Gastrulation Flashcards

1
Q

Week 3

“the week of threes”

A

Three cavities
Three “germ” layers
- rudimentary lineages from which all others will arise

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2
Q

Gastrulation

A
  • the process establishing the three germ layers and hence the origin of all tissues of the body
  • trilaminar disk
  • sets the axes observed in the adult
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3
Q

Beginning of gastrulation

A
Primitive streak appears (in the third week) 
- primitive node with primitive pit 
Cellular rearrangement 
- migration 
- invagination
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4
Q

Primitive streak, pit & node

A
  • streak is a narrow groove with bulging edges
  • node is located on the cranial end of the streak
  • pit is located at the centre of the node
    Once the three layers are established the primitive streak regresses
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5
Q

Formation the trilaminar disk

A

Epiblast cells migrate and differentiate to the edges of the primitive streak - they displace the hypoblast and create a third layer
- fate of the invaginating epiblast depends on where in the steak or node they invaginate

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6
Q

Trilaminar disk

A

The ectoderm/endoderm can be seen in two places when looking at the mesoderm from below/above

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7
Q

The notochord

A
  • defines the phylum Chordata
  • basis for axial skeleton - defines the midline
  • drives the formation of the nervous system
  • is what herniates when a disk slips
  • it regresses - vestigial remnant in the adult is the nucleus pulposus of intervertebral discs
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8
Q

Ectoderm

A
  • organs and structures that maintain contact with the outside world
    E.g. Nervous system, epidermis
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9
Q

Mesoderm

A
  • supporting tissue

E.g. Muscle, cartilage, bone, heart and vessels

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10
Q

Endoderm

A
  • internal structures

E.g. Epithelial lining of GI tract, respiratory tract

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11
Q

Side-specific signalling cascade

A
  • action of ciliated cells at the node results in a leftward flow of signalling molecules
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12
Q

Situs Inversus

heart appears on RHS

A
  • commonly results from immobile cilia

- problems arise if there is both normal and mirror-image disposition

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13
Q

Monozygotic Twins

A
  • embryo splits after first cleavage – two embryos each with its own placenta
    OR
  • inner cell mass duplicated – two embryos sharing a placenta
    Splitting can occur very late (embryos can share the same amniotic sac)
    Sometimes separation is not complete - conjoined twins
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14
Q

Teratogenesis

A
  • normal embryonic development is disrupted
  • week 3 to 8 is the most sensitive to teratogenic insult
  • each organ system will have a particular sensitive window
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15
Q

Teratogenic Agents

A

Chemical and infectious agents are known to cause developmental defects

  • thalidomide (limbs)
  • rubella (head and neck)
  • alcohol (CNS)
  • certain therapeutic drugs
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16
Q

Embryonic Period

A

weeks 3 to 8

  • period of greatest change
  • all major structures and systems are formed
  • the most perilous for the developing child