Pattern formation

1
Pattern Formation
2
Regeneration Models
Morphallaxis
Remodeling of tissue into a whole new organism
No new cell division
Hydra
Fig. 18.25, pg. 580
3
Epimorphosis
Addition of a part to the whole New growth
and cell division
Salamander
Fig. 18.19, pg. 574
4
(No Transcript)
5
Proposed Mechanisms for Pattern Formation

• Morphogen Gradient

• Haptotaxis Induction of cell growth
• and integrin expression

• Thermodynamic (Temporal) Model
• and cadherin expression

6
Ultimately defined by regional Hox gene
expression
Limb Field
7
Blastema
Regeneration
8
Fig. 16.3, pg. 507
9
Dorsal
Posterior
Distal
Proximal
Anterior
Ventral
10
Myotome cells moving in to the limb bud
3D reconstruction from in situ hybridization to
the myf5 mRNA in developing muscle cells
Myotome cells
Limb bud
11
DORSAL
VENTRAL
Right limb
Limb Outgrowth
12
Differential Control of 3 Axes
ZPA

• Proximal / Distal Axis Amount of time cells
• spend in progress zone

• Anterior / Posterior Axis Morphogen gradient
• from the Zone of Polarizing Activity

• Dorsal / Ventral Axis Ectodermal gradient

13
III
Forelimb
Femur Tibia Fibula Carpals Digits
Hindlimb
Chick Wing
Fig16.1, pg. 506
14
Apical Ectodermal Ridge influence on Mesenchyme
Wing
Regional Specificity
Fibroblast Growth Factor FGFs 12 genes with
100s of protein isoforms
Fig. 16.9, pg. 511
15
Apical Ectodermal Ridge influence on Mesenchyme
Wing
Fibroblast Growth Factor FGFs 12 genes with
100s of protein isoforms
Fig. 16.9, pg. 511
16
(No Transcript)
17
Limb Field
Forelimb
FGF10 expression throughout Lateral
plate mesoderm
Somatic mesoderm
Hindlimb
FGF10 stabilized regionally by Wnt8c (hindlimbs)
and Wnt2b (forelimbs)
Fig. 16.6, Pg. 509
18
Shh activates an antagonist of a repressor of
Fgf4 expression to positively regulate AER
signaling
Fig. 16.19, pg. 519
19
Equivalent genes on each chromosome Hoxa1,
Hoxb1 termed paralogues
4 chromosomes resulted from chromosome
duplications
20
Homeotic Hox genes Genes which regulate body
segmentation and proximal / distal
differentiation Hox genes in vertebrates
homologous to Homeotic genes in Drosophila
Similar to Fig. 16.14, pg. 515
21
Characteristics of Hox genes

• Mice and humans have 4 Hox clusters (a total
  of 39 genes
• in humans) located on 4 different
  chromosomes.

• In humans HOXA, HOXB, HOXC, HOXD

• Act along the developing embryo in the same
  sequence that they
• occupy on the chromosome.

• All genes in the mammalian Hox clusters show
  some sequence
• homology to each other (especially in their
  homeobox) but very
• strong sequence homology to the equivalent
  genes in Drosophila.
• HoxB7 differs from Antp at only two amino
  acids, HoxB6 at four.

- mouse HoxB6 gene inserted in Drosophila can
substitute for Antennapedia producing
legs in place of antennae
Conclusion?
22
Selector genes have retained, through millions of
years of evolution, function of assigning
particular positions in the embryo. The
structures actually built depend on a different
set of genes specific for a particular species.
Genetic Specificity
23
(No Transcript)
24
Proximal / Distal Axis Time cells spend in
Progress Zone
P/D positional value shifts accordingly the more
time spent in undifferentiated growth zone
25
Humerus
Radius
Radius
Ulna
Ulna
Humerus
26
X-irradiate
Remove bud and transplant to non-irradiated host
Limb Bud
Result ?
Lose proximal structures but distal structures
are normal
Conclusion ?
27
Thalidomide exposure between D20 D36 of
pregnancy
Glutamic acid derivative
Phocomelia
28
(No Transcript)
29
Cell fate already determined and cells simply
populate these regions
Fig. 16.13, pg. 514
30
Posterior
Distal
Proximal
Anterior
31
Anterior
Retinoic Acid
ZPA
Posterior
Zone of Polarizing Activity
32
III
Chick Wing
Fig. 16.1, pg. 506
33
(No Transcript)
34
4
3
2
2
Results of Graft I
3
4
2
3
4
4
3
Results of Graft II
4
35
4
4
3
3
2
2
36
4
4
4
3
3
2
37
Experimental Design - ZPA
from hamster transplanted to chick limb bud
Possible Results ?
Additional chick digits
Conclusion ?
38
2
ELCR - Enhancer regulates HoxD gene expression
3
Shh activates GCR enhancer and reverses Hox
expression
Gli3 repressor transcription factor dHand
activating transcription factor of shh
1
Fig. 16.18, pg. 518
39
Shh activates an antagonist of a repressor of
Fgf4 expression to positively regulate AER
signaling
Fig. 16.19, pg. 519
40
Sonic hedgehog (Shh) is the active agent
Fig. 16.16, pg. 516
41
Gene receptors for morphogen enhancer
42
Possible models of ZPA activity
43
Mouse
a) Progenitors of digit 4 (green) and 5
(red) are in ZPA and express Shh b) Digit 5
cells still expressing Shh in ZPA but digit
4 cells are not c) When digits form, digit 5
cells will have been exposed to Shh protein
for a longer time than digit 4 cells d)
Autocrine vs. paracrine exposure
Fig. 16.20, pg. 520
44
Shh may work through BMPs for digit identificatio
n
Homozygous for Gli3 and Shh deletions - No
order
Removal of interdigital (ID) space shown in red.
ID space expresses BMP. Addition of bead soaked
with Noggin, a BMP inhibitor.
Fig. 16.21, pg. 521
45
Dorsal / Ventral Axis Determination
Wnt7a
Lmx1b
En1 transcription factor represses Wnt7a
expression in ventral ectoderm
Wnt7a deficient mice --- Duplication
of ventral tendons and footpads
Fig. 16.22, pg. 521
46
Summary of proposed events in limb formation
Fgf-10
Induction
Maintenance
47
Maintenance of Shh in dorsal ectoderm
Shh blocks cleavage of GLi3 to GLi3R creating a
gradient of GLi activator and GLi3 repressor
Gremlin blocks BMPs which inhibit FGFs
Fig. 16.23, pg. 522
48
Aptosis - Programmed cell death
Noggin protein expressed
No expression of BMP4 Due to Noggin expression
Expression of BMP4
Fig. 16.24, pg. 522
49
BMP class of genes first identified as genes
that stimulate bone formation - Bone
Morphogenetic Proteins -
BMPs now shown to induce chondrogenesis and
aptosis
Context Dependency - Response depends on
the age of the target cells
50
Trematode cysts developing in limb
bud field