General Anatomy (Bilaminar And Trilaminar Germ Disc)

Question 1

What happens on the 5th day after fertilization?

Answer 1

• the morula acquires a cavity within itself
• the cavity is filled with fluid called blastocoele
• the embryo is called the blastocyst

Question 2

What are the 2 distinctive clumps of cells?

Answer 2

• Embryoblast - forming embryo proper
• Trophoblast - outer cell mass, forming supporting structures for the growing embryo

Question 3

Where does implantation takes place?

Answer 3

• the human blastocyst implants in the endometrium of the anterior or posterior wall of the uterus 6-7 days after fertilization
• it becomes embedded between the opening of the glands

Question 4

What are the two areas identified in the blastocyst in the second week of gestation?

Answer 4

• trophoblast - cells of the outer cell mass (placenta)
• embryoblast - cells of the inner cell mass (Bilaminar germ disc, trilaminar germ disc, embryo proper)

Question 5

What happens after fertilization by day 8?

Answer 5

The embryoblast differentiates into 2 layers
- Hypoblast - A layer of small cuboidal cells adjacent to blastocyst (blastocoele or primary yolk sac) - the blastocyst is now called a primary yolk sac
- Epiblast - A layer of high columnar cells adjacent to the amniotic cavity

Question 6

What is the cavity above the embryo called?

Answer 6

• amniotic cavity
• it is formed within the cells of the embryoblast
• the blastocoele is now called as primary yolk sac/exocoelomic cavity

Question 7

What does the trophoblast differentiates into?

Answer 7

• An outer syncytiotrophoblast - (syncytio = collection of cells, can’t see separation from one another) can erode into the endometrium and form connections with the arteries and the glands of the endometrium to establish a blood supply for the growing embryo
• an inner cytotrophoblast - (cytro = cell)

Question 8

What happens on day 10?

Answer 8

• cells from the hypoblast will start to form a thin exocoelomic Heuser’s membrane
• it will line the inner surface of the cytotrophoblast and outer surface of the primary yolk sac (blastocyst cavity)

Question 9

What is the fibrin clot?

Answer 9

• Its formed at the site of entry of the blastocyst in the uterine wall
• implantation bleeding is common during the process and is often mistaken for a menstrual period
• At this time the whole embryo will already be inside the endometrium
• the fibrin clot will be formed to ensure the bleeding is stopped

Question 10

What happens by day 11?

Answer 10

• New population of cells arise from the epiblast and form a fine loose connective tissue in between the cytotrophoblast externally and the amnion and Heuser’s membrane internally.
• Called as extra-embryonic mesoderm.
• But, as the embryo grows, there will be spaces that will be developed within the EM called lacunae. (Since EM is a loose CT and it will expand).

Question 11

What happens by day 12?

Answer 11

• the lacunae become confluent (unite with one another)
• when this happened, they will form a big space called extraembryonic coelom or the chorionic cavity. This will cause the formation of another 2 layers of mesoderm.
• the mesoderm that is nearer to primary yolk sax is called splanchnic mesoderm
• on the outer side, the mesoderm nearer to the cytotrophoblast is called somatic mesoderm

Question 12

What happens by day 13?

Answer 12

• the extra-embryonic coelom thus formed divides the extraembryonic mesoderm into:
  1. Extra-embryonic somatopleuric mesoderm - outer layer of cells lining the cytotrophoblast and amnion
  2. Extra-embryonic sphlancnopleuric mesoderm - inner layer of cells lining the yolk sac

Question 13

What is the chorionic cavity?

Answer 13

• The chorionic cavity (extraembryonic coelom) will also grow surrounding the top part as well
• it will surround the whole entire growing embryo except the connecting stalk

Question 14

What is the connecting stalk?

Answer 14

An area where the splanchnic and somatic layers of the mesoderm will be connected to each other.

Question 15

What happens at day 14?

Answer 15

• The hypoblast produces additional cells that migrate along the inside of the Heuser’s membrane.
• Primary yolk sac -> Secondary yolk sac
• They enclose the secondary or definitive yolk sac.

Question 16

What is the chronic plate?

Answer 16

The mesoderm lining inside of the cytotrophoblast

Question 17

What does the connecting stalk do?

Answer 17

The connecting stalk forms the future umbilical cord. [Later, the embryo will have a connection with the placenta (cytotrophoblast) via the umbilical cord).

Question 18

What is a hyadatiform mole?

Answer 18

Abnormal blastocysts with rapidly developing trophoblast with little embryonic tissue

Question 19

How does Hyadatiform mole form?

Answer 19

Happened due to fertilisation of an empty egg (egg w/o nucleus by a single sperm) followed by duplication of all the chromosomes.

Question 20

What is the appearance of hyadatiform mole?

Answer 20

• Gross (In Surgery) - Bunch of grapes appearance.
• In Ultrasound - snowstorm appearance.

Question 21

What are the complications of Hyadatiform mole?

Answer 21

• Benign (invasive mole)
• Malignant tumors (choriocarcinoma).

Question 22

What happens in Hyadatiform mole?

Answer 22

• No embryoblast but a huge amount of trophoblast that grows very fast.
• Trophoblasts will be the ones that secrete HCG. Therefore, there will be a higher amount of HCG.
• It can cause very severe morning sickness.

Question 23

What occurs in the 3rd week of gestation?

Answer 23

• Bilaminar germ disc (2 layers: Epiplast & Hypoblast) -> Trilaminar germ disc (3 layers: Ectoderm, Mesoderm, Endoderm)
• Beginning of morphogenesis (An embryo that has a little more shape to it)
• Axial orientation of the embryo is established.
• The embryo is now called as gastrula

Question 24

What does gastrulation means?

