L2: Second week of development

Question 1

what is implantation?

Answer 1

It is the embedding of the blastula in the endometrium.

Question 2

when does implantation take place?

Answer 2

at the end of the first week up to the end of the second week

Question 3

where does implantation take place?

Answer 3

in the endometrium of the posterior wall of the body of the uterus near the fundus.

Question 4

what are the steps of implantation?

Answer 4

1. Degeneration of the zona pellucida:
   - Cause: enzymes of acrosome reaction.
   - Result: exposure of the trophoblast.
2. Differentiation of the trophoblast into 2 layers:
   Inner: cytotrophoblast.
   Outer: syncytiotrophoblast
3. Small spaces called lacunae appear in the syncytiotrophoblast.
4. The syncytiotrophoblast has the following features:
   -Adhesive power: the trophoblast adheres to the endometrium.
   - Histolytic power: by releasing proteolytic enzymes.
5. Lysis of the glands and connective tissue: forms a defect in the endometrium for implantation of the blastula.
6. Lysis of the blood vessels: the blood fills the lacunae.
7. After complete implantation: the defect in the endometrium is filled with fibrin clot then the epithelium regenerates.

Question 5

what are the abnormal sites of implantation?

Answer 5

• intra-uterine (placenta praevia)
• etxra-uterine

Question 6

what are the intra-uterine sites of abnormal implantation?

Answer 6

Implantation in the lower segment of the uterus

-praevia centralis: cover the internal os completely
-praevia lateralis: away from the internal os
-praevia marginales: cover the internal os laterally

Question 7

what are extra-uterine abnormal sites of implantation?

Answer 7

-tubal pregnancy
-ovarian pregnancy
-abdominal (peritoneal) pregnancy

Question 8

what is the site of tubal pregnancy?

Answer 8

the ampulla

Question 9

what are the causes of tubal pregnancy?

Answer 9

-delay in the transport of the blastula
-early loss in zona pelucida

Question 10

what are the features of tubal pregnancy?

Answer 10

-Signs and symptoms of pregnancy

-rupture in the uterine tube which leads to internal hemorrhageand and abdominal pain

Question 11

what may be confused with tubal pregnancy?

Answer 11

appendicitis especially if the tubal pregnancy was in the right tube.

Question 12

what causes ovarian pregnancy?

Answer 12

: it is either 1ry or 2ry
1ry: the fertilized ovum is directly implanted on the ovary.

2ry: rupture tubal and the blastula is re-implanted on the ovary.

Question 13

what is the site of abdominal pregnancy?

Answer 13

on the surface of any abdominal organ.

Question 14

what is the cause of abdominal pregnancy?

Answer 14

usually 2ry to rupture tubal pregnancy.

Question 15

what are the features of abdominal pregnancy?

Answer 15

the placenta penetrates the wall of the organ leading
to internal hemorrhage.

Question 16

what is the decidua?

Answer 16

It is the endometrium of pregnant uterus.

Question 17

what is the structure of decidua?

Answer 17

The endometrium shows decidual reaction (like the secretory phase of the endometrium under the effect of progesterone hormone)

Question 18

what are the parts of decidua?

Answer 18

decidua basalis: Between the blastula and the muscle wall of the uterus

decidua capsularis: covers the blastula

decidua paretalis: lines the rest of the uterus

Question 19

what is the fate of decidua basalis?

Answer 19

forms the maternal part of the placenta

Question 20

what is the fate of decidua paretalis and capsularis?

Answer 20

they fuse and degenerate

Question 21

what are the changes that happen to the blastula in the second week?

Answer 21

During the 2nd week, the following changes occur in the blastula:
- The trophoblast differentiates into 2 layers:
 Syncytiotrophoblast: outer multi-nucleated layer with indistinct cell boundaries.
 Cytotrophoblast: inner mono-nucleated cells with clear cell boundaries.

-The embryoblast differentiates into bilaminar embryonic (germ) disc which is formed of:
 Epiblast: layer of tall columnar cells close to the cytotrophoblast.
 Hypoblast: layer of small cuboidal cells close to the blastocele.

Formation of 2 cavities:
 Amniotic cavity appears in the 7th or 8th day within the epiblast. Its roof is formed by amnioblasts (from the epiblasts), while its floor is formed by the epiblasts.

 Primary yolk sac cavity: in the 9th or 10th day the original blastocele is transformed into 1ry yolk sac. Its roof is formed by the hypoblast, while its sides and floor are formed by Hauser’s membrane (from the hypoblasts)

Question 22

explain the formation of the chorionic vesicle.

Answer 22

 Extraembryonic mesoderm develops (from cytotrophoblasts) between the trophoblast (externally) and both 1ry yolk sac and amniotic cavity (internally).

 Multiple cavities appear in the extraembryonic mesoderm.

 They fuse together forming single large cavity called extraembryonic coelom (chorionic cavity) which divides the extraembryonic into:
- Somatic layer: lines the trophoblast & covers the amniotic cavity.
- Splanchnic (Visceral) layer: covering the 1ry yolk sac.
- Connecting stalk: connecting the 2 layers at the caudal end of the embryo. It is the future umbilical cord.

Question 23

what is the chorion and what is the chorionic vesicle?

Answer 23

The trophoblast with the somatic layer of extraembryonic mesoderm is called chorion and the whole vesicle is now called the chorionic vesicle.