Chapter 3: Second Week Of Development

Question 1

What is implantation?

Answer 1

Process by which the blastocyst becomes embedded in the superficial layer of the endometrium.

Question 2

What is the time of implantation?

Answer 2

Starts at the 7th day and is completed on the 11th day.

Question 3

Site of implantation

Answer 3

In the endometrium of the upper part of the posterior wall of the uterus (near fundus). It occurs in the upper part of the anterior part less frequently.

Question 4

What kind of condition is the endometrium is at during implantation?

Answer 4

Endometrium (lining of uterus) is at the secretory phase of the endometrial (menstrual) cycle. This phase is characterized by:
1. Increased thickness of endometrium.
2. Increases number and size of endometrial cells.
3. Glands are spiral and rich with secretion.
4. Arteries are spiral showing arterio-venous anastomoses.

Question 5

Step 1 of implantation

Answer 5

Starts by adhesion of blastocyst by its embryonic pole to the endometrium at the implantation site.

Question 6

Step 2 of implantation

Answer 6

Trophoblast proliferate at the embryonic pole to form a new layer of cells that has no cell membranes called syncytiotrophoblast. This layer secretes proteolytic enzymes that erode the endometrium and form an implantation cavity.

Question 7

Step 3 implantation

Answer 7

Blastocyst becomes embedded inside the implantation cavity.

Question 8

Step 4 implantation

Answer 8

At the 9th day, the site of penetration by the blastocyst becomes blocked by fibrin clot.

Question 9

Step 5 implantation

Answer 9

On the 11th day, endometrial epithelium overgrows and covers the fibrin clot.

Question 10

What are the changes of the blastocyst during implantation?

Answer 10

1. Trophoblast differentiates into the outer synctiotrophoblast and inner cytotrophoblast, starting at the embryonic pole then extends all over the blastocyst.
2. Formation of amniotic and yolk sac cavities.
3. Formation of double layered embryonic disc (epiblast and hypoblast).

Question 11

Abnormal sites of implantation

Answer 11

Placenta Previa and Ectopic Pregnancy.

Question 12

Placenta Previa

Answer 12

Implantation occurs at the lower segment of the uterus. There are 3 types:
1. Placenta Previa Parietalis.
2. Placenta Previa Marginalis.
3. Placenta Previa Centralis.

Question 13

Placenta Previa Parietalis

Answer 13

The margin of the placenta is near the internal os.

Question 14

Placenta Previa Marginalis

Answer 14

The margin of the placenta covers the internal os.

Question 15

Internal os

Answer 15

The inner opening of the cervical canal of the uterus.

Question 16

Why is Placenta Previa life threatening?

Answer 16

1. It leads to antepartum (before birth) maternal hemorrhage.
2. May lead to death of fetus.

Question 17

What is a recommendation treatment for Placenta Previa.

Answer 17

Cesarean section is highly recommended in case of placenta Previa.

Question 18

Ecoptic Pregnancy

Answer 18

Blastocyst is abnormally implanted outdid the uterus. It has three sites:
1. Tubal.
2. Ovarian.
3. Omental.

Question 19

Tubal

Answer 19

Uterine tube, which occurs in ampulla, isthmus, or intramural parts. In this case, rupture of the tube is expected at the 8th week of the pregnancy leading to severe internal hemorrhage.

Question 20

Ovarian

Answer 20

Surface of the ovary.

Question 21

Omental

Answer 21

Peritoneum (abdominal or pelvic).

Question 22

Decidua

Answer 22

The endometrium of the uterus after implantation of the blastocyst. It is called decidua because it sheds during labor. It is exaggerated secretory phase of the endometrium.
- Latin word deciduus, meaning falling off or shedding.

Question 23

Features of Decidua

Answer 23

1. Increased thickness of the endometrium.
2. Increased number and size of endometrial cells.
3. Glands become spiral and show arterio-venous anastomoses.

Question 24

Parts of the Decidua

Answer 24

1. Decidua basalis.
2. Decidua capsularis.
3. Decidua Parietalis.

Question 25

Decidua basalis

Answer 25

Part of the decidua that lies between the implanted embryo and myometrium.

Question 26

Decidua capsularis

Answer 26

Part of Decidua that covers the embryo, separating the embryo from the uterine cavity.

Question 27

Decidua Parietalis

Answer 27

Part of Decidua that lines the rest of the uterine cavity.

Question 28

Fate of Decidua

Answer 28

1. Decidua basalis: persists as the maternal part of the placenta and is known as the decidual plate.
2. Decidua capsularis and Parietalism come in contact and fuse together obliterating the uterine cavity. They degenerate at last.

Question 29

8th day of the second gestational week

Answer 29

1. Blastocyst is partially implanted.
2. Trophoblast layer differentiates into inner cytotrophoblast and outer synctiotrophoblast, starting at the embroyonic pole of the blastocyst.
3. Cells of the inner cell mass (facing the blastocele) become cuboidal to form a layer known as hypoblast.
4. Amniotic cavity appears within the cells of the embryoblast. The cells adjacent to cytotrophoblast secrete amniotic fluid and are known as amino last cells. Amniotic cavity separates between the amnioblast and epiblast of the embryonic disc.
5. Epiblast and hypoblast form a bilaminar embryonic disc.

Question 30

9th and 10th days of the second gestational week

Answer 30

1. Blastocyst is completely embedded inside the endometrium.
2. Site of penetration is closed by fibrin clot.
3. Flat cells (Heuser’s membrane) from the hypoblast line the blastocele, which is transformed into primary yolk sac.
4. Synctiotrophoblast extends to surround the cytotrophoblast of the whole blastocyst.
5. Lacunae (empty spaces) appear in the synctiotrophoblast (lacunar stage).

Question 31

11th and 12th days of the second gestational week

Answer 31

1. Endometrial epithelium grows and covers the fibrin clot (implantation is completed).
2. Lacunae are filled with maternal blood to form utero-placental circulation.
3. Extra-embryonic mesoderm is formed from the wall of yolk sac. It is formed between cytotrophoblast (externally) and embryonic disc with amniotic and yolk sac cavities (internally). Multiple spaces appear in the extra embryonic mesoderm.

Question 32

13th day of the second gestational week

Answer 32

1. Fusion between spaces in the extraembryonic mesoderm leads to formation of extraembryonic coelom (chorionic cavity). Now the blastocyst is termed chorionic vesicles and it’s wall is called chorion. The chorionic cavity separated between the somatic (somatopleuric) and splanchnic (splanchnopleuric) mesoderm.
   Somatic mesoderm lines the cytotrophoblast and covers the amniotic cavity while splanchnic mesoderm covers the yolk sac.
2. Connecting stalk is part of the extraembryonic mesoderm that connects the caudal end of the embryonic with the chorion.
3. New generation of cells from the hypoblast line the primary yolk sac to form the secondary yolk sac.
4. Pinching off a large part of the secondary yolk sac with marked reduction of its size.
5. Formation of allantois that extends from caudal wall of yolk sac to the connecting stalk.
6. Primary chorionic villi start to appear from the chorion.

Question 33

Chorion

Answer 33

Wall of chorionic vesicle that secretes the chorionic gonadotropin (CG) which maintains the corpus luteum (source of progesterone in the ovary) for 4 months.

Question 34

What is the chorion formed of?xxx

Answer 34

1. Somatic extraembryonic mesoderm.
2. Cytotrophoblast.
3. Synctiotrophoblast.