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Advanced Developmental Biology > Vertebrate Early Development > Flashcards

Vertebrate Early Development Flashcards

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1
Q

What is the first step of early vertebrate development and what is it called in xenopus?

A
  • Generate a multicellular structure, no growth just cellular division
  • In Xenopus a series of rapid cleavage divisions produce the blastula
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2
Q

How does this first stage mass of cells turn in to a free swimming tadpole in xenopus?

A
  • Formation of the mesoderm, endoderm and ectoderm depends on the differences in the ‘Animal’ (upper) and ‘Vegetal’ (lower) poles
  • Vegetal poles are denser and produce a signal perceived by the animal hemisphere which characterises embryo formation
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3
Q

What is a blastopore?

A
  • opening that forms on the surface of the developing embryo
  • blastopore marks the site where cells begin to ingress and move inward to form the three germ layers
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4
Q

What do each of the germ layers become?

A
  • Endoderm = gut
  • Mesoderm = tissues, muscles, notochord
  • Ectoderm = neural and skin
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5
Q

How are the three germ layers simply arranged?

A

The endoderm tissue and mesoderm tissue ‘moves into’ the middle of the embryo and is covered by ectoderm (skin and neural tube) so its internal

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6
Q

What is the vegetal hemisphere and what does it produce?

A
  • The ‘yolk’
  • Contains ‘cytoplasmic determinants’
  • One of these is a mRNA which AFTER fertilisation encodes VegT
  • VegT binds to promoters of genes encoding molecules related to Nodal (Xnr genes) which is a morphogen
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7
Q

Once made, where does Nodal move to?

A
  • Moves past the equatorial region into the animal hemisphere
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8
Q

What are the effects of Nodal?

A
  • Cells producing Nodal becomes pharyngeal endoderm
  • Cells at a range of intermediate distances become become the mesoderm
  • Cells furthest from the source of Nodal (vegetal) become the ectoderm
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9
Q

During what process is tissue organisation and axial patterning established?

A

Gastrulation

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10
Q

How does Nodal work and what is it a family of?

A
  • TGF-B signals
  • They bind to TGF-B receptors causing
    conformational changes and phosphorylation, subsequent phosphorylation of Smad2/3
  • Once Smad 2 is phosphorylated it forms a hetero-dimer complex with Smad4 where it can enter the nucleus and activate the transcription of target genes
  • The amount of Nodal correlates to the signal they are producing and the conc. Of smad2 in the cell
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11
Q

How does Sperm entry at fertilisation impact the cytoplasm of the embryo and its cytoplasmic determinants (CDs)?

A
  1. Sperm entry - sperm enters egg typically at animal pole marked by pigment accumulation
  2. Cortical Rotation - Following sperm entry, the cortical cytoplasm rotates approximately 30 degrees clockwise
  3. Redistribution of the CDs - causes cytoplasmic determinants, including Wnt11, to move from the vegetal pole to the future dorsal side of the embryo (180 degrees from the sperm entry site)
  4. Axis Formation: The relocation of Wnt11 and other determinants sets up the dorsal-ventral axis
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12
Q

How do Wnt11 cells form the Niuewkoop centre and what is it?

A
  • activated B-catenin from Wnt activation accumulates in nuclei of cells on future D side of embryo causing transcription of Wnt target pathway genes
  • One of these targets is Nodal, so more Nodal in Nieuwkoop centre
  • Located in the vegetal tissue within the blastula and gives rise to the Spemann-Mangold organiser
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13
Q

How is the Spemann-Mangold Organiser (SMO) created? incl. what TFs are relevant

A
  • gradient distribution of Nodal activity going from Nieuwkoop centre side where its high and the opposite side where its low
  • high levels = where SMO is
  • Smad2 and Siamois are TFs and respond to coincident nodal and b-catenin signalling, causes formation of organiser tissue
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14
Q

What genes are transcribed as organiser specific genes?

A

Chordin
Noggin
Goosecoid

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15
Q

How does the Spemann-Mangold Organiser pattern the A-P and D-V axis of embryo?

A
  1. Involution (intercalation and migration of axial mesoderm convergent extension)
  2. Acquisition of distant cell fates by mesodermal cells at different axial positions: prechordal mesoderm, notochord, somites intermediate mesoderm etc.
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16
Q

What does the Early organiser become/ turn in to and what is it followed by?

A

The organiser turns into an extending rod of tissue across the blastocoel roof defining the future dorsal midline and a-p axis

  • Early organiser becomes pharyngeal endoderm , sitting most anteriorly
  • Followed by prechaudal mesoderm
  • Last is the notochord sitting most posteriorly
17
Q

What does the organiser do as it moves during convergent extension?

