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LS1 GI > Embryology - embryonic phase - gastrulation (week 3) > Flashcards

Embryology - embryonic phase - gastrulation (week 3) Flashcards

1
Q

Which weeks make up the embryonic period?

A

Weeks 3-8

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2
Q

What happens during the embryonic period?

A

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.

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3
Q

Why is week 3 called the week of threes?

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

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

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4
Q

What is gastrulation?

A

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.

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5
Q

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

A
  1. anterior/ posterior
  2. dorsal/ventral
  3. right/left
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6
Q

What indicates the beginning of gastrulation?

A

The primitive streak appears

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7
Q

When do researchers have to stop studying embryos?

A

When the primitive streak appears

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8
Q

What cellular rearrangement occurs during gastrulation?

A

Migration and invagination

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9
Q

What is the amnion?

A

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.

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10
Q

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

A

On the dorsal surface of the epiblast.

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11
Q

What does the primitive streak look like?

A

Narrow groove with bulging edges.

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12
Q

Where is the primitive node located?

A

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

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13
Q

Where is the primitive pit located?

A

At the centre of the primitive node.

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14
Q

What is the role of the primitive node?

A

It is the control centre, coordinating all movements.

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15
Q

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

A

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

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16
Q

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

A

It proceeds in a cranial to caudal direction.

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17
Q

In which direction does the primitive streak develop?

A

Cranial to caudal direction.

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18
Q

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

A

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

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19
Q

What two layers make up the bilaminar disk?

A

Epiblast and hypoblast,

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20
Q

What three layers make up the trilaminar disk?

A

Ectoderm, mesoderm and endoderm.

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21
Q

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

A

Laterally and cephalad (towards the head).

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22
Q

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

A

Future mouth and future anus.

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23
Q

What determines the ultimate fate of the invaginating epiblast?

A

Where in the streak or node they invaginate

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24
Q

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

A

Notochord

25
Q

What is the notochord a basis for?

A

Axial skeleton - it drives formation of the nervous system.

26
Q

Where in primitive streak/pit do prenotochordal cells invaginate?

A

Cranial part of primitive pit.

27
Q

Where do prenotochordal cells migrate?

A

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

28
Q

What is the significance of the notochord?

A
  1. Defines the midline
  2. Axial skeleton forms around it
  3. Drives neuralation
29
Q

What is the vestigial remenant of the notochord in adults?

A

Nucleus pulposa of intervertebral discs

30
Q

Anterior refers to which direction in an embryo?

A

Head not belly, like in adults

31
Q

Posterior refers to which direction in an embryo?

A

Tail not back, like in adults

32
Q

Which direction does rostral/cephalic/anterior describe?

A

Towards the head.

33
Q

Which direciton does caudal/posterior describe?

A

Towards the feet.

34
Q

Which direction does dorsal describe?

A

Towards the back.

35
Q

Which direction does ventral describe?

A

Towards the belly.

36
Q

Which direction does left describe?

A

45 degrees direction between rostral and ventral.

37
Q

Which direction does right describe?

A

45 degrees direction between caudal and dorsal.

38
Q

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

A

The primitive streak defines rostral and causal.

39
Q

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

A

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

40
Q

When does gastrulation occur?

A

Week 3

41
Q

Why does gastrulation happen?

A

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

42
Q

How does gastrulation happen?

A

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

43
Q

What is the fate of differentiated cells?

A

They are committed to one fate and one function.

44
Q

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

A

No

45
Q

Which three cell lineages does epiblast differentiate into?

A

Ectoderm, mesoderm and endoderm.

46
Q

What does ectoderm differentiate into?

A

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

47
Q

What does mesoderm differentiate into?

A

This is the most plastic of the three layers.

It differentiates into supporting tissues e.g. muscle, cartilage, vascular system (including heart and vessels).

48
Q

What does endoderm differentiate into?

A

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

49
Q

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

A

Liver on the right, stomach and spleen on the left etc…

Left lung has two lobes, right lung has three.

50
Q

How does the primitive node create left-right differences?

A

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

51
Q

Describe situs inversus

A

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.

52
Q

What is the difference between monozygotic and dizygotic twins?

A

Mono- from a single fertilised oocyte

Di- from two fertilised oocytes

53
Q

When can splitting occur in monozygotic twins?

A

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.
54
Q

What happens if separation of embryos is not complete?

A

Conjoined twins

55
Q

What is a teratogen?

A

A substance which interferes with normal development.

56
Q

Define teratogenesis

A

The process through which normal embryonic development is disrupted.

57
Q

Which weeks are the most sensitive to teratogenic insult?

A

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.

58
Q

List some teratogenic agents

A

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).

LS1 GI (17 decks)

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