Embryo: Second Week Of Development

Question 0

What 2 tissues does the trophoblast form?

Answer 0

1. Cytotrophoblast

2. Syncytiotrophoblast

Question 1

What are the rules of 2?

Answer 1

1. Embryoblast –> 2 germ layers
2. Trophoblast forms 2 tissues
3. Blastocyst cavity remodeled twice
4. Two new cavities formed
5. Extraembryonic mesoderm splits into 2 layers

Question 2

What is syncytiotrophoblast?

Answer 2

Invasive, digestive, ingestive

Question 3

What is cytotrophoblast?

Answer 3

Highly mitotic; contributes cells to syncytiotrophoblast

Question 4

What is placenta previa?

Answer 4

Pregnancy in which the placenta is implanted in the lower part of the uterus (instead of the upper part)

Question 5

What is an ectopic pregnancy?

Answer 5

Implantation of the fertilized egg outside the uterine cavity, often in the tube or ovary, or rarely in the abdominal cavity

Question 6

What causes a hydatidiform mole?

Answer 6

1. Trophoblast only; no embryonic tissue
2. Paternal genes only expressed
3. Produce high levels of hCG
4. May produce benign or malignant tumors

Question 7

What does the embryoblast become during the second week?

Answer 7

Bilaminar Germ Disc (at about day 8)

Question 8

The embryoblast forms what 2 layers?

Answer 8

1. Epiblast

2. Hypoblast

Question 9

What is the epiblast?

Answer 9

The primary ectoderm that differentiates into the endoderm, mesoderm, and ectoderm during Gastrulation
- has columnar cells and an amniotic cavity with epiblast/amnioblasts

Question 10

What is the hypoblast?

Answer 10

1. Cuboidal cells
2. Extraembryonic endoderm lines blastocyst (Heuser’s membrane); day 9
3. This is the primary yolk sac that eventually gets pushed aside
4. Definitive (secondary) yolk sac (day 13) forms smaller than primary

Question 11

What is the name of the remnants of the primary yolk sac?

Answer 11

Exocoelomic cysts

Question 12

What is the yolk sac for?

Answer 12

To provide nutrition for the embryo

Question 13

What is the exocoelomic cavity?

Answer 13

Formed from additional cells of the hypoblast that migrate along the inside of the exocoelomic membrane and form a new cavity

Question 14

What is the name of the vacuoles in the syncytiotrophoblast?

Answer 14

Lacunae primordial of intervillous spaces of placenta (day 9); concentrated at embryonic pole
- they act as barriers to maternal blood - are all fetal in origin

Question 15

What happens to the endometrium during week 2?

Answer 15

1. Blood vessels dilate to form sinusoids

2. Erosion of vessels and glands releases contents into lacunae

Question 16

What happens to the cytotrophoblast during week 2?

Answer 16

1. Forms primary villi; columns of cells extend into lacunae (day 13)
2. Induced by developing extraembryonic mesoderm

Question 17

What are the primary villi?

Answer 17

Extensions of cytotrophoblast grow out into the blood filled lacunae and carry with them a covering of syncytiotrophoblasts

Question 18

Where is the extraembryonic mesoderm?

Answer 18

In between the embryoblasts and trophoblast cells

Question 19

What happens to the extraembryonic mesoderm as it grows?

Answer 19

It splits in the middle and forms 2 layers

- layers form chorionic cavity

Question 20

What is the chorionic cavity?

Answer 20

Surrounds the amniotic cavity; the yolk sac is within the chorionic cavity
- it separates where the embryo is going to grow from where the placenta will be located

Question 21

What does the connecting stalk do?

Answer 21

Connects embryo properly to the developing placenta and becomes the umbilical cord

1. Somatopleuric mesoderm connects germ disc to lining of trophoblast (chorionic plate)
   - will become umbilicus

Question 22

What is the chorionic villi?

Answer 22

These are finger-like projections that form the fetal portion of the placenta

1. Primary = only trophoblast cells (day 13)
2. Secondary = core of mesoderm (day 16)
3. Tertiary = contain blood vessels (3rd week)

Question 23

How is the blastocyst cavity remodeled twice?

Answer 23

Primary and secondary yolk sac

Question 24

What are the 2 cavities formed?

Answer 24

1. Amnionic cavity - forms in the epiblast

2. Chorionic cavity - forms within the extraembryonic mesoderm

Question 25

Where does the extraembryonic mesoderm split into different layers?

Answer 25

Between the embryoblast and trophoblast

- this forms the chorionic cavity and connecting stalk