A

Organiser is a dynamic mix of progenitor cells

  • releasing signals that diffuse laterally and dorsally (into ectoderm to instruct to form neural tube rather than epidermis)
18
Q

How is this convergent extension/gastrulation process different in fish and chicks compared to xenopus?

A
  • Very similar in fish
  • Chick develops as a flat disc
  • In the chick embryo the pharyngeal endoderm extends along the dorsal midline FIRST followed by the prechordal mesoderm and then the notochord, in xenopus this is after
19
Q

What is the Spemann mangold Organizer Graft Experiment and how does it vary based on the development stage?

A
  • Organiser transferred to ectopic position (opposite) in a host
  • Induces formation of second A-P axis
  • Concluded that the SMO is releasing signals

Difference between early and late organiser?
- Early organiser led to complete second axis including head and trunk
- Late organiser only had partial second axis compromising of trunk only

20
Q

What does there being a difference in the effects of early and late organiser grafting show?

A
  • Early organizer produces specific signals crucial for early development.
  • Late organizer produces different signals as development progresses.
  • Signal production changes over gastrulation due to shifts in signaling molecule expression.
21
Q

What does the Spemann Mangold Organiser do and how?

A
  • Creates the A-P axis of the embryo
  • Induces neural tissue from ectoderm

** Secretes Wnt and BMP inhibitors e.g. Dickkopf, Chorddin and Noggin

22
Q

What is the difference between secreted molecules in the SMO?

A

The early organisers secrete Wnt inhibitors and the Late organiser secretes BMP inhibitors

23
Q

How do BMP antagonists and Wnt antagonists create A-P patterning?

A
  • BMP antagonists act on ectoderm to suppress epidermal fate and induce neural fate
  • Wnt antagonists induce anterior character within neural tissue promoting brain development by antagonising (inhibiting) the posteriorizing and ventralizing functions of wnt8 in the mesoderm - causing anterior transcription

**
both inhibitors work to create head and brain but the late organiser loses its Wnt inhibitory power so the trunk/tail is made

24
Q

What are the BMP antagonist and Wnt antagonist names?

A

BMP- Chordin, noggin, cerebrerus
Wnt - Cerebrus, Frzb and Dickkopf

25
Q

What is the Double Gradient Model of embryonic axis formation?

A

The SMO generates:

  1. A-P gradient of Wnt signalling activity, lowest in anterior where Wnt antagonist is highest
  2. A D-V gradient of BMP signalling activity, lowest in neural plate and dorsal axial mesoderm where BMP antagonist is highest
26
Q

What is the activation - transformation process?

A

Activation:
- SMO in early stage secretes Wnt and BMP antagonists
- Neuroectoderm exposed adopts forebrain fate

Transformation:
- Secretion of Wnt antagonists stops
- Posterior neural plate exposed to Wnts, RA and FGFs
- Forebrain fate suppressed over time and posteriorized neural fates are induced
- Neural plate elongates and transitions to neural tube

27
Q

What is the posterior neural tube exposed to and what does this induce?

A
  • RA and FGFs
  • Retinoic acid binds to retinoic acid receptor which is a TF that activates when Retinoic acid binds
  • RA gradient induces Hox gene transcription at different positions along the a-p axis
  • Specialises individual segment identities
  • Hox proteins are transcription factors expressed in partially overlapping expression domains
28
Q

How does Neural tube formation occur?

A
  • Neurulation as neural folds on lateral edges of neural plate fuse
  • Neural plate border cells (dorso-lateral hinge point) give rise to neural crest cells
  • Neural crest cells delaminate from the neural tube separating neural tube from epidermis and migrate to form neurons, glia etc.
29
Q

What are the roof and floor plate and what do they do?

A
  • Specialised regions of the neural tube, involved in its D-V patterning

Roof plate (dorsal):
- BMP4 release inducing dorsal neuronal fates
- Wnt released to promote BMP expression

Floor plate (ventral):
- Shh secreted inducing ventral neuronal fates

**The two opposing gradients produce formation of different types of neurons directly related to morphogen conc.

30
Q

How can you study the D-V patterning of the neural tube?

A
  • Immunocytochemistry to look at different TF to see specific cell types such as Pax7 and Pax6 (ventral Shh TFs)
  • You can see the ways cell respond to different concentrations of Shh, can do experimental manipulation adding different amounts

Advanced Developmental Biology (13 decks)

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