Embryology - embryonic phase - gastrulation (week 3)

Question 1

Which weeks make up the embryonic period?

Answer 1

Weeks 3-8

Question 2

What happens during the embryonic period?

Answer 2

It is the period of greatest change.
All major structures and systems are formed.
The most perilous stage for the developing child - many congenital defects have their root in this time.

Question 3

Why is week 3 called the week of threes?

Answer 3

1. 3 cavities - chorionic cavity est by end of second week

2. 3 germ layers - rudimentary lineages from which all others will rise

Question 4

What is gastrulation?

Answer 4

The process establishing three germ layers and hence the origins of all tissues in the body.
A trilaminar disk is formed and axes set which are observed in the adult.

Question 5

Which three axes are set in gastrulation which are observed in the adult body?

Answer 5

1. anterior/ posterior
2. dorsal/ventral
3. right/left

Question 6

What indicates the beginning of gastrulation?

Answer 6

The primitive streak appears

Question 7

When do researchers have to stop studying embryos?

Answer 7

When the primitive streak appears

Question 8

What cellular rearrangement occurs during gastrulation?

Answer 8

Migration and invagination

Question 9

What is the amnion?

Answer 9

The amnion is a membrane that when first formed, closely covers the embryo. It fills with the amniotic fluid which causes the amnion to expand and become the amniotic sac which serves to provide a protective environment for the developing embryo.

Question 10

Where does the primitive streak, pit and node first appear?

Answer 10

On the dorsal surface of the epiblast.

Question 11

What does the primitive streak look like?

Answer 11

Narrow groove with bulging edges.

Question 12

Where is the primitive node located?

Answer 12

Cranial end of primitive streak (end furthest away from the edge of the disk).

Question 13

Where is the primitive pit located?

Answer 13

At the centre of the primitive node.

Question 14

What is the role of the primitive node?

Answer 14

It is the control centre, coordinating all movements.

Question 15

What happens to the primitive streak once the three germ layers are established and gastrulation proceeds?

Answer 15

It regresses. If it doesn’t regress there are consequences.

Question 16

What is an important rule of thumb for the direction of development?

Answer 16

It proceeds in a cranial to caudal direction.

Question 17

In which direction does the primitive streak develop?

Answer 17

Cranial to caudal direction.

Question 18

Where do the epiblast cells surrounding the primitive streak migrate and invaginate to?

Answer 18

They displace the hypoblast creating a third layer (in the middle).

Question 19

What two layers make up the bilaminar disk?

Answer 19

Epiblast and hypoblast,

Question 20

What three layers make up the trilaminar disk?

Answer 20

Ectoderm, mesoderm and endoderm.

Question 21

As more and more cells migrate through the streak, which directions do they spread?

Answer 21

Laterally and cephalad (towards the head).

Question 22

Which are the two locations where the mesoderm does not spread and therefore the ectoderm is in direct contact with the endoderm?

Answer 22

Future mouth and future anus.

Question 23

What determines the ultimate fate of the invaginating epiblast?

Answer 23

Where in the streak or node they invaginate

Question 24

The presence or absence of which structure in the embryo determines whether a species is in the phylum Chordata or not?

Answer 24

Notochord

Question 25

What is the notochord a basis for?

Answer 25

Axial skeleton - it drives formation of the nervous system.

Question 26

Where in primitive streak/pit do prenotochordal cells invaginate?

Answer 26

Cranial part of primitive pit.

Question 27

Where do prenotochordal cells migrate?

Answer 27

They form a solid rod of cells running in the midline.

Question 28

What is the significance of the notochord?

Answer 28

1. Defines the midline 2. Axial skeleton forms around it 3. Drives neuralation

Question 29

What is the vestigial remenant of the notochord in adults?

Answer 29

Nucleus pulposa of intervertebral discs

Question 30

Anterior refers to which direction in an embryo?

Answer 30

Head not belly, like in adults

Question 31

Posterior refers to which direction in an embryo?

Answer 31

Tail not back, like in adults

Question 32

Which direction does rostral/cephalic/anterior describe?