Answer 24

the formative process by which the three germ layers and axial orientation are established in the embryo.

Question 25

What is the primitive streak?

Answer 25

Formation of primitive streak - The first sign of gastrulation.
The cells of the epiblast proliferate and migrate to the median plane of the embryonic disc.
These cells form an opacity.
This opaque primitive streak appears:
1. Caudally in the median plane
2. In the dorsal aspect of the embryonic disc.

Question 26

Gggggg

Question 27

What are mesenchyme?

Answer 27

• After the formation of the primitive streak, the cells deep to the surface form loose their epithelial cell properties (Polarity, resting on basement membrane) and become mesenchyme (more mobile, doesn’t have apical/basal, not resting on basement membrane) .
• These cells are amoeboid. Called as mesoendoderm.

Question 28

What os the formation of the 3 germ layers?

Answer 28

• The primitive pit and groove represent areas where cells are leaving the primitive streak and moving into the interior of the embryonic disc.
• Some of these cells invade the hypoblast, displacing the original hypoblast cells and replacing them with a layer of definitive endoderm.
• Others migrate bilaterally from the primitive streak and then cranially or laterally between endoderm and epiblast and coalesce to form the intraembryonic mesoderm.
• The remaining epiblast forms the ectoderm

Question 29

What is the fate map of the embryo?

Answer 29

• The cells of epiblast that migrate and ingress through the primitive streak have been mapped.
• Not all cells move caudally. Cells that ingress through the cranial region become the notochord.
• Cells just lateral to the node become the para-axial mesoderm.
• Cells migrating through the mid- streak region become the intermediate mesoderm.
• Cells migrating more laterally form the lateral plate mesoderm

Question 30

What is the notochordal process?

Answer 30

• Some mesenchymal cells migrate cranially from the primitive node and pit to form a median cellular cord-The notochordal process.
• This later forms a flat notochordal plate.
• The plate in-folds on itself to form a tube-like notochord.

Question 31

What is the importance for the formation of the notochord?

Answer 31

❖ Defines primordial axis of the embryo.
❖ Gives some rigidity to the embryo.
❖ Serves as a basis for the development of the axial skeleton
❖ Indicates the future site of the vertebral bodies.
❖ Induces overlying ectoderm to thicken and form neural plates.

Question 32

What is the overall growth of the embryonic disc?

Answer 32

• The embryonic disc initially flat and almost round gradually elongates with a broad cephalic and narrow caudal end.
• Invagination and migration of the cells in the primitive streak forward (begins at 3rd week) and laterally continue till the end of the 4th week.
• By the end of the 4th week, the Trilaminar Germ Discs have been fully formed.
• After the 4th week, the streak regresses, shrinks, and disappears. There will be no more migration of cells.
• The embryo grows craniocaudally. The cranial (head) part always grows bigger and faster than the caudal end (tail/lower limbs).

Question 33

What stage is most vulnerable in gestation?

Answer 33

• The third week of gestation is the most vulnerable stage to teratogenic insult.
* Teratogen = Materials/Substances that can cause malformation in a growing embryo

Question 34

What is fetal alcohol syndrome?

Answer 34

• High doses of alcohol kill the cells of the anterior midline, hence a defect in the craniofacial structures of the midline.
• Defective cranium, small head (microcephaly) • Mental retardation
• Small face, thin lips, small jaw
• Atrial/ ventricular septal defects.

Question 35

What is sirenomilia?

Answer 35

• Mermaid syndrome.
• Due to insufficient mesoderm in the caudal region (Caudal Dysgenesis)

Question 36

What is sacrococcygeal teratoma?

Answer 36

• Remnants of primitive streak or primordial germ cells.
• Common in female.
• Tissues of all 3 germ layers are present. • Good prognosis.

Question 37

What is chordomas?

Answer 37

• Remnants of notochord.
• Malignant, local bone infiltrating tumors.
• Seen in the base of brainnasopharynx and lumbo-sacral region.

Question 38

What is situs inversus?

Answer 38

• Transposition of the viscera in the thorax and abdomen in the sagittal plane.
• If the cells which make our various organs are programmed incorrectly the body’s left-right plan or ‘laterality’ is disturbed.

Laterality sequences:
- Left-sided bilaterality- polysplenia
- Right-sided bilaterality- a/hyposplenia.

Question 39

What is kartagener’s syndrome?

Answer 39

• Situs inversus patients have ciliary dysfunction.
• Associated with sinusitis, bronchiectasis, male infertility.

Question 40

What is ectoderm?

Answer 40

Epidermis, central and peripheral nervous system

Question 41

What is mesoderm?

Answer 41

smooth/striated muscles, vessels, skeletal system, connective tissue, reproductive and excretory organs.

Question 42

What is endoderm?

Answer 42

lining of GIT, respiratory tract, glands and glandular cells of liver and pancreas.

Question 43

What is the first sign of gastrulation?

Answer 43

Formation of primitive streak

Question 44

What is primitive streak

Answer 44

• The cells of the epiblast proliferate and migrate to the median plane of the embryonic disc.
• These cells form an opacity.
• This opaque primitive streak
appears
1. caudally in the median plane
2. in the dorsal aspect of the embryonic disc.

Question 45

What do the cells in the caudal end do?

Answer 45

It elongates the streak

Question 46

What is the cranial end do?

Answer 46

The cranial end of the streak proliferates to form the primitive/ Hensen node.

Question 47

What is primitive pit?

Answer 47

• A small depression forms in the primitive node called primitive pit.
• It is continuous with the primitive groove a narrow groove in the primitive streak.

Question 48

What can you identify as soon as the primitive streak is formed?

Answer 48

1. The embryo’s cranio-caudal axis.
2. Dorsal and ventral sides,
3. The right and left sides.