Early Embryogenesis

Question 1

What is embryogenesis?

Answer 1

Embryogenesis is the development of the zygote up to the end of eight week of intra-uterine life

Question 2

Early embryogenesis begins with the formation of ………………, and ends when …………….. is formed

Answer 2

Early embryogenesis begins with the formation of zygote and ends when the primitive streak forms

Question 3

Implantation is completed by the end of the ……… week of development

Answer 3

Implantation is completed by the end of the second week of development

Question 4

Blastocyst, partially embedded in the endometrial stroma on day ……..

Answer 4

8

Question 5

Trophoblast in the embryonic pole differentiates into cytotrophoblast and syncytio-trophoblast on day ……..

Answer 5

8

Question 6

The inner cell mass or embryoblast differentiates into hypoblast layer (small cuboidal cells) and epiblast layer (high columnar cells) on the ……….day

Answer 6

8th

Question 7

The two layers of the inner cell mass form a flat disc, hence the bilaminar germ disc on the ………. day

Answer 7

8th

Question 8

The amniotic cavity develops within the epiblast on the ……… day

Answer 8

8th

Question 9

The amnioblasts derived from the epiblast and epiblast form the wall of the amniotic cavity on day ………

Answer 9

8

Question 10

Outline the developmental changes that occur on the 8th day of IUL

Answer 10

• Blastocyst, partially embedded in the endometrial stroma.
• Trophoblast in the embryonic pole differentiates into cytotrophoblast and syncytio-trophoblast.
• Inner cell mass or embryoblast differentiates into hypoblast layer (small cuboidal cells) and epiblast layer (high columnar cells).
• The two layers of the inner cell mass form a flat disc, hence the bilaminar germ disc
• Amniotic cavity develops within the epiblast.
• Amnioblasts derived from the epiblast and epiblast form the wall of the amniotic cavity.

Question 11

the extraembryonic somatic mesoderm and the two trophoblast layers collectively form ……….

Answer 11

Chorionic plate

Question 12

The ………….. later becomes the umbilical cord

Answer 12

connecting stalk

Question 13

Growth of the bilaminar germ disc is relatively slow in comparison with the trophoblast, true or false

Answer 13

True

Question 14

What is the size of the bilaminar germ disc during early embryogenesis?

Answer 14

The disc remains very small, 0.1 to 0.2mm, during the early embryogenesis

Question 15

The blastocyst becomes more deeply embedded in the endometrium on day …….

Answer 15

9

Question 16

…………… closes the defect at the implantation site.

Answer 16

Fibrin coagulum

Question 17

Vacuoles appear in the syncytio-trophoblast layer at the embryonic pole on day ……

Answer 17

9

Question 18

The vacuoles fuse to form large lacuna on day …….

Answer 18

9

Question 19

How is the primitive yolk sac (excoelomic cavity) formed?

Answer 19

Primitive yolk sac (exocoelomic cavity) forms at the embryonic pole by flattened cells probably derived from the hyboblast
The flattened cells form a thin membrane called the Heuser’s membrane or exocoelomic membrane.
The membrane & the hyboblast form the wall of the primitive yolk sac.

Question 20

The Heuser’s membrane and the hyboblast form ………..

Answer 20

the wall of the primitive yolk sac.

Question 21

Outline the events that occur on the 9th day of IUL

Answer 21

Blastocyst , more deeply embedded in the endometrium .

Fibrin coagulum closes the defect at the implantation site.

Vacuoles appear in the syncytio-trophoblast layer at the embryonic pole.

The vacuoles fuse to form large lacuna – the lacunae stage of placental development

Primitive yolk sac (exocoelomic cavity) forms at the embryonic pole by flattened cells probably derived from the hyboblast
The flattened cells form a thin membrane called the Heuser’s membrane or exocoelomic membrane.
The membrane & the hyboblast form the wall of the primitive yolk sac.

Question 22

Exocoelomic cysts refer to ………..

Answer 22

portions of the primitive yolk sac pinched off into the chorionic cavity.

Question 23

What are the forms of abnormal blastocysts?

Answer 23

• Some consist of syncytium only
• Others show trophoblastic hypoplasia
• Abnormal orientation of the germ disc
• Absence of embryoblast (blighted ovum)

Question 24

Mention the features of hyadatiform mole

Answer 24

• Little or no embryonic tissue
• Trophoblast develops and forms placental membrane
• Benign tumour may be produced by the mole
• Or malignant or invasive mole/choriocarcinoma may develop

Question 25

Genetic analysis of the moles shows that ……………..

Answer 25

most moles arise from fertilization of an oocyte without a nucleus (duplication of the male chromosomes)

Question 26

Microdeletion on chromosomes 15 leads to ………

Answer 26

• Prader - Willi syndrome - when inherited from the father
• Angelman’s Syndrome - when inherited from the mother.

Question 27

The uteroplacental circulation is established on ……….

Answer 27

Beteween day 11-12

Question 28

When does the blastocysts become completely embedded in the endometrial stroma?

Answer 28

Beteween day 11 and 12

Question 29

Site of implatation, almost covered by the surface epithelium on …………

Answer 29

The period between day 11 and 12

Question 30

The blastocyst slightly protrudes into the endometrial cavity on ……..

Answer 30

The period between day 11 and 12

Question 31

How is a communication is established between the lacunae?

Answer 31

The syncytial lacunae become continuous with the congested and dilated maternal capillaries (maternal sinusoids).
Syncytio-trophoblast cells erode maternal sinusoids.
Then blood enters the lacunae system leading to establishment of utero-placental circulation.

Question 32

The lacunae system is established on ……….

Answer 32

The period beteween day 11 and 12

Question 33

Where is the extraembryonic mesoderm formed?

Answer 33

Extraembryonic mesoderm forms between the outer surface of the primitive yolk sac and the inner surface of the cytotrophoblast

Question 34

When is the extraembryonic mesoderm formed?

Answer 34

Between day 11 and 12

Question 35

……………….. forms when vacuoles that formed within the extra-embryonic mesoderm coalesce.

Answer 35

Chorionic cavity or extra-embryonic cavity

Question 36

The chorionic cavity surrounds both the amniotic cavity and the primitive yolk sac except at a point called ……….

Answer 36

The connecting stalk

Question 37

where is the bilaminar germ disc connected to the trophoblast layer?

Answer 37

At a point called the connecting stalk

Question 38

Extraembryonic somatic mesoderm can be defined as?

Answer 38

the extraembryonic mesoderm covering the amnion

Question 39

…………… is the extraembryonic mesoderm covering the yolk sac

Answer 39

Extraembryonic splanchnic mesoderm

Question 40

The decidual reaction occurs on …….

Answer 40

The period between day 11 and 12

Question 41

Describe the decidual reaction.

Answer 41

Endometrial cells become polyhedral, loaded with glycogen and lipids.

Extravasate fill the intercellular spaces.

The endometrium becomes oedematous.

The reaction spreads from the implantation site to the entire endometrium.

Question 42

The site of implantation is usually healed by day …..

Answer 42

13

Question 43

The trophoblast develops villous structures on day …….

Answer 43

13

Question 44

Villi are ………

Answer 44

cellular columns of cyto-trophoblast covered by syncytio-trophoblast

Question 45

What are excoelomic cysts?

Answer 45

portions of the primitive yolk sac pinched off into the chorionic cavity