surface is dark red in color and oozes blood due to torn maternal blood vessels

Question 35

Describe the fetal component of the placenta

Answer 35

1. Consists of tertiary chorionic villi derived from both the trophoblast and extraembryonic mesoderm, which collectively become known as the villous chorion. 2. The villous chorion develops most prolifically at the site of the decidua basalis. The villous chorion is in contrast to an area of no villus development known as the smooth chorion (which is related to the decidua capsularis). 3. The fetal surface of the placenta is characterized by the well-vascularized chorionic plate containing the chorionic (fetal) blood vessels. 4. The fetal surface has a smooth, shiny, light-blue or blue-pink appearance (because the amnion covers the fetal surface), and 5–8 large chorionic (fetal) blood vessels should be appear.