SAQs Flashcards

1
Q

In mammalian development, the first cell fate decision produces two cell types from
blastomeres. Name the cell types, and for each, give one gene that is a marker for
that cell type (2 marks)

A

Trophectoderm. Marker = Cdx2, Gata4/6, Tead4

Inner Cell Mass. Marker = Sox2, Oct3/4, Sall4, Nanog

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2
Q

Name two places (each in a different tissue) in mammalian embryos where
epithelial-to- mesenchymal transformation takes place. (2 marks)

A
Heart valve formation
myogenesis
neural crest formation
gastrulation
neurulation- form neural plate
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3
Q

Give one advantage and one disadvantage for both a genetic and non-genetic means
of lineage labelling in the mouse embryo. (4 marks)

A

Genetic Adv: Label not diluted by divisions/accurate/inherited by all progeny/ as accurate as the promtor to drive expression
Genetic Dis: Expensive, difficult, time consuming, reliant on promoter activity, complicated
Non-Genetic Adv: Easy, cheap, quick
Non-Genetic Dis: label diluted by cell divisions, not equally inherited, limited accessibility of embryo, accuracy of injection/meand of introduction

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4
Q

Briefly describe how (a) determination and (b) specification of embryonic tissue can
be assessed. (2 marks)

A

a) Determination: using genetic lineage labelling e.g. green fluorescent protein and observing cell fates from a single lineage
b) Specification of embryonic tissue: using non-genetic dye which is cell specific to follow cell migration

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5
Q

Explain how Cre recombinase with ROSA26 locus transgenics can be used for
lineage analysis. (4 marks)

A
  • Rosa26 is normally ubiquitously expressed
  • R26R is a modified Rosa26 containing STOP sequence upstream of the reporter gene (originally lacZ, also now fluorescent proteins now flanked by loxP sites)
  • LacZ is not expressed in R26R because of the stop sequence
  • In the presence of Cre, stop sequence is removed and lacZ is expressed
  • Promoter x-cre mice crossed to R26R can be used to trace lineage of cells where promoter X is active

E.g. Wnt1-Cre is not expressed in neural crest cells, so Wnt1-Cre xR26R marks all neural crest cells and their derivatives

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6
Q

Which receptors bind wnt proteins? (0.5 marks) Name two molecules that can
directly antagonize wnt signalling in the embryo. (1 mark) What happens to betacatenin in the absence of active wnt signalling? (0.5 marks)

A

Frizzled Receptor

2 molecules: WIF-1, sFRP, Cerberus, dikkopf (Dkk)

beta catenin: destruction complex targets it for ubiquitylation and so degradation by the proteosome

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7
Q

Smad1 is phosphorylated following BMP binding to the BMP receptor. Phosphorylated
Smad1 commonly forms a complex with one of two proteins. What are these
proteins and what is the consequence of the formation of each of these complexes? (2
marks)

A

SMAD 4 = translocation of complex from cytoplasm to nucleus where target genes are switched on

SMAD 6 = inhibitory so target genes are not switched on

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8
Q

Mouse embryos that are homozygous null mutant for the gene vangl1 develop with
an abnormally wide floor-plate. Say, briefly, why this occurs and what is the
phenotypic consequence. (3 marks)

A

Occurs due to a neural tube defect where cranial neuropore closure failure occurs because of a broader floor plate;
Vangl1 drives PCP
KO Vangl1 causes Abnormal PCP
this:
disrupts normal actin-mysoin rearragnement,
this prevents constriction of the cytoskeleton in the apical surface of the neural plate cells,
This prevents normal wedging, neural closure and convergence-extension of the neural tube.

Phenotype is craniorachishisis

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9
Q

Describe, briefly, how a graded signal can cause a homogeneous group of cells within
the embryo to become sub-divided into two distinct populations with a sharp boundary between them. (4 marks)

A
  • Graded signal uses different concentrations of a morphogen to induce a rough pattern of 2 domains with different identities e.g. high/low [BMP]
  • Mutual repression transcription factors creates strict complementarity of identity
  • This is further sharpened by segregation and restriction of intermingling across the border to form 2 distinct compartments rather than a fuzzy border
  • cell switching occurs- cells on the incorrect side of the boundary are switched to majority side
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10
Q

What is clonal analysis?