Answer 32

Towards the head.

Question 33

Which direciton does caudal/posterior describe?

Answer 33

Towards the feet.

Question 34

Which direction does dorsal describe?

Answer 34

Towards the back.

Question 35

Which direction does ventral describe?

Answer 35

Towards the belly.

Question 36

Which direction does left describe?

Answer 36

45 degrees direction between rostral and ventral.

Question 37

Which direction does right describe?

Answer 37

45 degrees direction between caudal and dorsal.

Question 38

What is the first thing to set the axes in a developing embryo?

Answer 38

The primitive streak defines rostral and causal.

Question 39

The presence of what ensures correct dorso-ventral and left-right development?

Answer 39

Molecular signals emanating from the primitive node ensure symmetry of limbs and assymetry of viscera.

Question 40

When does gastrulation occur?

Answer 40

Week 3

Question 41

Why does gastrulation happen?

Answer 41

To ensure the correct placement of precursor tissue to allow subsequent morphogenesis to take place.

Question 42

How does gastrulation happen?

Answer 42

Primitive streak appears in the caudal epiblast, leading to migration and invagination of epiblast cells.

Question 43

What is the fate of differentiated cells?

Answer 43

They are committed to one fate and one function.

Question 44

Does cross-over between the cell types in ectoderm, mesoderm or endoderm occur?

Answer 44

No

Question 45

Which three cell lineages does epiblast differentiate into?

Answer 45

Ectoderm, mesoderm and endoderm.

Question 46

What does ectoderm differentiate into?

Answer 46

Organs and structures that maintain contact with the outside world e.g. nervous system, epidermis.

Question 47

What does mesoderm differentiate into?

Answer 47

This is the most plastic of the three layers. | It differentiates into supporting tissues e.g. muscle, cartilage, vascular system (including heart and vessels).

Question 48

What does endoderm differentiate into?

Answer 48

Internal structures e.g. epithelial lining of GI tract, respiratory tract; parenchyma of glands (functional elements of glands e.g. secretory cells).

Question 49

Give some examples of left-right asymmetry found in the body

Answer 49

Liver on the right, stomach and spleen on the left etc... | Left lung has two lobes, right lung has three.

Question 50

How does the primitive node create left-right differences?

Answer 50

By the action of ciliated cells at the node, a left-ward flow of signalling molecules is created. This creates a side-specific signalling cascade to be initiated. Presence of signalling molecules = LHS signals Absence of signalling molecules= RHS signals

Question 51

Describe situs inversus

Answer 51

It is an occurence, that is not that rare, of a complete mirror image of viscera in the body. It is commonly the result of immotile cilia and normally has no associated morbidity. Problems arise if there is normal and mirror-image disposition because this is a sign of more complex and compound congenital problems.

Question 52

What is the difference between monozygotic and dizygotic twins?

Answer 52

Mono- from a single fertilised oocyte | Di- from two fertilised oocytes

Question 53

When can splitting occur in monozygotic twins?

Answer 53

At many different times. 1. If embryo splits after first cleavage, both embryos have their own placenta. 2. If embryo splits by duplication of inner cell mass - there will be two embryos sharing the same placenta. 3. Splitting can occur very late with duplication of the primitive streak. - two embryos share a placenta and the same amniotic sac.

Question 54

What happens if separation of embryos is not complete?

Answer 54

Conjoined twins

Question 55

What is a teratogen?

Answer 55

A substance which interferes with normal development.

Question 56

Define teratogenesis

Answer 56

The process through which normal embryonic development is disrupted.

Question 57

Which weeks are the most sensitive to teratogenic insult?

Answer 57

The embryonic phase - weeks 3 to 8. Each organ system will have a particular 'sensitive window' depending on the time at which most of its development occurs.

Question 58

List some teratogenic agents

Answer 58

A large number of chemical and infectious agents are known to cause developmental defects: Thalidomide - interferes with limb development Rubella - interferes with head and neck development. Alcohol - no safe threshold for use in pregnancy Certain therapeutic drugs - paradine and warfarin (cross placenta and cause problems).