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Embryology > Trilaminar Disc Formation > Flashcards

Trilaminar Disc Formation Flashcards

1
Q

What day does gastrulation commence?

A

day 15

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2
Q

What is gastrulation?

Why must it be correct?

A

Development from two layers into three layers
(blastula to gastrula)

It must be correct as all body systems develop from the trilaminar disc

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3
Q

What is the first stage in gastrulation?

A

Formation of the primitive streak

This consists of the primitive pit, node and groove

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4
Q

What is established after the primitive streak forms?

Where does the streak develop?

A

A right and left side of the embryo

The primitive streak forms on the dorsal face of the developing embryo, toward the caudal end

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5
Q

What is situs inversus?

A

The right to left asymmetry of the body is reversed

This can be partial or complete

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6
Q

How is complete situs inversus usually found?

Why?

A

It is usually an incidental finding

There are still all the normal connections between organs, so there aren’t usually any issues

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7
Q

What is Kartagener syndrome and how does it arise?

A

It affects the cilia, which sweep the correct cells to the right or left sides

If the cilia do not sweep the cells correctly, some organs may be reversed

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8
Q

What types of mutations usually affect the cilia?

A

Mutations may be in different genes that encode proteins that are important to the structure and function of the cilia

This may cause them to become immotile or dysmotile

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9
Q

Why is Kartagener syndrome more problematic than complete situs inversus?

A

Some organs have been reversed but others haven’t

This leads to abnormal connections in the body

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10
Q

What happens on day 15 of development?

A

formation of the definitive endoderm

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11
Q

What is the first stage in development of the definitive endoderm?

A

cells of the epiblast begin to move towards the primitive streak

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12
Q

What is the second stage in the development of the definitive endoderm?

A

Epiblasts migrate through the primitive streak, from the dorsal side, and displace the hypoblast cells

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13
Q

Why are no body systems formed from the hypoblast layer?

A

All systems are formed from the epiblast cells

The epiblast cells replace the hypoblast cells during formation of the definitive endoderm

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14
Q

What is the definitive endoderm?

A

The layer of epiblast cells that completely replaces the hypoblast

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15
Q

How is the mesoderm formed?

A

Epiblast cells continue to migrate through the primitive streak

They form a middle layer between the epiblast and definitive endoderm

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16
Q

On what day does the mesoderm form?

A

16

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17
Q

How is the ectoderm formed?

A

The remaining epiblast cells do not migrate and form the ectoderm

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18
Q

What is complete after the ectoderm has formed?

A

Gastrulation

Three distinct layers that make up the trilaminar disc have been formed

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19
Q

What are the two types of mesoderm in the trilaminar disc?

A

Intraembryonic mesoderm that is continuous with extraembryonic mesoderm

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20
Q

What structures are formed from the endoderm?

A
  1. epithelium of the digestive system and respiratory system

2. organs associated with digestive system e.g. pancreas and liver

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21
Q

What structures are formed from the mesoderm?

A
  1. musculoskeletal system
  2. cardiovascular system
  3. dermis of the skin
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22
Q

What structures are formed from the ectoderm?

A

It is involved in development of the nervous system, neural crest and the epidermis of the skin

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23
Q

On what day does the mesoderm become highly organised?

What is it organised in to?

A

day 17

  1. paraxial mesoderm
  2. intermediate mesoderm
  3. lateral plate mesoderm
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24
Q

What does paraxial mesoderm form?

A

This is mesoderm parallel to the midline

It forms the majority of the skeleton, skeletal muscle and dermis of the skin

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25
Q

What does the intermediate mesoderm form?

A

Gonads, reproductive tract and the kidneys

26
Q

What does the lateral plate mesoderm form?

A

Lining of body cavities

27
Q

How does the paraxial mesoderm undergo further organisation?

On what day does this occur?

A

It begins to divide into blocks called somites

This occurs on day 20

28
Q

At what rate are somites formed?

How many somites are formed in total?

A

They are formed in a cranial-caudal fashion at a rate of 3 or 4 pairs per day

This continues until 42-44 pairs of somites have been formed

29
Q

Why can somite formation be used to date an embryo?

A

It is a very highly regulated and predictable process

30
Q

What systems is mesoderm important in developing?

A

Musculoskeletal and genitourinary system

They cannot form properly if there is not enough mesoderm

31
Q

What is sirenomelia?