A

Examination of the position and nature of daughter cells following the labelling of a single cell

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11
Q

Explain how tamoxifen inducible gene knockout works in the mouse (4 marks)

Also worded as: Explain how temporal control of Cre recombinase in vivo in the mouse can be achieved using tamoxifen.

A
  • Transgenic construct encodes a fusion protein of Cre with a mutant E2 receptor ligand binding domain (ERLBD) driven by a promoter (ubiquitous/ tissue specific)
  • In the absence of tamoxifen mutant LBD is not activated by natural Oestrogen at physiological levels
  • The fusion protein is held in the cytoplasm in a complex with HSP90
  • Treatment with tamoxifen releases the HSP90, allowing the Cre to move into the nucleus and excise any floxed sequences
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12
Q

Explain how simple autoregulation can generate regulary cyclic oscillations in gene expression, such as that observed in pre-somitic mesoderm (draw a diagram, if it helps) 4 marks

A

-Bottom tier: single cell oscilators- so gene expression increases protein synthesis; once protein expression reaches a certain level, this negatively feeds back to inhibit further gene expression for the same protein.
This produces peaks and troughs in gene expression.

Gene X is negatively auto-regulated- repressor x binds in its own transcription.

Genes can be positively or negatively autoregulated

e.g. in somitogenesis the oscillation of Hes7 –> express mRNA–> protein –> which repress the transcription of its own encoding gene via cyclic expression and negative feedback

Notch (forms before delta when inhibiton is lost) delta

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13
Q

what is meant by the terms symmetric and asymmetric cell division as applied to progenitor cells (within the developing cortex, for example)? 2 marks

A

Symmetric cell division: Proliferative division e.g. of neural epithelial cells to produce 2 identical cells
Asymmetric: Neurogenic division that produces 2 different cells e.g. pre-plate neurons and RGC

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14
Q

Name two different progenitor cell-types that exist within the cortical germinal zones. For each of these give an example of one cell type that they give rise to (0.5 marks each)

A

Neuroepithelial cells give rise to radial glial cells
Radial glial cells give rise to basal neurones

Neocortical radial glial cells can become intermediate progenitor cells –> basal neurones

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15
Q

Two opposing models have been proposed for proximo-distal patterning of the vertebrate limb. These are the progress and pre-specification models. Describe briefly the experimental evidence that first suggested the progress zone model. How can these findings be reconciled with the pre-specification model? (5 marks)

A

Progress zone model is maintained as proliferative by FGF8 from AER. As cells leave the progress zone they have a proximo-distal identity. The first cells to leave are proximal, the last to leave are distal. Removal of AER= dorsal truncation. Evidence for the progress zone showed Removing the AER at a later period of development results in less disruption of distal structures than if the AER was removed early in development.

Pre-specification model states cells are specified for each segment in early limb bud; this population of cells expand out as limb bud grows. Cell division is throughout the limbs= able to rescue limb development when the AER is removed by preventing cell death

AER= expression of HoxB13- remove AER, dont get digits

Experimental evidence:

Labeled cells in different position of an early limb bud were restricted to single segments of the limb.
Limbs lacking expression of required FGF4 & FGF8 showed all structures of the limb and not just the proximal parts.
More recently, however, the investigators primarily responsible for both the Progress Zone and Prespecification models have acknowledged that neither of these models accounts adequately for the available experimental data

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16
Q

What is the origin of the uteric bud and which tissue induces its outgrowth? (1 mark)

Name the growth factor and receptor responsibile for this induction (1 mark)

The uteric tip causes the condensation of mesenchymal cells to form which structure? Name the two intermediate structures and the final structure that results. (2 marks)

A

Origin = nephric duct
Tissue inducing outgrowth= metanephric mesenchyme

GF= GDNF - acts on Ret Receptor

Nephrons

Bowman’s capsule; proximal convoluted tubule

S shape collecting duct and comma shaped collecting duct

17
Q

What is the congenital heart defect commonly found in Down’s Syndrome, what is its morphology and what process was abnormal in embryonic development? (4 marks)

A
  • Down’s = atrio-ventral septal defect.
  • deficiency of the atrioventricular septum of the heart.
  • failure of formation of the endocardial cushions.

