Organogenesis and limb formation

Question 1

Name the vertebrae segments from A to P and corresponding Hox genes.

Answer 1

cervical - Hox5
thoracic - Hox6
lumbar - Hox9
sacral - Hox10
caudal - Hox11

Question 2

At which positions do fore and hindlimbs form?

Answer 2

forelimbs: border between cervical and thoracic
hindlimbs: border between lumbar and sacral

Question 3

Which 3 gradients that induce Hox gene expression arise along rostro-caudal axis?

Answer 3

RA
Wnt
Fgf

Question 4

What are Hox genes?

Answer 4

homeodomain transcription factors
can repress or activate gene expression programs
are selector genes
single mutations cause homeotic transformations (mutations result in transformation of a whole segment or structure into another one)

Question 5

Examples of homeiotic transformations in Drosophila.

Answer 5

antennapedia

bithorax (ultrabithorax)

Question 6

How many Hox genes and Hox gene clusteres are in humans?

Answer 6

4 clusters
13 Hox genes
(not all genes in all clusters)

Question 7

Is there a colinearity between order of Hox genes and their affected regions in body segments? If so, who discovered it?

Answer 7

E. Lewis:
genes located on the 3’ end of each cluster are activated early in development and in anterior tissues

genes located on the 5’: later, more posterior

(no Hox in the head)

Question 8

What causes transcriptional activation during axis extension?

Answer 8

transition in histone modifications:
from repressive H3K27me3
to activating
H3K4me3

higher order chromatin organisation:
in forebrain = Hox clusteres are silent
= adopt compact structures (heterochromatin)

Question 9

What happens when you KO all copies of a Hox gene (e.g. Hox10)?

Answer 9

Hox10 = lumbar

KO: no lumbar vertebrae, thoracic extends posteriorly

Question 10

How do you make a snake out of a mouse?

Answer 10

Express Hox6 ectopically

= thoracic vertebrae with ribs extending to the tail

Question 11

Which 2 roles does HOX cluster play in limb development?

Answer 11

1. limb field patterning

2. patterning of the limb along proximal-distal axis

Question 12

Describe limb field patterning.

Answer 12

Hox proteins in lateral plate mesoderm
induce Tbx expression below surface ectoderm (= in mesoderm)
Tbx -> Fgf10 (-> Wnt) -> Fgf8 in surface ectoderm
creates apical ectodermal ridge

Type of Tbx specifies limb type:
Tbx5 = wing
Tbx4 = leg in chick

Question 13

Which Hox genes determine which part of the limb?

Answer 13

Stylopod = Meis
Zeugopod = Hox11
Autopod = Hox13

Question 14

Which 2 morpho-regulatory signaling centers control vertebrate limb-bud development?
Which gradient defines each of them?

Answer 14

a) apical ectodermal ridge -> Fgf8 & Fgf10 expression

b) zone of polarizing activity -> Shh on P side

Question 15

Is RA concentration higher in proximal or distal part of the limb?

Answer 15

proximal

Question 16

Which gradient defines the identities of the digits?

Answer 16

Shh from ZPA:
digit 1 never encounters Shh
2 only paracrine
3 is half paracrine, half directly Shh secreting cells
5 exposed to Shh longer than 4

Question 17

Which two roles does Shh play in patterning of the limb?

Answer 17

1. ) acts as a morphogen: specification and patterning of tissue by duration of exposure to Shh and its concentration
2. ) mitogen: induces cell proliferation of the limb bud mesenchyme and therefore shaping the growing limb bud

Question 18

Which gradient is opposing the Shh gradient?

Answer 18

Gli3R

Question 19

Is there a difference between ZPA genome regions between different species?

Answer 19

yes
ZPA enhancer is different
in snakes, Fgf8 is degraded = no limbs
can create serpentized mice