A

“mermaid syndrome”

There is insufficient mesoderm in the causal region of the embryo

32
Q

What are the 2 reasons why sirenomelia might result?

A
  1. insufficient numbers of epiblasts have migrated through the primitive streak
  2. the primitive streak has regressed too early
33
Q

What are the consequences of sirenomelia?

A

Abnormalities of the urogenital system and lower limbs

34
Q

What is a sacrococcygeal teratoma?

A

The remnants of the primitive streak may persist and give rise to a tumour

35
Q

What is the tumour in sacrococcygeal teratoma made from?

A

The abundance of mesoderm forms a large mass

36
Q

Why does sacrococcygeal teratoma occur?

A

Too many epiblast cells have migrated through the primitive streak

or the primitive streak has regressed too late

37
Q

What is the prognosis after resection of a sacroccocygeal teratoma?

A

Prognosis tends to be good as most sacroccocygeal teratomas diagnosed in newborns are benign

38
Q

Why is the primitive streak described as a ‘transient structure’?

A

It is only temporary

It begins to regress as soon as sufficient amounts of epiblast cells have migrated

39
Q

What happens as the primitive streak regresses?

A

It gets progressively shorter and lays down a layer of cells beneath it

the primitive node forms the notochordal process

40
Q

What happens to the notochordal process as the primitive streak regresses further?

A

the notochordal process is a hollow tube which gets longer as the streak regresses

41
Q

On day 18, what does the notochordal process transiently fuse with?

A

It transiently fuses with the endoderm to form the flattened notochordal plate

42
Q

What happens to the notochordal plate once it is formed?

A

It separates from the endoderm and fuses to form a solid notochord

43
Q

What is the role of the notochord?

A

It is a signalling centre

It starts to signal and cause components of the body to begin to develop

44
Q

How is neurulation initiated?

What is this process called?

A

Signals from the notochord that cause the ectoderm to thicken and form the neural plate

This is induction

45
Q

How does the notochord cause the ectoderm to thicken?

What is significant about this process?

A

It signals to cause the ectoderm to start proliferating

the notochord induces formation of the CNS by signalling for the ectoderm to thicken

46
Q

What is neurulation?

A

the first event in the development of the central nervous system

47
Q

How does the notochord influence differentiation of the somites?

A

It induces the differentiation of the somites into 3 different structures:

  1. sclerotome
  2. myotome
  3. dermatome
48
Q

What does the sclerotome form?

A

bone and cartilage

49
Q

What does the myotome form?

A

skeletal muscle

50
Q

What does the dermatome form?

A

dermis of the skin

51
Q

How does the notochord further organise the sclerotome?

A

It further organises the sclerotome to form vertebral bodies

52
Q

How are vertebral bodies and vertebral arches formed?

A

Sclerotome cells surround the notochord to form the vertebral body

Sclerotome cells surround the neural tube to form the vertebral arch

53
Q

What is the fate of the notochord?

A

The majority of the notochord degenerates as bodies of vertebrae form

The remnants of the notochord form the nucleus pulposus

54
Q

What is the purpose of the intervertebral discs?

A

They are shock absorbers that prevent bone from clashing and becoming damaged

55
Q

What are the 2 components of the intervertebral disc?

A

Strong outer rim is the annulus fibrosis

The jelly-like centre is the nucleus pulposus

56
Q

Why do spinal defects occur?

A

Abnormal induction of the sclerotomes

This means that vertebral bodies will not form correctly

57
Q

What is the least severe form of spina bifida?

How is it identified?

A

Spina bifida occulta

Identified by dimpled skin and a tuft of hair

58
Q

What is the process behind spina bifida occulta?

A

The vertebral arches have failed to form

The spinal cord is only protected by a layer of subcutaneous fat and skin

59
Q

What is meningocele?

A

It involves the 3 meninges that surround the CNS

Failure to form vertebral arches leads to meninges protruding through the gap

The meninges sac is filled with CSF

60
Q

What is myelomeningocele?

A

This is where failure to form the vertebral arches leads to the meninges and part of the spinal cord protruding through the gap

61
Q

Why is myelomeningocele dangerous?

How can it be detected?

A

some of the spinal cord is open to the external environment

It can be detected on ultrasound and in utero surgery can fix it

Embryology (4 decks)

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