The endocardial cushions are responsible for separating the central parts of the heart near the tricuspid and mitral valves (AV valves), which separate the atria from the ventricles. The structures that develop from the endocardial cushions include the lower part of the atrial septum (wall that divides the right atrium from the left atrium) and the ventricular septum (wall that divides the right ventricle from the left ventricle) just below the tricuspid and mitral valves. The endocardial cushions also complete the separation of the mitral and tricuspid valves by dividing the single valve between the embryonic atria and ventricles. An atrioventricular septal defect may involve failure of formation of any or all of these structures.

Endocardial cushions between atria + ventricles dont form. cushions dont fuse? Cushions normally form as endocardial cells undergo EMT and populate cardiac jelly?

18
Q

Describe the process that results in “Cardiac jelly” (extracellular matrix) of the atrio-ventricular region of the heart tube becoming populated with cells to make the AV cushions? (3 marks)

A

Cardiac jelly (acellular ECM) made by myocardium initially throughout heart tube.

Undergoes EMT

Endocardium cells lose cell-cell junctions.

Apical-basal polarity causing change in cytoskeleton to change shape, increasing motility and gaining invasive properties to move into cardiac jelly.

19
Q

What is the main source of the cells that make the coronary vessels in the ventricles of the heart (1 mark)

A

neural crest cells

20
Q

Which signalling molecule is required for proper separation of the eye fields and what is the consequence if this signal is absent? (1 mark)

A

Shh; = cyclopia/holoprosencephaly in humans

21
Q

From what embryonic structures are the external ear and the ear canal formed? (2 marks)

A

External ear = 6 surface hillocks (auricular hillocks) 3 on each of pharyngeal arch 1+2
Ear canal= Endoderm from 1st pharyngeal pouch

22
Q

What is the consequence of a loss of Notch activity/lateral inhibition at the time of hair cell production in the inner ear? (1 mark)

A

Excess hair cells from when lateral inhibition is disrupted

23
Q

Fibroblast Growth Factor binding to its receptor leads to receptor activation by biochemical modification of specific amino acids within the cytoplasmic domain. What is this biochemical modification and which amino acid is modified? (1 mark)

A

Specific tyrosine residues phosphorylated upon ligand binding

24
Q

Name the three main classes of effector proteins that bind to the activated FGF receptor and, for each, give an example of an activated protein immediately downstream (3 marks).

A

EGFs, PDGFs, FGFs,

Proteins downstream: PLC, Jak Stat, Ras Raf, PI3K

25
Q

Explain briefly, why sonic hedgehog signalling depends on the presence of a functional primary cilium (4 marks)

A

Key components for Shh e.g. Gli transcription factors, smoothened and patched 1 is enriched in primary cilia. Intraflagella transport required for Gli activation in limb development. IFT mutants results in polydactyl (common feature of human ciliopathies). Analysis of basal body proteins shows proteins for Shh are localised to pre-centriolar material needed for cilia. Basal body mutations cause human ciliopathies and detramental to Shh signalling.

26
Q

Describe, briefly, the molecular control of the formation of the median (MHP) and dorso-lateral (DLHP) hinge points during neurulation (4 marks).

