Gastrulation

Question 1

When does gastrulation commence?

Answer 1

In week 3

Question 2

How does gastrulation commence?

Answer 2

Formation of the primitive streak

Question 3

What makes up primitive streak?

Answer 3

1. Primitive pit: top, rounded end
2. Primitive node: outer edges
3. Primitive groove: middle

Question 4

What does gastrulation involve?

Answer 4

Formation of 3 germ layers

Question 5

Cranial/caudal and right/left axes of embryo are firmly established in this period.

What forms towards cranial end?

Answer 5

Oropharyngeal membrane (mouth and throat)

Question 6

What is the cranial end?

Answer 6

Head end

Question 7

What is the caudal end?

Answer 7

Tail end

Question 8

What forms towards caudal end?

Answer 8

Cloacal membrane (GI and urinary systems)

Question 9

What is primitive streak?

Answer 9

A groove formed on dorsal aspect of embryo in epiblast. Appears at caudal end at bilinear disc stage. Marks beginning of gastrulation. Epiblast cells migrate through primitive streak to form 3 germ layers.

Question 10

What is situs inversus?

Answer 10

Complete reversal of left axis - left and right side of body are switched around (e.g. liver on left not right)

Question 11

Is complete situs inversus dangerous?

Answer 11

Sometimes harmless (asymptomatic)

20% of patients suffer from Kartagener syndrome

Question 12

What is Kartagener syndrome?

Answer 12

Problem with cilia cells - sweep in wrong direction cause cells to go to wrong side of body

Question 13

What are dangers of partial situs inversus?

Answer 13

Associated with malformations

Question 14

What is an example of a malformation due to partial situs inversus?

Answer 14

Dextrocardia

Question 15

What is involved in dextrocardia?

Answer 15

Isolated right sided heart causes problems with circulatory systems

• If correct blood vessels are attached to correct chambers it can cause no problems as oxygenated blood still goes to body and deoxygenated blood still goes to lungs
• If this isn’t correct, oxygenated blood can go to lungs and deoxygenated blood can go to body

Question 16

How are 3 germ layers formed?

Answer 16

Epiblast cells migrate through primitive streak

Question 17

What are 3 germ layers?

Answer 17

1. Endoderm
2. Mesoderm
3. Ectoderm

Question 18

How is endoderm formed?

Answer 18

1. Epiblast cells move through primitive streak
2. Push hypoblast cells out of way and replace them
3. Once completely replaced, forms 1st germ layer (definitive endoderm)

Question 19

What are adult structures developed from?

Answer 19

Epiblast cells - hypoblast cells are only transient (replaced)

Question 20

How is mesoderm formed?

Answer 20

1. Epiblast cells continue to migrate though streak (keep on replicating)
2. Form middle layer between epiblast and endoderm - 2nd germ layer (mesoderm)

Question 21

How is ectoderm formed?

Answer 21

1. Epiblast cells left behind make up 3rd and final layer (ectoderm)
2. This completes gastrulation

Question 22

What do we end up with after gastrulation?

Answer 22

Trilaminar disc consisting of 3 germ layers all derived from epiblast cells

Question 23

What is purpose of these germ layers?

Answer 23

Form all our body systems

Question 24

What does endoderm go on to form?

Answer 24

• Epithelial lining of respiratory tract, gut tube (GI tract), urinary bladder, urethra
• Liver
• Pancreas
• Gallbladder

Question 25

What does mesoderm go on to form?

Answer 25

- Dermis of skin - Epithelial lining of blood vessels, lymph vessels, body cavities, joint cavities - Muscle tissue - Heart - Connective tissue - Kidneys & ureters - Spleen - Internal reproductive organs

Question 26

What does ectoderm go on to form?

Answer 26

- Epidermis of skin & epidermal derivatives (hair, nails) - Nervous tissue & sense organs - CNS & spinal cord & brain - Pituitary gland - Enamel of teeth - Lens of eye

Question 27

The mesoderm becomes highly organised into what structures?

Answer 27

1. Paraxial mesoderm 2. Intermediate mesoderm 3. Lateral plate mesoderm

Question 28

What does paraxial mesoderm go on to form?

Answer 28

Majority of skeleton, skeletal muscles, dermis of skin

Question 29

What does intermediate mesoderm go on to form?

Answer 29

Gonads, internal reproductive tracts, kidney

Question 30

What does lateral plate mesoderm go on to form?

Answer 30

Lining of body cavities

Question 31

Paraxial mesoderm starts dividing into blocks. What are these blocks called?

Answer 31

Somites

Question 32

When does this dividing into somites happen?

Answer 32

Day 20 post fertilisation

Question 33

In what direction does this dividing into somites occur?

Answer 33

Begins in cranial region and progresses towards caudal region

Question 34

Why is this dividing into somites useful in estimating the age of the embryo?

Answer 34

Very predictable process

Question 35

When does this dividing into somites stop?

Answer 35

When 42-44 pairs have formed

Question 36

What is the feature of Sirenomelia?

Answer 36

'Mermaid syndrome' - legs appear stuck together

Question 37

What is Sirenomelia caused by?

Answer 37

Insufficient mesoderm formed in caudal region of embryo as primitive streak regresses too early - don't have sufficient migration of epiblast cells

Question 38

Why is the cranial end normal during Sirenomelia?

Answer 38

Mesoderm forms cranial to caudal end

Question 39

What is the feature of Sacrococcygeal Teratoma?

Answer 39

Tumours

Question 40

What is cause of Sacrococcygeal Teratoma?

Answer 40

Remnants of primitive streak persist - too much mesoderm formed

Question 41

What happens to the primitive streak once sufficient migration occurs?

Answer 41

It regresses