Segmentation in Vertebrate Embryos Flashcards
A “module” of the embryo that consists of a group of adjacent cells that do not mix with cells from neighbouring compartments is defined as what?
A compartment
Cells in a compartment display what similar trait?
The same gene expression profile
A border/region between two compartments that cells do not cross is defined as what?
A compartment boundary
Some clones of cells within a compartment have _____________ or ______________ borders
Jagged; straight
What are the two functions of the boundaries?
Prevent intermingling of cells and provide positional information to flanking cells
How can compartment boundaries be visualized?
With a lineage marker
What are the different adhesion properties at compartment boundaries?
Quantitative and qualitative
How does a field of cells become subdivided?
Their interpretation of a morphogen gradient
What induces transcription factors, adhesion affinities within a field of cells?
Morphogen gradient
Subdivisions are maintained and refined by what?
Local cell-cell interactions
How is a boundary formed in a field of subdivided cells?
Two distinct populations lead to the formation of specialized cells at borders
How does the boundary influence surrounding cells?
Long-range signaling
Where do organizers form?
At compartment boundaries
What initiates the segment polarity genes in Drosophila?
Stripes of pair-rule genes
What is a parasegment?
Anterior end of one stripe to anterior end of next stripe
Where is Wingless expressed in Drosophila?
Between the stripes of Eve and Ftz
How many cells expressing Engrailed are found per stripe of Eve and Ftz?
One
Cell expressing _____________ is the posterior cell of each segment
Engrailed
Segments allow for repetition of ________
Form
What are somites?
Masses of mesoderm formed from presomitic mesoderm
How many pairs of somites are formed in chicks?
55 pairs
Somites differentiate to give rise to which structures?
Dermis, skeletal muscle and vertebrae
Which part of somite differentiation gives rise to muscle, endothelia, cartilage, connective tissue and dermis?
Dermomyotome
Which part of somite differentiation gives rise to vertebral column, meninges, connective tissue and ribs?
Sclerotome
Signaling of what molecule is important for paraxial mesoderm specification?
BMP
What experiment was done to test the role of BMP signaling in somite formation?
Transplant Noggin-secreting cells into LPM
Formation of somites
What experiment was done to observe the role of Tbx6 in paraxial mesoderm differentiation?
Tbx6 knockout in mice led to paraxial mesoderm transformation into neural tubes
How do somites know when and where to form?
Embryological clock measures development
Rate of somite segmentation must be coordinated with overall development and occur in parallel on both left and right sides
What patterns neuromesodermal progenitors during paraxial mesoderm development?
Antagonistic signals along the anterior-posterior axis
Which antagonistic signal is more prominent at the anterior axis of NMPs?
Retinoic acid
Which antagonistic signal is more prominent at the posterior axis of NMPs?
Fgf8
Which molecule is expressed at the boundary of low RA/low Fgf8 and indicates where the next somite will form?
Mesp
Signaling of what molecules regulates epithelialization during somite boundary formation?
Ephrin
Place the events of epithelialization during somite boundary formation in order
a. Upregulates EphA4 in anterior half
b. Mesp restricted to anterior half of S-1
c. Triggers epithelialization and formation of a boundary
d. EphA4 upregulates its binding partner ephrinB2 in presumptive posterior S-0
b, a, d, c
What signaling pathway controls when the somite boundary is formed?
Notch signaling pathway
Which molecular oscillator in the PSM modifies Notch signaling to regulate the pace of vertebral segmentation?
Lunatic Fringe (Lfng)
What is the mechanism of Notch signaling in boundary formation?
Notch target gene oscillates in segmentally defined pattern
Mesp (Notch target) in anterior half represses Notch activity in anterior half
Genes that form the boundary are activated
High Fgf8 in posterior end prevents cells from being competent to respond to Notch signal
The spatiotemporal collinearity of Hox gene expression in the presomitic mesoderm is correlated with what?
Chromatin remodelling
What happens when you transplant presomitic mesoderm from region that would normally form thoracic vertebrae caudal to the 1st somite in a younger embryo of chick?
Formation of ribs in the neck vertebrae (gain-of-function anterior to posterior transformations)
What happens if cervical somites were transplanted into thoracic region?
Ribs don’t form on vertebrae derived from cervical somite
True or false: Snakes have almost 1-2x more somites than vertebrae
True
Why do snakes have more somites than mice?
They have 3-fold more oscillations in Lunatic Fringe expression in the PSM
Identify the definitions to the axis in the human body
a. Tips of digits
b. Closest to the body
c. Back of the limb (top of the foot)
d. Front of the limb (bottom of the foot)
e. Thumb/big toe
e. Pinky/little toe
a. Distal
b. Proximal
c. Dorsal
d. Ventral
e. Anterior
f. Posterior
In the limb bud, the posterior LPM and somite cells form which structures?
Posterior LPM forms skeletal precursors and somite cells form muscle precursors
Between which days do the limb buds continue to grow in mice?
9 and 11
What is visible at E11 of the formation of the limb buds in mouse?
Apical ectodermal ridge (AER)
What event occurs for the interdigit region to be formed?
Programmed cell death
Digit growth in human embryo occurs between which days?
48 and 56
Where is limb bud formed?
Outpouching of lateral plate mesoderm at the “correct” position along the AP axis
What do hox genes expressed along AP axis regulate?
Expression of retinoic acid and Fgf
RA and Fgf8 act as antagonistic signals to induce ___________ expression
Tbx5
What is Tbx5 responsible for?
Position of forelimb bud
The forelimb field belongs to which hox gene?
Hoxc6
What is RARE?
RA response element
What is Rdh10 important for?
Synthesis of retinoic acid
What experiment was done to observe the role of RA and Fgf8 along the AP axis?
In situ hybridization for RARE expression in the forelimb field of the wild type and mutant Rdh10
In situ hybridization for Fgf8 in the heart and caudal progenitor zone of the wild type and mutant Rdh10
What were the results from the experiment with Rdh10 mutant?
RARE was expressed in wildtype Rdh10, but not in mutant Rdh10, indicating that RARE is found in the heart
Fgf8 was expressed in both wildtype and mutant Rdh10, indicating that Fgf8 is found in the heart and caudal progenitor zone
Tbx5 was expressed in wild type Rdh10, but not in mutant Rdh10. What is the significance of this?
Tbx5 requires RA signaling to be induced
What signals position of LPM outgrowth?
Fgf10
Between RA and FGFs, which one induces ectopic limbs?
FGFs