A

MHP: induced by notochord- Shh from notochord induces floor plate formation which secretes Shh required for MHP formation.
DLHP: inhibited by Shh via Noggin inhibitor. Noggin in secreted from tips of neural folds overcomes BMP-2-mediated inhibition and enables DLHP formation

(HELP THIS IS BAD)

27
Q

Which two signalling molecules are required for the initiation of the limb buds in the flank of the tetrapod embryo? (1 mark)

A
  • FGF10

- Retanoic acid

28
Q

Explain the role of Tbx transcription factors in forming

the atrio-ventricular region of the heart tube. (2 marks)

A

tbx3- conductive tissue
tbx5 - ballooning of chambers
tbx 18 - inflow tract
tbx 1 - outflow tract

29
Q

Which protein family predominate in the vertebrate lens? What is the consequence of mutations occurring within this family? Which two transcription factors are responsible for expression of these lens-specific genes? (2 marks)

A

Crystallins

Crystallins mutations cause congenital cataract

Pax6, Sox2

30
Q

Usher Syndrome can be caused by different gene mutations that participate in the formation of a common structure. What is this structure? Name THREE genes that have been associated with Usher Syndrome.(2 marks)

A

The retina

USH1B-G, USH2A-C, USH3A

31
Q

Which two tissues induce the formation of the otic placode? (1 mark)

A

The neural tube and the cephalic mesenchyme.

32
Q

What embryonic tissue gives rise to the neural crest?
(1 mark)
What are the two, major subdivisions of the neural crest along the anterior-posterior axis? (1 mark)
Name four different (cell-type) derivatives of the neural crest, clearly indicating one derivative that arises ONLY from the anterior-most subdivision. (2 marks)

A

Ectoderm

Cranial and trunk neural crest

Only from anterior most sub-division= bone/cartilage

  • sensory nerve
  • schwann cell
  • melanocyte
  • smooth muscle
  • enteric nerve
  • autonomic nerve
  • chromaffin cells
33
Q

Describe two distinct roles of retinoic acid signalling in somitogenesis. (3 marks)

A

Khadija - Recent studies indicate that RA action involves an interplay between diffusion (morphogen-like) gradients and the establishment of signalling boundaries due to RA metabolism, thereby allowing RA to finely control the differentiation and patterning of various stem/progenitor cell populations.

A Wnt, FGF & RA threshold represents the wavefront with Wnt3a particularly important
High levels (above threshold): clock on
Low levels (below threshold): clock off.

In a RA-signaling-deficient mouse or zebrafish embryo, somite formation is consistently delayed on the right side.

RA gradient formed by RALDH2 is in the opposite direction and antagonises FGF.

RA- against CYP26 across AP axis = Hox gene patterning hence somites are predetermined.
Forms caudal part of somite in wavefront.

34
Q

It is generally advised that, for maximum protective effect against birth defects, folate supplements should be started before pregnancy. Why before pregnancy? (2 marks)

A

folate is required to reduce the occurence of neural tube defects. Neurulation occurs around day 21 which is before people usually find out they are pregnant. Therefore starting them before pregnancy means the folate levels will be high enough when needed

35
Q

What happens to the autopod of the chick limb following application of retinoic acid to the anterior margin of the developing limb bud? How does the outcome vary with increasing concentrations of retinoic acid? (2 marks)

A

RA anteriorly increases dHAND increases Shh duplicating ZPA –> opposng gradient Gli3 so digits form symmetrically duplicated
- the more equal the conc of the ZPA the exact digit mirror image form
lower conc = only digit 1 or st

36
Q

Explain the development of “persistent foramen ovale”(PFO)? (4marks)

A

primitve embryonic atrium starts with a single cavity

primary atrial septum growing caudally towards the AV cushion

the trailing edge breaks down

this closes at secondary atral septation
PFO is failure of this closure causing right to left shunting

37
Q

Which small, intracellular signalling molecule is responsible for maintaining the oocyte in meiotic arrest? How are the levels of this molecule altered to permit the resumption of meiosis? 4 marks

A
  • cyclic AMP
    Elevated intracellular cAMP levels
    in denuded mouse oocytes prevent spontaneous maturation.
    Intracellular cAMP appears to decrease rapidly under some conditions at the start of mammalian oocyte maturation.
38
Q
Match the heart region with the Tbx gene that regulates its formation: 2 marks 
Inflow: 
Chambers: 
Conduction system:
Outflow:

Tbx 1 5 18 3

A

Inflow: Tbx 18
Chambers: Tbx 5
Conduction system: Tbx 3
Outflow: Tbx 1