Development of the placenta

Question 1

Describe the function of the placenta

Answer 1

Facilitates nutrients and gas exchange between maternal and fetal compartments

Question 2

Describe the composition of the placenta

Answer 2

1) Fetal portion (chorionic frondosum)

2) Maternal portion (decidua basalis)

Question 3

Placental tissue is derived from which embryonic structure?

Answer 3

Trophoblasts, and the syncytial lacunae become continuous with the sinusoids (forming the uteroplacental circulation)

Question 4

How is the cytotrophoblastic shell formed?

Answer 4

Once the syncytiotrophoblast invaginates the endometrium they forms villi’s with the maternal sinusoids, Then the cytotrophoblast cells penetrate progressively into the syncytium reaching the endometrium coming into contact with close villous systems forming the thin outer cytotrophoblastic shell

Question 5

How is the villi formed by the “embryo”?

Answer 5

1) Primary villi (cytotrophoblast covered by syncytiotrophoblasts)

2) Secondary villi (when mesodermal cells penetrate the primary villi)

3) Tertiary villi (when blood vessels appear within the secondary villi)

1. The extraembryonic mesoderm surrounding blood vessels will disappear gradually which will result in the blood vessels coming closer to the cytotrophoblast layer
2. Syncytiotrophoblast in between the maternal sinusoids and the cytotrophoblast will become very thin
   The barrier left is 2 layers only which are: the syncytiotrophoblast & the endothelium of the blood vessel

Question 6

What are the types of villi found in the embryo?

Answer 6

• Cytotrophoblast “shell” – syncytiotrophoblast – cytotrophoblast – extraembryonic mesoderm

1) Anchoring villi (villi that extends from the chorionic plate to the decidua basalis)

2) Terminal villi (villi that extend from the anchoring villi, to be closer to the lacuna)

Question 7

What is the difference between the villi developing in the embryonic pole and the one developing in the abembryonic pole?

Answer 7

1) In the embryonic pole villi continue to grow and expand forming the chorionic frondosum, which will later form the placenta

2) In the abembryonic pole there are less villi and they are thinner, forming the chorion leave that will later disappear and won’t be part of the placenta

Question 8

What is a decidua?

Answer 8

it is the endometrium after implantation

Question 9

Describe the decidual reaction

Answer 9

1) Cells of the endometrium become polyhedral
2) Loaded with glycogen and lipids
3) Tissue becomes edematous

Question 10

What are the types of deciduas?

Answer 10

1) Decidua basalis (surrounds the chorion frondusom that will form the maternal portion of the placenta)
2) Decidua capsularis (capsules the embryo from the abembryonic pole)
3) Decidua parietalis (remaining portion of the endometrium)

Later on in pregnancy the decidua capsularis and decidua parietalis will fuse (amniotic membrane will fuse with chorionic leave)

Question 11

What is a placenta previa?

Answer 11

It is when the placenta covers the internal OS, which causes repeated bleeding episodes, and the baby must be delivered by a C-section

Question 11

Describe the structure of a full-term placenta

Answer 11

From inside to outside:
Anchoring villi – terminal villi – syncytiotrophoblast – cytotrophoblast - Decidua basalis

• Decidua basalis sends down (deeper into the placenta) projections called decidual septum (like a wall dividing a building into rooms) which causes compartmentalization of the placenta
• Compartmentalization of the placenta: called cotyledons, if one compartment of the placenta is not working the rest can make up for it

there are typically 15-20 cotyledons in a placenta

Question 11

What are the types of twin conception?

Answer 11

1) Dizygotic twins (two separate ovum that are fertilized by two sperms – two zygotes, from LH & FSH treatment)

2) Monozygotic twins (genetically identical, one ovum fertilized by two sperms)

Question 12

Describe a dizygotic twin

Answer 12

TWO ZYGOTES

1) Fusion
- One placenta and chorion
- Two umbilical cords
- Two amniotic sac

2) Non-fusion
- 2 Placenta
- 2 Chorion
- 2 Amniotic sacs
- 2 Umbilical cords

Question 12

Describe a monozygotic twin

Answer 12

One ovum that is fertilized by two sperms divides later in one of three different stages:
- ONE ZYGOTE

1) Two cell stage
- Two placenta
- Two chorion
- Two amniotic cavities
- Two umbilical cords
- Similar to non-fusion monozygotic twins

2) Early blastocyst
- 1 placenta
- 1 chorion
- 2 amniotic cavities
- 2 umbilical cords
- Resembles fused dizygotic twins

3) Bilaminar disk
- One placenta
- One chorion
- One amniotic cavity
- Two umbilical cords
- Before the primitive streak forms, the epiblast and hypoblast will fight and make two groups that will move to different sides – in this case, the embryos may fuse with each other and result in conjoined twins

Question 13

During a later period of pregnancy, the maternal blood is separated from the fetal blood by the?

Answer 13

Syncytiotrophoblast and fetal endothelium